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Strategies to Mitigate the Effects of Soil Physical Disturbances Caused by Forest Machinery: a Comprehensive Review

Strategies to Mitigate the Effects of Soil Physical Disturbances Caused by Forest Machinery: a... Purpose of Review Ground-based mechanized forest operations can cause severe soil disturbances that are often long lasting and detrimental to the health of forested ecosystems. To reduce these soil disturbances, focus is being increasingly directed at identifying and using appropriate mitigation techniques. This systematic review considered 104 scientific articles and reported the main findings according to four core themes: terrain-related factors, operational planning, machine modifica - tions, and types of amendments used to mitigate machine-induced soil impacts. Recent Findings For terrain-related factors, most severe disturbances occur on machine operating trails exceeding 20% slope and that soil bulk density and rut depth show greater increases in fine-textured soils. When considering operational planning, trafficability maps proved to be helpful in reducing the frequency and magnitude of soil damages as well as the length of trails needed within harvest sites, especially if they are regularly updated with weather information. Machine modi- fications, through high flotation tires, use of extra bogie axle, lower inflation pressure, and use of steel flexibles tracks, are highly researched topics because of the considerable upside in terms of machine ground pressure distribution and increased traction. Two main types of amendments emerged to mitigate soil disturbances: brush mats and mulch cover. Brush mats created from harvesting debris can spread the load of a machine to a greater area thereby lowering peak loads transferred −2 to the soil. Brush mats of 15–20 kg  m are being recommended for adequate soil protection from harvesting operations. Summary To conclude, we outline recommendations and strategies on the use of soil mitigation techniques within cut-to- length forest operations. New research opportunities are also identified and discussed. Considering single factors causing machine-induced soil disturbances remains important but there is a pressing need for having a multi-disciplinary approach to tackle the complex problems associated with machine/soil/plant interactions. Keywords Harvesting · Planning · Rutting · Soil compaction · Mitigation techniques · Sustainability This article is part of the Topical Collection on Forest Engineering * Eric R. Labelle Skogforsk, the Forestry Research Institute of Sweden, eric.labelle@sbf.ulaval.ca Uppsala Science Park, 751 83 Uppsala, Sweden Linnea Hansson Department of Forest Ecology, Swedish University linnea.hansson@skogforsk.se of Agricultural Sciences, 90103 Umea, Sweden Lars Högbom Department of Forestry and Forest Economics, lars.hogbom@skogforsk.se Faculty of Natural Resources, College of Agriculture and Natural Resources, University of Tehran, P. O. Box, Meghdad Jourgholami 31585-4314 Karaj, Alborz, Iran mjgholami@ut.ac.ir Department of Agricultural, Food and Forest Sciences, Andrea Laschi University of Palermo, Viale delle Scienze ed. 4, andrea.laschi@unipa.it 90128 Palermo, Italy Department of Wood and Forest Sciences, Université Laval, 2405 Rue de la Terrasse, Québec, Québec G1V 0A6, Canada 1 3 Current Forestry Reports (2022) 8:20–37 21 strategies to control and minimize the negative effects of Introduction mechanized harvesting operations on forest soils should be properly planned and implemented [1, 2]. A higher share of forest mechanization has been needed Many papers have dealt with the direct impact of for- to meet the growing needs of the world’s population for est operations on soils, water regime, vegetation, and for- wood and wood-derived products [1, 2]. Despite some est growth. However, as forest mechanization is increasing, clear advantages of mechanized harvesting systems, such so is the need for appropriate and tailored mitigation tech- as higher productivity and improved safety, concerns sur- niques to reduce the environmental impacts of machines on rounding the use of heavy machines on forest soils remain the soil. The overall aim of this paper was to systematically [3]. Forest machines are commonly operated on machine review scientific literature concerning soil impact mitiga - operating trails designed to allow machine movement in tion techniques used in a context of mechanized harvesting a stand. This off-road traffic can lead to significant envi - operations. ronmental effects on forest ecosystems, particularly for - In addition to the overall aim, the paper also: est soils [4, 5]. From the perspective of sustainable forest management and the concept of sustainable forest opera- (1) provides recommendations and strategies to mitigate tions [1], forest soils serve as one of the basic components machine-induced soil disturbances through terrain- assuring the sustainability and productivity of forest stands related factors, operational planning, machine modifi - [6, 7]. cations, and types of amendments, Compaction, rutting, and displacement (a biproduct of (2) highlights key opportunities for future research. rutting) are the main impacts caused to the soil by forest harvesting operations [2, 8, 9]. Compaction occurs when the mechanical forces exerted to the soil cause soil parti- Material and Methods cles to be pushed closer together at the detriment of pore space, which leads to increased soil bulk density [6, 10]. Databases and Search Strategy During this process and when soil water content is high, soil deformation can occur through repeated machine A systematic review was performed in Scopus and Web passes [11–13]. Ground-based traffic of forest machines of Science databases. After several iterations, the follow- can adversely affect the following key functions of forest ing search strings were used as they provided the most rel- soils: increase soil strength and soil bulk density [14, 15], evant results regarding soil mitigation techniques to reduce decrease macroporosity [16], infiltration [ 14], saturated machine-induced soil disturbances. In Scopus, we searched hydraulic conductivity [17], and impede soil fauna colo- for the terms ("forest* operations" OR "forest* harvesting" nization [14]. In turn, these effects can lead to decreased OR "forwarder*" OR "skidder*") ("soil mitigation" OR "soil oxygen availability, increased surface waterflow [ 18], soil disturbance" OR "soil protection" OR "ground protection" loss and sedimentation [19]. Ultimately, these adverse OR "rutting" OR "compaction") in the title, abstract and effects can hinder root and tree growth [ 20], and nega- keywords. In Web of Science, the search was (forest* opera- tively affect site productivity [ 21]. The degree and extent tions OR "forest* harvesting" OR “forwarder*” OR “skid- of soil compaction and rutting during mechanized logging der*”) AND ("soil mitigation" OR "soil disturbance" operations is often influenced by the type and characteris - OR "soil protection" OR "ground protection" OR "rutting" tics of the harvesting system, number of machine passes, OR "compaction") Refined by: WEB OF SCIENCE CATE - terrain slope, soil type, texture and water content as well GORIES: (FORESTRY). Indexes: SCI-EXPANDED, SSCI, as silvicultural treatments [2, 3, 22, 23]. CPCI-S, ESCI. The undesirable effects of machine traffic on forest soils Studies were only considered if words used in the search can persist for a few years to several decades [24–26]. For string were present in the title, keywords or abstract and if example, DeArmond et al. [5] concluded that soil physical they were: properties did not return to preharvest level 30 years after machine-induced compaction had occurred in the Ama- • published between 2010 and 2020, in order to reflect the zon Basin. In the Hyrcanian forests, Jourgholami et al. actual state of the art and recent developments; [14] reported that soil compaction effects persisted over a • peer-reviewed scientific articles or conference proceed - 25-year period. Similarly, Ezzati et al. [27] found that soil ings; bulk density and total porosity did not recover 20 years • written in English. after ground-based machine traffic. In general, the impli - cations to restore a compacted soil to a preharvest level are time-consuming and costly. Therefore, measures and 1 3 22 Current Forestry Reports (2022) 8:20–37 3. Full articles were read and inclusion rested on the use Search Query and Article Analysis of techniques to mitigate machine-induced soil physical disturbances (primarily soil compaction and rutting). The search query resulted in a total of 273 articles of which 90 were reported by Scopus, 101 by WOS and an additional In total, 104 articles fulfilled all three criteria (Fig.  1; 82 articles common to both databases but only counted once (Fig. 1; black line). The number of published articles has grey line) and are part of this review. To structure the main findings and associated recommendations, the articles been increasing in the past decade with two noticeable peaks occurring in 2015–2016 and from 2018 onwards with the were divided into four main themes: terrain-related fac- tors, operational planning, machine modifications, and use highest frequency of 45 articles reported in 2020. Each of the 273 articles resulting from the search queries of amendments as mitigation techniques (Fig. 2). These themes are then presented and assessed in a chronological were evaluated in a three-step process. order from the point of view of forest operations. 1. Titles were first verified to make sure that the field of study was forestry, 2. Abstracts and keywords were read and articles that did not refer to soil mitigation techniques were omitted, Fig. 1 Overall search results of Scopus and Web of Science databases (black line) and search results according to three-step refinement process (grey line) 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Year of publication Unfiltered results Filtered results Fig. 2 Thematic classification along with article count 1 3 Number of articles Current Forestry Reports (2022) 8:20–37 23 soil water content at the time of harvesting [9, 11, 13, 30], Results soil texture [23, 32], initial soil bulk density, relative bulk density [21], organic matter content, and duff thickness [ 31] Terrain‑Related Factors can all play an important role in how a soil will physically respond to ground-based machine traffic (Table  1). In the Terrain-related factors are normally inherent to a harvest site context of this article, slope is referred to as the longitudinal and are thereby difficult to adjust, particularly once opera - incline or decline of the terrain located on machine operat- tions are on-going. However, despite the reduced possibili- ing trails. ties for alterations, factors such as slope [12, 23, 28–31], Table 1 Summary of terrain-related factors and their associated mitigation recommendations Factors References Country Category Properties Impacts Mitigation recommenda- tions Slope [31, 33–37] Iran 0–20, > 20% BD, PR, TP ↑↑↑ Limit operations to gentle Italy slope, restrict trails to slopes < 25%, importance of preplanning and using designated trails [30] Slovakia 5, 10, 20, 25% RD − Limit operations to gentle slope [6, 12, 16, 23] Iran < 10, 10–20, > 20% RD, BD, TP ↑↑↑ Limit traffic to slopes < 20% [29] Iran 15, 25, 35% BD, TP ↑↑ Restrict trails to slopes < 25% [28, 38] Iran flat, 10 (downhill), 10, RD, BD ↑↑ Limit traffic to 20% (uphill) slopes < 20%, limit traffic from uphill direct when loaded Soil water content [13, 30] Slovakia n.c RD, PR ↑↓ Schedule operations to dry and Czech Republic periods or frozen soil whenever possible [12] Iran 19, 33% GWC RD, BD, TP ↑↑↑ Plan logging operations when soil water content is low, limit excessive machine passes on moist soils [11] Iran 20 − 30, 30 − 40, RD ↑ Plan operations in dry soil 40 − 50% GWC conditions, limit exces- sive machine passes on moist soils Soil texture [32] Brazil Sa. Cl. Lo., Cl., Sa. Lo., PS ↑ Consider soil load bearing Lo. Sa., Sa capacity [23] Iran Cl. Lo. + Sa. Lo RD, BD ↑↑ When possible, exclude areas with fine textured soils, apply topsoil rein- forcement (i.e., logging residues, brush mats) [39] U.S.A S. Lo., Sa., Cl BD ↑ Limit skidding operations to times of low soil water content Relative soil bulk density [21] Germany n.c RBD ↑ Exclude driving on sensi- tive areas Link between RBD thresh- old and biomass growth impediments GWC gravimetric water content, BD bulk density, PR penetration resistance, TP total porosity, RD rut depth, PS precompression stress; Sa. Cl. Lo. sandy clay loam; Sa. Lo. sandy loam; Lo. Sa. loamy sand; n.c. not considering the category 1 3 24 Current Forestry Reports (2022) 8:20–37 Slope Jourgholami and Majnounian [11] concluded that average rut depth were 35 cm, 22 cm, and 17 cm on soils with gravi- Terrain slope often governs the trafficability of forest metric water contents of 40 − 50%, 30 − 40%, and 20 − 30%, machines and can result in increased soil bulk density [12, respectively. 23, 28, 29, 31, 33, 38] due to an unbalanced load distribu- Studies also highlight that when soil water content tion during logging operations. Naghdi et al. [23] found that exceeds the field capacity, an important measure to miti - soil bulk density and rut depth increased as the slope of a gate soil compaction and rutting is the cessation of harvest- machine trail increased. Conversely, Jankovský et al. [30] ing activities until soil conditions improve (Table 1). Other reported that trail slope had little to no effect on mean rut implications include reducing the ground contact pressure, depth and the rehabilitation of rut depth five years following topsoil reinforcement (i.e., logging residues, mulching), harvesting operations. and scheduling operations to dry period or on frozen soils Sohrabi et  al. [31] reported that, due to slippage of a [11–13, 30]. wheeled skidder, soil bulk density and penetration resist- ance were higher on slopes exceeding 20% as compared to Soil Texture and Other Factors lower gradients (0–20%). These same authors also reported that the recovery rate of soil properties including bulk den- Several studies focused on the effects of texture on soil sity, total porosity, and penetration resistance was slower in compactibility [32, 39]. Naghdi et al. [23] concluded that slopes exceeding 20%. Similarly, Naghdi et al. [33] deter- soil bulk density and rut depth showed greater increases mined that soil bulk density, porosity, and rut depth were in clay loam soil than in a sandy loam soil during ground- significantly affected by trail slope. based skidding. The content of fine-grained particles (silt According to the abovementioned results, several authors and clay) is an important driver that determines the degree concluded that ground-based logging operations should be of soil compaction [9]. The resistance to compaction was limited to machine operating trails with slopes below 25% higher in soils with low clay content [32]. Slesak et al. [39] [12, 23, 28, 29, 31, 33, 38]. However, in some instances, stated that a well-structured loam soil was more susceptible few machine passes can be allowed on trails steeper than to soil compaction. Martins et al. [32] found that higher lev- 25% [12]. By considering two skidding directions through els of resistance to soil compaction were observed on soils downhill slopes (i.e., -10%, -20%) and uphill slopes (10%), with greater amount of sand content. Accordingly, Naghdi Majnounian and Jourgholami [38] and Jourgholami et al. et al. [23] reported that rut depth on slopes exceeding 20% [28] explained that soil bulk density showed substantial were 28.3 and 15.4 cm, on clay loam and sandy loam soils, increases in uphill skidding as compared to downhill skid- respectively. ding direction. In uphill skidding, wheeled ground-based Soil structure, soil organic matter content, and initial machines slipped on the surface soil, resulting in the expo- bulk density were also crucial in assessing soil compaction sure of mineral soil due to the applied vibration and shear caused by heavy forest machines [31]. Forest soils with an strength [28, 38]. initial low (pre traffic) bulk density can be more easily com - pacted, at the same compaction energy, than a denser soil. Soil Water Content However, a denser soil might require a lower compaction energy in order to trigger tree growth impediments [21]. Soil water content plays a key role in soil consistency as soil changes from one state to another (i.e., Atterberg lim- Operational Planning its) [30]. Seasonal changes in rainfall and temperature have direct effects on soil water conditions that influence load Decision Support for Predicting Trafficability and Route bearing capacity, which in turn determines the degree of soil Planning disturbance and compaction [11]. Normally, moist soils are more susceptible to compaction than dry soils [30]. Soil dis- turbances and average rut depth can effectively be predicted Theoretical Predictions of Machine Impact In Russia, several by soil water content [12, 13]. At a given compaction energy, studies have focused on predicting machine impact by theo- increasing the soil water content will increase soil bulk retical mathematical models. Manukowskii [40], presented density up to the maximum density, after this critical point, a model to calculate rutting by caterpillar forest machinery soil bulk density decreases as soil water content increases based on number of passes, subsoil and topsoil properties. [30]. A strong correlation exists between soil water content Grigorev et al. [41] developed a model for dynamic soil and mean rut depth [11–13]. Allman et al. [13] found that compaction by wheeled forestry machines where the impact when soil water content exceeded the plastic limit, an aver- of topography, weight and type of wheel system could be age rut depth > 16 cm occurred during logging operations. studied. However, the model did not include parameters 1 3 Current Forestry Reports (2022) 8:20–37 25 such as soil water content and soil type. A detailed study of susceptibility for either compaction, displacement or rut- the impact of logs on three different soils during skidding, ting. The model was based on GRASP (generalized regres- by the same research group, was suggested to be used for sion analysis and spatial prediction) and included a.o. soil drawing up a technological map for projecting a logging properties, slope, and forest type. It accurately predicted area based on skidding system specifics and soil type [ 42, compaction, displacement and rutting with 97 − 98% in the 43]. The model results were compared with experimental test area in Iran. studies in the laboratory and the differences were less than In 2012, Mohtashami et al. [49] demonstrated a model 10% [42]. Rudov et al. [44] studied the lifting capacity of in ArcGIS where elevation, slope, aspect and soil type were frozen grounds of different textures, soil water contents and used to build up a cost-index surface, where trafficability was temperatures under the action of static loads and pointed out classified in five levels. In the case study presented, it was the increase in sensitivity of the soils between -1 and 0 °C demonstrated that both time and money could be saved when and its dependence on soil salinity. using the model and building a corduroy road on a piece of A decision support tool used in agriculture is the Soil- wetland to reduce the forwarding distance and at the same Flex-model which was developed for predictions of tire/ minimize the disturbance on soils and waters for the entire soil interactions: Goutal et al. [45] tested if it could also be extraction road network. Following this work, Mohtashami used on forest soils in France. The predicted bulk densities et al. [50] tested if depth-to-water maps alone or combined after two passes with a forwarder were higher than the meas- with other information such as soil type, could predict where ured ones in the upper layer (0 − 10 cm) and the opposite ruts occurred during forest operations. The results pointed but smaller differences in the deepest layers (20 − 35 cm). to that DTW-maps alone did not predict rut formation but According to the authors, the overestimation in the upper combined with other information, their relevance increased. layers could be due to the buffering effect of the forest floor Slash reinforcement was used by the operators, when they due to the organic matter content and persistent root mat found it necessary, at the 16 sites included in the inventory which is not present in agricultural soils. Goutal et al. [45] which may have influenced the results. suggested that an increased rebound could improve the Whereas trafficability maps that are not modified over model performance and that this parameter depends on the time despite changing weather can be referred to ‘static’, soil organic carbon in the soil layer. They conclude that the ‘dynamic’ trafficability maps are updated using weather model estimations well predicted the compaction range after data. In a study by Salmivaara [51], a dynamic map for forwarder traffic, even for forest soils with a non-negligible rut depth or rolling resistance prediction was developed root and gravel content. by using hydrological modelling to account for changing weather conditions. The map had a resolution of 16 × 16 m and a wide range of open-source spatial data was included, Trafficability Maps Several trafficability maps have been e.g. soil texture, a digital elevation model and stand charac- developed and tested in the field in the recent years. In Fin - teristics from the Finnish forest inventory, as well as trans- land, a static trafficability map was developed by Arbonaut ported mass through a cell. In the same study, harvester OY and tested by Kankare et al. [46]. The model was based CAN (Controlled Area Network)-bus data was pointed out on topographic wetness index (TWI), amount of vegeta- as a useful tool to measure rolling resistance coefficient in tion, ground water height and ditch depth. The main point the field [ 51]. was to provide information about in which season different areas may be harvested with standard machinery (harvesters Predicting Trafficability Based on Field Measurements Cone and forwarders) with a 16 m × 16 m resolution. When the penetrometers are popular and simple tools for measure- maps were used in thinning operations, ~ 70% of the evalu- ments of bearing capacity in soil disturbance research, ated stands were harvested without causing any damage if where Cone index is the penetrometer resistance at a given they were harvested in the correct season; the correspond- depth (average or maximum). Allman et al. [52] found that ing percentage for incorrect timing was ~ 40%. In Western Cone index of the upper 30 cm of the mineral soil was a Montana (US), Reeves et al. [47] developed a model based weak predictor of soil disturbance (rutting and soil com- on landscape characteristics and the season of harvest that paction) in the temperate forests in the mountains of central with local calibration can be used for prediction of detri- Europe. There was a weak relationship between maximal rut mental soil disturbance and help adapt management strate- depth and cone index for the eight harvested forest stands, gies. Aspect, slope, land type and the interaction between but not between cone index and mean rut depth [52]. In a harvest season (winter or non-winter) and land type were study in Croatia, both cone index and shear strength were significant variables in the model. Another decision support found to following the same pattern over a year as the volu- tool for predicting forest harvesting impact on soil properties metric water content (VWC) measured by a portable TDR was developed by Shabani et al. [48] generating maps with (time-domain reflectometry); high VWC resulted in low 1 3 26 Current Forestry Reports (2022) 8:20–37 cone index and shear strength and vise verse [53]. All three Operational Methods parameters were related to precipitation and temperature and their fluctuations over the year. There are several means by which soil compaction and rut- Different soil properties can be measured and available ting can be minimized within an operational context. At the inventory data used for predicting trafficability [ 54–56]. forefront, this includes improved planning and choice of har- Uusitalo et  al. [54] presented a model for predicting rut vesting method. In the literature there are examples of fol- depth by an 8-wheeled forwarder in fine-grained boreal for - low-up techniques and methods to counteract soil compac- est soil. The best predictors were VWC, the thickness of tion by site preparation to enhance seedling growth. There humus layer, cumulative mass of machine passes, and bulk are two main harvesting methods dominating; cut-to-length density. For a forwarder on fine grained soil, rut depth was (CTL) and tree length (TL). The CTL method is normally found to be related to total overdriven mass, VWC, cone performed with a harvester-forwarder system, while the TL penetration resistance and the harvester rut depth [56]. Five method usually includes felling (by machine or manual) and different models were tested, and the authors conclude that skidders of different types. These differences can be consid - the model including harvester ruth depth and cumulative ered when evaluating the results. Furthermore, differences overdriven mass is a good predictor of rut depth. The impact in management strategies, final felling or thinnings could of the harvester on the soil was a good predicter of the for- explain some of the results since transported volume per warder rut formation also on mid-grained soils [55]. Here, area differs. the number of machine passes, VWC in the mineral soil and depth of the organic layer was controlling factors for Reduced Compaction by Improved Planning The way har- rut formation. vesting operations are planned could have a large impact on soil damages. Improved planning could be done by a Route‑Planning A model called “Direct Skid Trail Pattern better distribution of machine operating trails [59], mini- (DSTP) has been developed for planning skidding opera- mizing driving by optimizing distances between harvested tions with farm tractors in Turkey [57]. The aims were to trees [60] or establish permanent designated trails [61]. As minimize skidding time, achieve a high extraction productiv- presented in the results of the terrain-related factors, it is ity and optimize the skid trail pattern in order to minimize demonstrated that the direction uphill or downhill could spatial soil compaction and soil loss. Working time perfor- affect soil disturbances. There are indications that driving mance/productivity increased by 17% when the “Direct Skid downhill reduces the negative impacts on the soil [28, 62]. Trail Pattern”-model was used compared to not using the The extent of machine turning can also influence the severity model. At the same time, the average length of skid trails per of soil disturbances, particularly when the change in direc- hectare decreased by 33%, and thus the spatial area affected tion is pronounced [63]. Driving distances within a harvest by soil compaction and the potential soil loss decreased site could also have an impact on the extent and severity of accordantly [57]. compaction or rather, that all traffic is funneled to landing Picchio et al. [58] demonstrated a method for strip road sites leads to higher soil compaction at the perimeters of planning, where a digital elevation model (DEM) of 2 m res- harvest sites [64]. olution was used, together with data on no-go areas within the forest stands. Driving steeper than 45% forward-back- Reduced Compaction by Logging Methods Literature con- wards or steeper than 25% sideways were not allowed (based cerning reduced compaction by harvesting methods are quite on the tipping risk of the machine used). From each position diverse. A study from Poland reported of less damages on of the forwarder, it was assumed that logs up to 12 m from remaining trees and soil following CTL as compared to TL the center of the strip-road could be reached. The harvested [65 ]. Similar results were obtained from a study in Maine area was divided into pixels, based on the total harvested (USA) were the harvester-forwarder system had the low- mass for the entire harvested area, where each pixel was one est impact on soil bulk density compared to skidding with forwarder load (assuming that the harvested mass was evenly tractor, bulldozer, or skidder [66]. This is in contrast to a spread over the clearcut). The strip road pattern and routes study by Dudakova et al. [67] in Slovakia where a harvester- were optimized with various GIS-tools to minimize surface forwarder system caused the largest damages. Cambi et al. impact of the operation. Finally, the routes were transferred [68] used a photogrammetry method to compare effect of to the machine and followed by the operator. When com- forwarder traffic vs. skidder traffic and concluded that the paring the GIS-planned routes with the harvester strip road forwarder caused more damages to the soil. In a Turkish pattern, the area of impacted soil was reduced between 50 study by Eroğlu et al. [69], four different extraction meth - and 70% (three different stands). ods were compared: skyline, ground skidding by manpower, skidders and a chute system. The results imply that both ground-based system had an impact on soil permeability, 1 3 Current Forestry Reports (2022) 8:20–37 27 bulk density and soil water balance. Marchi et al. [70] meas- it is fundamental to choose appropriate machines consider- ured the difference between two carrying systems compared ing local conditions in terms of logistics and soil properties to two winching system and concluded that dragging the [79–81]. Several studies investigated the use of machines logs on the ground caused more severe disturbances. applying different technological solutions to mitigate the negative effects on soil. Coppice Forestry Coppice or short rotation forestry is a common forest management system in the Mediterranean Technical Features area, where about 23 million ha are managed as coppice [71]. The coppice system is characterized by short rotation, The reduction of ground pressure exerted to the soil is key 6–10 years between cutting meaning that the time for soil in minimizing soil compaction and rutting [15]. For this recovery is very short between operations. Spinelli et al. [72] reason, several strategies have been tested and reported in published an extensive review covering 65 stands in central the literature. The addition of steel flexible tracks (SFT), and Southern Italy and Laschi et al. [22] made a thorough created from steel cross-members joined by chain links Life Cycle Analysis (LCA) and concluded that the extraction that span the entire length of a bogie axle, are common for phase had the largest impact on the end result. Venanzi et al. wheeled machines as they contribute to reduce soil com- [71, 73] compared two skidder systems with light weight paction in many conditions by increasing the contact area machines and a harvester/forwarder system and reported between machines and soil surface [82–84]. In this context, some soil recovery already one year after operations. In Labelle and Jaeger [85] investigated the load distribution on contrast, Naghdi et al. [74] found no sign of recovery dur- a load test platform using a forwarder in different conditions ing the first year after skidding. Paya et al. [ 71] found that (loaded/unloaded and with/without SFT). They assessed following skidding operations, microporosity was reduced for the first time the dynamic load distribution below SFT, by 62.5 and 53.8% for 0–10 cm and 10–20 cm, respectively, obtaining 30% lower values of dynamic peak loads mounting in comparison to unaffected soil surfaces. SFT than without SFT. In another study, Haas et al. [82] compared three configu - Site Preparation In four cases of the articles in this review, rations of tires – 940 mm width, 710 mm with and without site preparation has been reported to counteract soil com- steel flexible tracks (SFT)- on an approx. 30 000 kg loaded paction. Silveira França et al. [75] showed that planting in forwarder to assess the effects of rutting. Results indicated borehole through the compacted layer improved seedling that deeper ruts were caused by narrower tires and also growth. Following a wind throw event on a wet soil in South higher lateral soil displacement, causing more consistent Carolina, long-term effects of different types of site prepara - bulges than wider tires. The use of wider tires as mitiga- tion were studied [76, 77]. The result showed that so-called tion strategy for soil compaction has also been confirmed by bedding (i.e., creating a string of soil on top of the soil sur- Cudzik et al. [83] and Solgi et al. [86], which found a posi- face for better aeration at wet sites) led to improved seedling tive effect in terms of tractive efficiency [ 83] and soil physi- growth. Long-term studies are rare, but Cerise et al. [78] cal properties [86]. According to Starke et al. [84], reduc- sampled a 45-year old felling where different types of site ing tire inflation pressure of a ten-wheeled forwarder caused preparation had been tested and could not detect any residual lower rut depth. Moreover, authors evidenced the potential effects. positive effects of an added bogie to reduce soil compaction, as suggested also by Solgi et al. [86], especially on soft soil Machine Modifications conditions [84]. An interesting improvement on forwarders to reduce rutting has been tested in Sweden by applying a Twenty-seven articles investigated the effects of various pendulum arm technology to a 6-wheel forwarder. It has machines operating in different conditions on their impacts been compared with a traditional 8-wheel forwarder; results on soils and identifying possible mitigation strategies linked of rutting measurements were not better than traditional to machine modic fi ations. In fact, the use of machines in for - technology, mainly due to the lower number of wheels, but est operations is the direct cause of impacts on soil. For this the good potential of this technology was reported [87]. Fur- reason, many studies investigated different factors to find thermore, this potential was partially confirmed in another solutions to reduce the impacts, mainly in relation with soil study comparing different forwarders equipped with stand - compaction. In this context, machine size, traction type, tire ard wheels, SFT, rubber tracks, and pendulum arms. In fact, size, axle load and the total number of passes (traffic) are the pendulum arm technology obtained promising results in most important variables affecting soil compaction; the role terms of rutting, but rubber tracks obtained the most encour- and the effects of these factors can be increased or reduced aging results. In fact, rubber tracks used under loaded con- depending on soil properties. In particular, slope, soil water ditions were able to lower rut depth by up to three times content and soil texture are the most crucial. In this context, as compared to conventional tracks [88], highlighting the 1 3 28 Current Forestry Reports (2022) 8:20–37 importance to distribute the applied load. In this context, the difference was identified as a poorer load distribution and response of tracks in relation to wheels of crawler tractors higher ground pressure of the tractor. Other studies obtained on soils exposed to different conditions of slope and traffic, different results, as reported in Solgi et al. [ 97], comparing caused significantly lower increases of bulk density and rut - skidding operations with a light farm tractor (3 100 kg) and ting [79, 89–91]. In this sense, similar results were obtained a skidder (11 500 kg) in clay loam soil. In this case study, by Cambi et al. [90] comparing the effects of a tracked and a the negative effects of traffic on soil properties in terms of wheeled agricultural forest tractor adapted for use in forestry bulk density, porosity and rut depth after the same number on forest soil physical properties (bulk density, penetration of passes were higher for skidder than tractor [97]. Similar resistance, porosity and shear resistance) at different soil results and conclusions are reported in Nikooy et al. [99]. In water contents. general, the results obtained comparing different machines or solutions in terms of impacts on soils are strictly related Solutions for Steep Terrains to local conditions; this is also valid for comparing different harvesting systems as reported by Venanzi et al. [71], which Slope is one of the most important factors affecting the obtained lower soil impacts by winching in comparison with impacts of forest operations on soils. This is one of the rea- forwarding in coppice. sons why, together with productivity and safety improve- ment, cable-assisted machines have been developed and increasingly used in the last years, and why this technology Machines Use is considered as “one of the major innovations in steep ter- •• rain harvesting in the twenty-first century” [ 92 ]. In fact, Beyond the above-mentioned technical aspects, a strong thanks to this technology, slip during machine traffic can be influence on the effective impacts on soil derives from the reduced [93], together with reducing maximum ground pres- proper or unproper use of machines, which is influenced •• sure [92 , 94]. Moreover, Garren et al. [94] in an experi- by the workers’ experience and the operational conditions. mental area in Brazil, found similar impacts of bulk density In this sense, several authors have reported a series of rec- for both forwarding on gentle slope and cable-assisted for- ommendations to mitigate impacts on soil physical proper- warding on steep slope [94]. Furthermore, it could have pos- ties. Many authors agree on the fundamental need to have •• itive effects also on flat terrains and soft soils [ 92 ]. Finally, a correct and rational planning of forest roads and machine Green et al. [93] reported that cable-assisted machines main- operating trails in particular [38, 87, 96, 100]. On the other tain consistent travel paths concentrating the impacts on a hand, some authors recommend to reduce the number of smaller surface than non-tethered ones, confirming also a trails in order to concentrate impacts on limited surfaces lower effect in terms of bulk density increase after traffic. [38], while others suggest to limit the number of passes in Nevertheless, comparing tethered equipment and traditional a single trail recommending the creation of more trails to systems for felling and extraction, Chase et al. [95] observed distribute soil compaction in the forest [100]. Solgi et al. lower soil disturbance and stream-adjacent disturbance [86] included both options suggesting to limit the number applying motor-manual felling and extraction by conven- of passes in a trail up to 15, or to create high-traffic areas. tional cable yarder than using cable-assisted machines. Uusitalo et al. [101] suggested to create wider trails in peat- lands to avoid to pass with wheels or tracks in the same area, Machine Choice thus distributing the traffic on a larger path. In any case, machines should avoid operating outside the planned trails Regarding machines, another important factor to miti- [102]. It is important considering that the negative effects gate the impacts on soil properties is strictly related with of traffic in terms of increased bulk density are extended the importance in choosing the most suitable machine for up to 1 m far from tracks, enlarging the total damaged for- specific worksite conditions [ 79, 80, 96, 97]. In this con- est surface [15]. Moreover, trails must be maintained and text, the debate on the best choice between traditional or restored to minimize erosion risks [79] and they should be mechanized harvesting systems is very active, where “tra- used in dry conditions [15, 80, 96, 97, 102]. When it is not ditional” means low mechanized systems including the use possible and work is necessary in wet conditions, the use of of chainsaw for felling and processing, and farm tractors tracked tractors is recommended and preferred to wheeled adapted for forestry for bunching and extraction [97–99]. [90]. However, to evaluate if soil water content allows forest In some cases, mechanization is considered better than tra- operations, Allman et al. [103] suggest to test if Atterberg ditional systems in terms of impacts on soils; Rejšek et al. plasticity limits of soil are exceeded or not. [98] applying a dynamic penetration test obtained lower effects operating with a three-axle harvester and four-axle forwarder than with a universal wheeled tractor. The main 1 3 Current Forestry Reports (2022) 8:20–37 29 can also mitigate the increase of soil bulk density during Use of Amendments as Mitigation Techniques ground-based machine traffic [ 110]. Specifically, Labelle et al. [105], Solgi et al. [108], Agherkakli et al. [111], and In total, 15 articles dealt directly with amendments that can be placed on the soil surface during or after harvesting oper- Matangaran [112] all reported a reduction in the compac- tion rate (difference between pre- and post-impact soil bulk ations to act as a protective layer against machine-induced soil disturbances (Table 2). From these articles, two main density) as brush amount increased. However, this finding was not supported by all studies as Parkhurst et al. [107] groups of amendments emerged: brush (12 articles) and mulch (3 articles). These amendments were tested to see and Hashimoto et al. [114] were not able to report any clear trends as to the ability of brush mats to reduce soil bulk how they could impact load distribution, bulk density, rut depth, penetration resistance, and total porosity following density increases as compared to a no brush scenario. Since the compaction rate is often linked to the initial (pre-harvest) ground-based machine traffic. Brush is defined as harvesting debris (treetops, branches, and foliage) that originates from state of the soil, the concept of relative bulk density has also been applied to obtain a qualitative assessment of the the processing phase and is placed on machine operating trails in order mitigate the effect of harvesting and forward - disturbance [15, 105]. In this context, relative bulk density is referred to as the ratio between field bulk density and ing machines on soils as they are being operated in a harvest block. The use of brush is more commonly linked to cut-to- maximum bulk density achieved by the standard Proctor test. A relative bulk density threshold of 0.8 has been linked to length operations where harvesting debris remains in the stand. Once placed on a machine operating trail, harvest- reduced growth of Norway spruce (Picea abies Karst) [21]. Using this relative bulk density concept, Labelle et al. [105] ing debris forms a brush mat. When a machine is in direct contact with a brush mat, the friction between the pieces reported that 40.5%, 17.9%, 14.3%, 15.5%, and 3.6% of soil relative bulk density measurements covered with 0, 5, 10, constituting the brush mat increases and creates a reinforced –2 surface that can expand the contact area between a machine’s 15, and 20 kg  m of brush, respectively, exceeded the 0.8 threshold when performing operations with a Timberjack running gear and the soil surface [10]. The other main group of amendment, referred to as mulch, includes sawdust, straw 1110D 8-wheel forwarder (32 860 kg loaded) on silty soils. and litter. Unlike brush, which is a direct raw biproduct of forest operations, mulch types are normally brought to the Rut Depth and Penetration Resistance As the distance between the point of impact (below the machine running harvest site and originate from a secondary transformation. gear) and the soil surface increases through the use of brush mats, rut depth caused by machine traffic can also be reduced Brush [104, 106, 108, 111–113]. In a study performed in eastern Canada with a 35 800 kg loaded 8-wheel forwarder, brush As bioenergy markets are expanding, the competition −2 between using harvesting debris as a soil protective layer mats of 5, 10, 15, and 20 kg  m offered reductions of indent areas (depression of the soil below the pre-impact level) of during forest operations or for bioenergy operations is on the rise [15]. Finding a certain equilibrium between the two 0.0, 14.3, 71.4, and 90.5%, respectively, as compared to the no brush treatment [106]. Moreover, brush mats of 15 and uses remains complicated and is normally a function of bio-  −2 mass markets and a site’s susceptibility to disturbances. The 20 kg  m were able to statistically reduce rut depth as com- pared to when the forwarder was driven directly on the for- quantity and quality of brush available for soil protection is directly related to site quality, stand characteristics (species, est floor, a finding that was also supported by Ilintsev et al. [104] when using an 8-wheel John Deere 1210 (31 080 kg age, diameter at breast height (dbh), and height), silvicultural treatment, and degree of tree removal [117]. loaded). On highly susceptible soils (clayey silts), research by Matangaran et al. [112] showed that average rut depth Load Distribution and Bulk Density Brush mats are able to created by a Valmet 860.1 forwarder weighing 29 900 kg decreased from 24 cm down to 0 cm when using 1 m thick distribute applied loads to a greater area below the mats and thus decrease peak pressures exerted to the soil [10, 109]. brush mats composed of black wattle (Acacia mangium Willd.) as compared to no brush. In a controlled test, Labelle and Jaeger [10] demonstrated −2 that brush mats of ≥ 20 kg  m were able to statistically In addition to soil bulk density, penetration resistance measured pre and post harvest can also be used to gain fur- lower peak pressures exerted below an 8-wheel forwarder (30 180 kg loaded) as compared to a no brush scenario and ther insight soil disturbances. According to Labelle et al. [105] who performed field trials with a Timberjack 1110D that these thicker mats offered better protection against repeated forwarder passes as compared to lighter brush mats 8-wheel forwarder weighing 32  980  kg, soil penetration −2 values > 3.0 MPa represented 23.7%, 15.0%, 9.4%, 4.6%, (< 20 kg  m ). Those tests were performed on a so-called load test platform equipped with high-capacity load cells. and 0.7% of all post-forwarding test plots that were covered –2 by 0, 5, 10, 15, and 20 kg  m of brush, respectively. The Beyond their capabilities of load distribution, brush mats 1 3 30 Current Forestry Reports (2022) 8:20–37 1 3 Table 2 Summary of studies focused on the use of amendments for soil mitigation Amendment type Ref Country Machine Mass (kg) Amendment amount Soil properties measured Key findings −2 Brush [104] Russia John Deere 1210 8-wheel forwarder 31 080 (loaded) average of 12 kg  m RD Exponential regression explains the relation- ship between rut depth and amount of brush −2 −2 [105] Canada Timberjack 1110D 8-wheel forwarder 32 860 (loaded) 5, 10, 15, 20 kg  m BD, PR Recommending brush mats of 15–20 kg  m to statistically reduce soil density increase −2 [106] Canada Logset 8F 8-wheel forwarder 35 800 (loaded) 5, 10, 15, 20 kg  m RD Rut depth significantly decreased when using −2 15–20 kg  m brush mats [107] USA John Deere 450E bulldozer / Caterpillar 8 606 and 21 192 (both loaded) 1 m thickness BD, TP No clear link reported between the influence 525D wheeled grapple skidder of brush for mitigating bulk density increases −2 −2 [108] Iran Timberjack 450C 4-wheel cable skidder 10 257 (unloaded)10, 20 kg  m BD, RD20 kg  m brush mats significantly reduced bulk density increases as compared to no brush and both tested brush amounts statistically reduced rut depth −2 [109] Canada Not relevant Not relevant 10, 20, 30, 40 kg  m Microstrains below brush Softwood brush showed a slightly better capacity at distributing loads as compared to hardwood mats −2 [110] Germany Timberjack 1110 8-wheel forwarder 26 000 (loaded) average of 27 kg  m BD, TP Increases in soil bulk density were significantly higher in the upper 10 cm following opera- tions on brush as compared to no brush −2 [111] Iran LTT-100A steel-tracked skidder 11 200 (unloaded)7.5, 17.5 kg  m BD, RD Both tested brush amounts statistically lowered rut depth as compared to no brush. Benefits of brush to mitigate compac- tion was limited to 5 machine passes and following passes were only helpful to reduce rutting −2 −2 [10] Canada Timbo TF820D 8-wheel forwarder 30 180 (loaded) 5, 10, 15, 20, 25, 30 kg  m Peak loads below brush Recommending brush mats of 15–20 kg  m [112] Indonesia Valmet 860.1 8-wheel forwarder 29 900 (loaded) 1 m thickness BD, RD Soil compaction rates were reduced by half with the use of brush as compared to no brush. Ruts were not present with brush and averaged 24 cm without brush −2 [113] Russia Ponsse Elk 6-wheel forwarder and John 31 800 / 30 500 (both loaded)15 kg  m BD, RD Brush constrained rut formation after all Deere 1410 8-wheel forwarder forwarder passes and was deemed neces- sary for environmentally sensitive wood harvesting on soft soils −2 [114] Japan Komatsu PC120 excavator with GC-40CA 13 8005, 15 kg  m BD, PR No clear link reported between the influence feller-buncher head of brush for mitigating bulk density and penetration resistance increases −2 −2 Mulch [115] Iran TAF E655 wheeled skidder 6 800 (unloaded)1.27 kg  m straw and 1.67 kg  m BD, PR Three years after mulch application, bulk litter density recovery values were significantly higher when using straw as opposed to litter mulch −2 [116] Iran Timberjack 450C 4-wheel cable skidder 10 300 (unloaded)3.65 kg  m straw BD, TP, RD, PR On trails with a slope of 10%, straw mulch allowed for faster recovery of bulk density, rut depth and penetration resistance as compared to untreated trails −2 −2 [108] Iran Timberjack 450C 4-wheel cable skidder 10 257 (unloaded)10, 20 kg  m sawdust BD, RD20 kg  m mats are significantly more effec- tive in reducing adverse effects than lighter −2 10 kg  m mats BD bulk density, PR penetration resistance, TP total porosity, RD rut depth Current Forestry Reports (2022) 8:20–37 31 penetration resistance threshold of 3.0 MPa was used since soil compaction and rut depth were significantly higher in it has been linked interrupted root growth [118]. moist soils as compared to drier soils [11, 17, 33]. In many regions, ground-based logging operations were conducted on frozen soils, a natural barrier that greatly reduces soil Mulch disturbances [2]. However, in changing climate that triggers highly variable weather conditions, forest operations can be Mulch has been used as a post-impact amendment to try presented with shortened windows of operations, particu- and accelerate the rehabilitation of certain soil physical larly in sensitive areas of high water content [1, 104, 119]. properties. Jourgholami et  al. [115] tested the influence In fact, studies revealed that fine-textured soils coupled with of straw and litter mulches applied to the machine operat- high water content are easily prone to soil compaction and −2 −2 ing trail at rates of 1.27 kg  m and 1.67 kg  m , respec- rutting [23, 32, 39]. Forest soils with low intrinsic bulk den- tively, on the recovery of soil bulk density and penetration sity, well-structured, and high organic matter content might resistance measured three years after mechanized opera- be more vulnerable to compaction and rutting, which appro- tions. Results indicate that both bulk density and penetra- priate measures should be applied to prevent the adverse tion resistance values recovered at a faster rate with the use effects on forest soil. of litter mulch as compared to straw mulch but that both mulch types improved recovery as compared to the uncov- Operational Planning ered plots. Jourgholami et al. [116] reported that applying −2 sawdust mulch (3.65 kg  m ) allowed for a faster recovery The different theoretical mathematical models, described measured six years after harvesting operations of soil bulk above, need to be incorporated in a user-friendly decision density (8%), rut depth (13%) and penetration resistance support tool to be of use for operational planning in forestry. (19%) as compared to untreated trails. It is important to note The static and dynamic trafficability maps summarized in that these findings are for trails with a longitudinal gradient this paper appear to be useful tools for operational planning. of 10%. In a study performed in Iran with a 4-wheel skidder However, their use may be limited to the region/country in operated on a silty loam, Solgi et al. [108] only reported which they were developed, as they are based on conditions statistical differences in soil bulk density when using the and data availability in that region. As pointed out by some −2 thicker 20 kg  m sawdust mat as compared to unprotected studies included in this review [55, 56], the rut depth of the soil but statistical differences were noted when using the harvester is a useful predictor of where more severe rutting −2 thicker 20 kg  m sawdust mats. is expected by the forwarder. There are several techniques of directly measuring the forwarder rut depth, but they all require additional sensors to be mounted at the rear of the Discussion and Recommendations harvester. A more cost-efficient method could be to use harvester CAN-bus data (i.e., data on the power used for Terrain‑Related Factors driving) to predict trafficability of the forwarder, as tried by Salmivaara et al. [51]. This technique has been further Terrain steepness was an important factor that influences tested by Ala-Ilomäki et al. [120 ] and they found that the the trafficability of ground-based machines on forest soils CAN-bus data was efficient for mapping site trafficability. and has a direct impact on changes in soil physical proper- In addition, assessments of harvester rolling resistance by ties (i.e., bulk density, porosity, perpetration resistance, rut using CAN-bus data has the potential to be used in a larger depth) [23, 31, 37]. Both unbalanced load distribution and scale (Big Data), as indirect power recording is possible in wheel slippage on the soil surface are the main contributors all modern harvesters and the cost of automatically creating to excessive soil compaction on steeper terrains [34, 36]. trafficability maps is negligible, according to Ala-Ilomäki Studies highlighted that ground-based harvesting operations et al. [120 ]. should be planned whenever possible on slopes less than A couple of tools for route-planning of an area to be 20% with downhill directions [28, 30, 36, 38]. By increasing harvested were within the search string of this review [57, the slope of machine operating trails, environmental impacts 58]. However, there are other tools with high potential for on soils appeared to be intensified, thereafter; several studies efficient and gentle route-planning, where less soil distur - concluded that a critical slope can be appointed coupled with bance could be an important biproduct. Rönnqvist et  al. soil texture and water content conditions [2, 31, 33]. [121] demonstrated a decision support tool (BestWay) for Changes in soil water content during the year by seasonal primary extraction routes to minimize forwarding distance alterations and precipitation has a significant effect on soil while avoiding steep terrain and wet areas. The input data vulnerability to compaction and rut formation, especially in the optimization model were harvesting block perimeter, in fine-textured soils [ 9, 13, 30]. Studies demonstrated that forest volume density (from LiDAR-data), a digital terrain 1 3 32 Current Forestry Reports (2022) 8:20–37 model (2 m resolution), a depth-to-water map and no-go of brush on machine operating trails, at least not to the areas (e.g., historical or nature conservation areas). The suggested extent, but rather try to reach the recommended −2 BestWay tool has been developed to also suggest landings targets (15–20 kg  m ) on sensitive areas (depressions, based on the forest road network, closeness to crossings, moist soils, fine-textured soils, high traffic areas, etc.) power lines, streams, and too steep areas for storage [122]. where soils are most susceptible. Most brush studies per- If wood can be extracted through shorter distances and trans- formed in a context of soil protection presented brush −2 port times and at the same time with less impact on soils and amounts in kg m . This unit is practical for scientific waters, there is a high potential for acceptance and a smooth experiments but remains quite cumbersome to translate implementation in operational forestry. into an operational context. To circumvent this, Labelle −2 et al. [10] determined that kg m brush amounts can be Machine Modifications converted to cm thickness by applying a factor of 2 or 2.5, −2 hence, a 20 kg  m brush mat would approx. equal a 40 to The analysis of the wide existing literature in terms of 50 cm loose thickness. impact on soils due to machine traffic highlighted how the Brush generally provided the highest protection against different aspects are always directly or indirectly related soil bulk density increases and rut depth as compared to with the pressure exerted on the ground. In fact, the mitiga- the other reported mulch types. This is particularly the tion techniques reported, such as wider tires, reduced tire case when machine traffic is low. When brush is exposed inflation pressure, additional axles and wheels, steel flexible to repeated machine passes, it compacts and a high propor- tracks, etc. are all systems aimed at reducing ground pres- tion of branches are broken or sheared, which can in turn sure and consequently the negative effects on soil physi - decrease the overall strength of the mat [10]. Despite this cal properties. As reported, many factors affect the perfor - lowered strength, Labelle and Jaeger [10] reported that −2 mances of the existing solutions limiting impacts on soil. thicker brush mats of 15 and 20 kg  m still offered ben - In particular, soil texture, soil bearing capacity and terrain eficial load distributing capabilities after 12 loaded passes. slope have to be considered as fundamental to choose the Another advantage of brush is that it is readily available at best technical solution. In this context, the right option is the harvest site, especially when no bioenergy markets are not certain, even if some solutions often resulted better than developed in the proximity to the harvest area. others. In general, harvester-forwarder are considered the least impactful machine-based forest operations [2]. Never- theless, conflicting results emerged from some studies that General Management Recommendations compared traditional tractors with highly mechanized sys- and Strategies tems. In many cases, using traditional tractors resulted in lower forest soils disturbances than with highly mechanized Pre ‑har vest Operational planning of forest operations systems. This result could be attributed to the fact that com- is the key to avoiding severe soil disturbances, including parisons were made in terms of similar operational condition which forest units to be cut in which season, which machine and comparing the same number of passes. To gain further resources to be used (including machine modifications), and insight, it would be interesting to compare the impacts on the planning of landings, main extraction routes (machine soil after the same wood quantity processed and transported operating trails), no-go areas, and amendments to be used, by the two machines (and consequently a different number such as portable bridges and brush. Following recommenda- of passes). Moreover, it would be useful to investigate if tions should be considered during planning: workers’ level of both training and experience can affect the impacts on soil during operations. This is demonstrated in Adapting the forest practice to local conditions and spe- terms of productivity [123], but not in terms of efficiency to cific site characteristics with different soil types, hydrol - reduce impacts on soil, even if remarked in some studies as ogy, and slope. a factor to be considered [2, 124]. Planning landings and forest road network in steep terrain in such a way that the timber extraction is done in down- Use of Amendments as Mitigation Techniques hill directions; upward movement of untethered loaded machines should be prevented as far as possible. The frequency of partial harvests is on the rise as more Limiting logging operations to gentle slopes, particu- forest operations are performed under close-to-nature/ larly < 25% appeared to be an effective way to minimize continuous cover forestry [125, 126]. During such opera- the degree and extent of soil compaction and rutting. tions, it might become more difficult to reach the suggested Lowering the ground pressure of the machines by using brush amounts for adequate soil protection [117]. How- larger tires, reduced inflation pressure, additional axles ever, it is not necessary to aim for a uniform distribution and wheels, and/or bogie tracks. 1 3 Current Forestry Reports (2022) 8:20–37 33 Limiting logging operations on fine-textured soils to dry mitigation techniques. The studies reviewed were wide periods or frozen soil in addition to minimizing ground ranging, both in terms of research scope and geographical pressure. location. The latter undoubtably influences the direction of Use route-planning tools based on various geo-data for the research through local policies and operational guide- finding potential landings and effective wood extraction lines. Below is a list of selected research opportunities that routes to increase productivity (by shortening off-road either remain untapped or offer possibilities of expanded transports) at the same time as soil disturbances are mini- research. mized. • • Plan the operation beforehand as regards soil conditions Digital forestry provides almost endless possibilities of and remaining trees to avoid turning. research. Now that trafficability maps are available, the challenge will be shifted to providing dynamic infor- During harvest One of the most pivotal issue to minimize mation to machine operators on where to travel, under soil compaction and rutting is to prohibit machine entrance which loading condition, and at which traffic frequency on wet soils. to minimize soil physical disturbances. A lingering research question remains regarding Using real time mapping of sensitive soil based on local the contrasting concepts of reducing the number weather forecasts could be useful for help avoiding oper- of machine operating trails as much as possible via ations in areas where soil disturbances could be severe. increased trail spacing, thus further concentrating Measuring soil water content at the beginning of forest machine-induced disturbances to a reduced surface or operations with some field methods could help machine distributing the stress of machine passes over a larger operators to take decisions to concentrate the routes area of a harvest site. This question is not trivial as the where high volume is extracted to drier areas and of answer might be influenced by soil bearing capacity, increased use of amendments such as brush, mulch, cor- operational considerations and machine use. duroy mats in more sensitive areas. The presence of stones and boulders can increase soil With the increase of load capacity on new forwarder bearing capacity but can also cause shifts in machine models, improved load distribution becomes paramount. weight distribution if they are large enough, thereby In this context, the use of harvesting debris (brush mats) increasing the exerted loads to certain wheels. We obtained from the processing phase, remains one of the could not find studies that addressed this dual influ - most beneficial methods of distributing applied loads. ence. −2 • • Applying between 15 and 20 kg  m of brush on machine Consider new soil mitigation techniques that are in line operating trails to reduce soil bulk density increases and with the effects of changing climate (increased calami - rut formation. ties, insect outbreaks, draughts, etc.). For instance, a better understanding of how different mitigation tech - Post harvest It is advisable to avoid rutting and soil compac- niques can be used simultaneously. Furthermore, the tion in the first place, but given that severe soil disturbances competition for use of brush between soil protection in have occurred, there are some ways to try to mitigate the forest stands and bioenergy operations as a source of effect or speed up the recovery rate: biofuel should be studied further. Can an equilibrium be reached between the two uses? The answer probably The usage of mulch types as a biproduct of a secondary rests on the status of biomass markets and the need to transformation can help to accelerate the recovery of soil assign a value to the protection of forest soil. properties post harvest. One caveat is that is does require Most studies have focused on quantifying one or two material to be brought to a harvest site and spread to factors causing soil disturbances. There is a need for machine operating trails post harvest, thus adding costs. having a multi-disciplinary approach to tackle the Mechanical site preparation could be used for loosening complex problems associated with machine/soil/plant the topsoil after soil compaction and also backfill wheel interactions. Certain relationships between mitigation ruts with the use of excavators. Such methods are nor- techniques and their performance can only be fully mally applied in areas of high population (e.g., frequently understood through holistic projects. accessed forest roads, areas of high visibility, etc.). Acknowledgements The authors wish to thank the Université Laval Further Research and Skogforsk for covering the open-access fees. This work was con- ducted within IUFRO division 3.05. Despite the wealth of published literature, it is clear that extensive research is still required in the field of soil 1 3 34 Current Forestry Reports (2022) 8:20–37 10. Labelle ER, Jaeger D. Quantifying the use of brush mats in Declarations reducing forwarder peak loads and surface contact pressures. Croat J For Eng. 2012;33:249–74. Conflict of Interest The authors declare that they have no conflict of 11. Jourgholami M, Majnounian B. Effects of wheeled cable skidder interest. on rut formation in skid trail - a case study in Hyrcanian forest. J Forestry Res. 2011;22:465–9. Human and Animal Rights and Informed Consent This article does not 12. Naghdi R, Solgi A, Zenner EK, Tsioras PA, Nikooy M. Soil dis- contain any studies with human or animal subjects performed by any turbance caused by ground-based skidding at different soil mois - of the authors. ture conditions in Northern Iran. Int J For Eng. 2016;27:169–78. 13. Allman M, Jankovský M, Messingerová V, Allmanová Z. Soil moisture content as a predictor of soil disturbance caused by Open Access This article is licensed under a Creative Commons Attri- wheeled forest harvesting machines on soils of the Western Car- bution 4.0 International License, which permits use, sharing, adapta- pathians. J For Res. 2017;28:283–9. tion, distribution and reproduction in any medium or format, as long 14. Jourgholami M, Ghassemi T, Labelle ER. Soil physio-chemical as you give appropriate credit to the original author(s) and the source, and biological indicators to evaluate the restoration of compacted provide a link to the Creative Commons licence, and indicate if changes soil following reforestation. Ecol Indic. 2019;101:102–10. were made. The images or other third party material in this article are 15. Labelle ER, Jaeger D. 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Strategies to Mitigate the Effects of Soil Physical Disturbances Caused by Forest Machinery: a Comprehensive Review

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10.1007/s40725-021-00155-6
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Abstract

Purpose of Review Ground-based mechanized forest operations can cause severe soil disturbances that are often long lasting and detrimental to the health of forested ecosystems. To reduce these soil disturbances, focus is being increasingly directed at identifying and using appropriate mitigation techniques. This systematic review considered 104 scientific articles and reported the main findings according to four core themes: terrain-related factors, operational planning, machine modifica - tions, and types of amendments used to mitigate machine-induced soil impacts. Recent Findings For terrain-related factors, most severe disturbances occur on machine operating trails exceeding 20% slope and that soil bulk density and rut depth show greater increases in fine-textured soils. When considering operational planning, trafficability maps proved to be helpful in reducing the frequency and magnitude of soil damages as well as the length of trails needed within harvest sites, especially if they are regularly updated with weather information. Machine modi- fications, through high flotation tires, use of extra bogie axle, lower inflation pressure, and use of steel flexibles tracks, are highly researched topics because of the considerable upside in terms of machine ground pressure distribution and increased traction. Two main types of amendments emerged to mitigate soil disturbances: brush mats and mulch cover. Brush mats created from harvesting debris can spread the load of a machine to a greater area thereby lowering peak loads transferred −2 to the soil. Brush mats of 15–20 kg  m are being recommended for adequate soil protection from harvesting operations. Summary To conclude, we outline recommendations and strategies on the use of soil mitigation techniques within cut-to- length forest operations. New research opportunities are also identified and discussed. Considering single factors causing machine-induced soil disturbances remains important but there is a pressing need for having a multi-disciplinary approach to tackle the complex problems associated with machine/soil/plant interactions. Keywords Harvesting · Planning · Rutting · Soil compaction · Mitigation techniques · Sustainability This article is part of the Topical Collection on Forest Engineering * Eric R. Labelle Skogforsk, the Forestry Research Institute of Sweden, eric.labelle@sbf.ulaval.ca Uppsala Science Park, 751 83 Uppsala, Sweden Linnea Hansson Department of Forest Ecology, Swedish University linnea.hansson@skogforsk.se of Agricultural Sciences, 90103 Umea, Sweden Lars Högbom Department of Forestry and Forest Economics, lars.hogbom@skogforsk.se Faculty of Natural Resources, College of Agriculture and Natural Resources, University of Tehran, P. O. Box, Meghdad Jourgholami 31585-4314 Karaj, Alborz, Iran mjgholami@ut.ac.ir Department of Agricultural, Food and Forest Sciences, Andrea Laschi University of Palermo, Viale delle Scienze ed. 4, andrea.laschi@unipa.it 90128 Palermo, Italy Department of Wood and Forest Sciences, Université Laval, 2405 Rue de la Terrasse, Québec, Québec G1V 0A6, Canada 1 3 Current Forestry Reports (2022) 8:20–37 21 strategies to control and minimize the negative effects of Introduction mechanized harvesting operations on forest soils should be properly planned and implemented [1, 2]. A higher share of forest mechanization has been needed Many papers have dealt with the direct impact of for- to meet the growing needs of the world’s population for est operations on soils, water regime, vegetation, and for- wood and wood-derived products [1, 2]. Despite some est growth. However, as forest mechanization is increasing, clear advantages of mechanized harvesting systems, such so is the need for appropriate and tailored mitigation tech- as higher productivity and improved safety, concerns sur- niques to reduce the environmental impacts of machines on rounding the use of heavy machines on forest soils remain the soil. The overall aim of this paper was to systematically [3]. Forest machines are commonly operated on machine review scientific literature concerning soil impact mitiga - operating trails designed to allow machine movement in tion techniques used in a context of mechanized harvesting a stand. This off-road traffic can lead to significant envi - operations. ronmental effects on forest ecosystems, particularly for - In addition to the overall aim, the paper also: est soils [4, 5]. From the perspective of sustainable forest management and the concept of sustainable forest opera- (1) provides recommendations and strategies to mitigate tions [1], forest soils serve as one of the basic components machine-induced soil disturbances through terrain- assuring the sustainability and productivity of forest stands related factors, operational planning, machine modifi - [6, 7]. cations, and types of amendments, Compaction, rutting, and displacement (a biproduct of (2) highlights key opportunities for future research. rutting) are the main impacts caused to the soil by forest harvesting operations [2, 8, 9]. Compaction occurs when the mechanical forces exerted to the soil cause soil parti- Material and Methods cles to be pushed closer together at the detriment of pore space, which leads to increased soil bulk density [6, 10]. Databases and Search Strategy During this process and when soil water content is high, soil deformation can occur through repeated machine A systematic review was performed in Scopus and Web passes [11–13]. Ground-based traffic of forest machines of Science databases. After several iterations, the follow- can adversely affect the following key functions of forest ing search strings were used as they provided the most rel- soils: increase soil strength and soil bulk density [14, 15], evant results regarding soil mitigation techniques to reduce decrease macroporosity [16], infiltration [ 14], saturated machine-induced soil disturbances. In Scopus, we searched hydraulic conductivity [17], and impede soil fauna colo- for the terms ("forest* operations" OR "forest* harvesting" nization [14]. In turn, these effects can lead to decreased OR "forwarder*" OR "skidder*") ("soil mitigation" OR "soil oxygen availability, increased surface waterflow [ 18], soil disturbance" OR "soil protection" OR "ground protection" loss and sedimentation [19]. Ultimately, these adverse OR "rutting" OR "compaction") in the title, abstract and effects can hinder root and tree growth [ 20], and nega- keywords. In Web of Science, the search was (forest* opera- tively affect site productivity [ 21]. The degree and extent tions OR "forest* harvesting" OR “forwarder*” OR “skid- of soil compaction and rutting during mechanized logging der*”) AND ("soil mitigation" OR "soil disturbance" operations is often influenced by the type and characteris - OR "soil protection" OR "ground protection" OR "rutting" tics of the harvesting system, number of machine passes, OR "compaction") Refined by: WEB OF SCIENCE CATE - terrain slope, soil type, texture and water content as well GORIES: (FORESTRY). Indexes: SCI-EXPANDED, SSCI, as silvicultural treatments [2, 3, 22, 23]. CPCI-S, ESCI. The undesirable effects of machine traffic on forest soils Studies were only considered if words used in the search can persist for a few years to several decades [24–26]. For string were present in the title, keywords or abstract and if example, DeArmond et al. [5] concluded that soil physical they were: properties did not return to preharvest level 30 years after machine-induced compaction had occurred in the Ama- • published between 2010 and 2020, in order to reflect the zon Basin. In the Hyrcanian forests, Jourgholami et al. actual state of the art and recent developments; [14] reported that soil compaction effects persisted over a • peer-reviewed scientific articles or conference proceed - 25-year period. Similarly, Ezzati et al. [27] found that soil ings; bulk density and total porosity did not recover 20 years • written in English. after ground-based machine traffic. In general, the impli - cations to restore a compacted soil to a preharvest level are time-consuming and costly. Therefore, measures and 1 3 22 Current Forestry Reports (2022) 8:20–37 3. Full articles were read and inclusion rested on the use Search Query and Article Analysis of techniques to mitigate machine-induced soil physical disturbances (primarily soil compaction and rutting). The search query resulted in a total of 273 articles of which 90 were reported by Scopus, 101 by WOS and an additional In total, 104 articles fulfilled all three criteria (Fig.  1; 82 articles common to both databases but only counted once (Fig. 1; black line). The number of published articles has grey line) and are part of this review. To structure the main findings and associated recommendations, the articles been increasing in the past decade with two noticeable peaks occurring in 2015–2016 and from 2018 onwards with the were divided into four main themes: terrain-related fac- tors, operational planning, machine modifications, and use highest frequency of 45 articles reported in 2020. Each of the 273 articles resulting from the search queries of amendments as mitigation techniques (Fig. 2). These themes are then presented and assessed in a chronological were evaluated in a three-step process. order from the point of view of forest operations. 1. Titles were first verified to make sure that the field of study was forestry, 2. Abstracts and keywords were read and articles that did not refer to soil mitigation techniques were omitted, Fig. 1 Overall search results of Scopus and Web of Science databases (black line) and search results according to three-step refinement process (grey line) 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Year of publication Unfiltered results Filtered results Fig. 2 Thematic classification along with article count 1 3 Number of articles Current Forestry Reports (2022) 8:20–37 23 soil water content at the time of harvesting [9, 11, 13, 30], Results soil texture [23, 32], initial soil bulk density, relative bulk density [21], organic matter content, and duff thickness [ 31] Terrain‑Related Factors can all play an important role in how a soil will physically respond to ground-based machine traffic (Table  1). In the Terrain-related factors are normally inherent to a harvest site context of this article, slope is referred to as the longitudinal and are thereby difficult to adjust, particularly once opera - incline or decline of the terrain located on machine operat- tions are on-going. However, despite the reduced possibili- ing trails. ties for alterations, factors such as slope [12, 23, 28–31], Table 1 Summary of terrain-related factors and their associated mitigation recommendations Factors References Country Category Properties Impacts Mitigation recommenda- tions Slope [31, 33–37] Iran 0–20, > 20% BD, PR, TP ↑↑↑ Limit operations to gentle Italy slope, restrict trails to slopes < 25%, importance of preplanning and using designated trails [30] Slovakia 5, 10, 20, 25% RD − Limit operations to gentle slope [6, 12, 16, 23] Iran < 10, 10–20, > 20% RD, BD, TP ↑↑↑ Limit traffic to slopes < 20% [29] Iran 15, 25, 35% BD, TP ↑↑ Restrict trails to slopes < 25% [28, 38] Iran flat, 10 (downhill), 10, RD, BD ↑↑ Limit traffic to 20% (uphill) slopes < 20%, limit traffic from uphill direct when loaded Soil water content [13, 30] Slovakia n.c RD, PR ↑↓ Schedule operations to dry and Czech Republic periods or frozen soil whenever possible [12] Iran 19, 33% GWC RD, BD, TP ↑↑↑ Plan logging operations when soil water content is low, limit excessive machine passes on moist soils [11] Iran 20 − 30, 30 − 40, RD ↑ Plan operations in dry soil 40 − 50% GWC conditions, limit exces- sive machine passes on moist soils Soil texture [32] Brazil Sa. Cl. Lo., Cl., Sa. Lo., PS ↑ Consider soil load bearing Lo. Sa., Sa capacity [23] Iran Cl. Lo. + Sa. Lo RD, BD ↑↑ When possible, exclude areas with fine textured soils, apply topsoil rein- forcement (i.e., logging residues, brush mats) [39] U.S.A S. Lo., Sa., Cl BD ↑ Limit skidding operations to times of low soil water content Relative soil bulk density [21] Germany n.c RBD ↑ Exclude driving on sensi- tive areas Link between RBD thresh- old and biomass growth impediments GWC gravimetric water content, BD bulk density, PR penetration resistance, TP total porosity, RD rut depth, PS precompression stress; Sa. Cl. Lo. sandy clay loam; Sa. Lo. sandy loam; Lo. Sa. loamy sand; n.c. not considering the category 1 3 24 Current Forestry Reports (2022) 8:20–37 Slope Jourgholami and Majnounian [11] concluded that average rut depth were 35 cm, 22 cm, and 17 cm on soils with gravi- Terrain slope often governs the trafficability of forest metric water contents of 40 − 50%, 30 − 40%, and 20 − 30%, machines and can result in increased soil bulk density [12, respectively. 23, 28, 29, 31, 33, 38] due to an unbalanced load distribu- Studies also highlight that when soil water content tion during logging operations. Naghdi et al. [23] found that exceeds the field capacity, an important measure to miti - soil bulk density and rut depth increased as the slope of a gate soil compaction and rutting is the cessation of harvest- machine trail increased. Conversely, Jankovský et al. [30] ing activities until soil conditions improve (Table 1). Other reported that trail slope had little to no effect on mean rut implications include reducing the ground contact pressure, depth and the rehabilitation of rut depth five years following topsoil reinforcement (i.e., logging residues, mulching), harvesting operations. and scheduling operations to dry period or on frozen soils Sohrabi et  al. [31] reported that, due to slippage of a [11–13, 30]. wheeled skidder, soil bulk density and penetration resist- ance were higher on slopes exceeding 20% as compared to Soil Texture and Other Factors lower gradients (0–20%). These same authors also reported that the recovery rate of soil properties including bulk den- Several studies focused on the effects of texture on soil sity, total porosity, and penetration resistance was slower in compactibility [32, 39]. Naghdi et al. [23] concluded that slopes exceeding 20%. Similarly, Naghdi et al. [33] deter- soil bulk density and rut depth showed greater increases mined that soil bulk density, porosity, and rut depth were in clay loam soil than in a sandy loam soil during ground- significantly affected by trail slope. based skidding. The content of fine-grained particles (silt According to the abovementioned results, several authors and clay) is an important driver that determines the degree concluded that ground-based logging operations should be of soil compaction [9]. The resistance to compaction was limited to machine operating trails with slopes below 25% higher in soils with low clay content [32]. Slesak et al. [39] [12, 23, 28, 29, 31, 33, 38]. However, in some instances, stated that a well-structured loam soil was more susceptible few machine passes can be allowed on trails steeper than to soil compaction. Martins et al. [32] found that higher lev- 25% [12]. By considering two skidding directions through els of resistance to soil compaction were observed on soils downhill slopes (i.e., -10%, -20%) and uphill slopes (10%), with greater amount of sand content. Accordingly, Naghdi Majnounian and Jourgholami [38] and Jourgholami et al. et al. [23] reported that rut depth on slopes exceeding 20% [28] explained that soil bulk density showed substantial were 28.3 and 15.4 cm, on clay loam and sandy loam soils, increases in uphill skidding as compared to downhill skid- respectively. ding direction. In uphill skidding, wheeled ground-based Soil structure, soil organic matter content, and initial machines slipped on the surface soil, resulting in the expo- bulk density were also crucial in assessing soil compaction sure of mineral soil due to the applied vibration and shear caused by heavy forest machines [31]. Forest soils with an strength [28, 38]. initial low (pre traffic) bulk density can be more easily com - pacted, at the same compaction energy, than a denser soil. Soil Water Content However, a denser soil might require a lower compaction energy in order to trigger tree growth impediments [21]. Soil water content plays a key role in soil consistency as soil changes from one state to another (i.e., Atterberg lim- Operational Planning its) [30]. Seasonal changes in rainfall and temperature have direct effects on soil water conditions that influence load Decision Support for Predicting Trafficability and Route bearing capacity, which in turn determines the degree of soil Planning disturbance and compaction [11]. Normally, moist soils are more susceptible to compaction than dry soils [30]. Soil dis- turbances and average rut depth can effectively be predicted Theoretical Predictions of Machine Impact In Russia, several by soil water content [12, 13]. At a given compaction energy, studies have focused on predicting machine impact by theo- increasing the soil water content will increase soil bulk retical mathematical models. Manukowskii [40], presented density up to the maximum density, after this critical point, a model to calculate rutting by caterpillar forest machinery soil bulk density decreases as soil water content increases based on number of passes, subsoil and topsoil properties. [30]. A strong correlation exists between soil water content Grigorev et al. [41] developed a model for dynamic soil and mean rut depth [11–13]. Allman et al. [13] found that compaction by wheeled forestry machines where the impact when soil water content exceeded the plastic limit, an aver- of topography, weight and type of wheel system could be age rut depth > 16 cm occurred during logging operations. studied. However, the model did not include parameters 1 3 Current Forestry Reports (2022) 8:20–37 25 such as soil water content and soil type. A detailed study of susceptibility for either compaction, displacement or rut- the impact of logs on three different soils during skidding, ting. The model was based on GRASP (generalized regres- by the same research group, was suggested to be used for sion analysis and spatial prediction) and included a.o. soil drawing up a technological map for projecting a logging properties, slope, and forest type. It accurately predicted area based on skidding system specifics and soil type [ 42, compaction, displacement and rutting with 97 − 98% in the 43]. The model results were compared with experimental test area in Iran. studies in the laboratory and the differences were less than In 2012, Mohtashami et al. [49] demonstrated a model 10% [42]. Rudov et al. [44] studied the lifting capacity of in ArcGIS where elevation, slope, aspect and soil type were frozen grounds of different textures, soil water contents and used to build up a cost-index surface, where trafficability was temperatures under the action of static loads and pointed out classified in five levels. In the case study presented, it was the increase in sensitivity of the soils between -1 and 0 °C demonstrated that both time and money could be saved when and its dependence on soil salinity. using the model and building a corduroy road on a piece of A decision support tool used in agriculture is the Soil- wetland to reduce the forwarding distance and at the same Flex-model which was developed for predictions of tire/ minimize the disturbance on soils and waters for the entire soil interactions: Goutal et al. [45] tested if it could also be extraction road network. Following this work, Mohtashami used on forest soils in France. The predicted bulk densities et al. [50] tested if depth-to-water maps alone or combined after two passes with a forwarder were higher than the meas- with other information such as soil type, could predict where ured ones in the upper layer (0 − 10 cm) and the opposite ruts occurred during forest operations. The results pointed but smaller differences in the deepest layers (20 − 35 cm). to that DTW-maps alone did not predict rut formation but According to the authors, the overestimation in the upper combined with other information, their relevance increased. layers could be due to the buffering effect of the forest floor Slash reinforcement was used by the operators, when they due to the organic matter content and persistent root mat found it necessary, at the 16 sites included in the inventory which is not present in agricultural soils. Goutal et al. [45] which may have influenced the results. suggested that an increased rebound could improve the Whereas trafficability maps that are not modified over model performance and that this parameter depends on the time despite changing weather can be referred to ‘static’, soil organic carbon in the soil layer. They conclude that the ‘dynamic’ trafficability maps are updated using weather model estimations well predicted the compaction range after data. In a study by Salmivaara [51], a dynamic map for forwarder traffic, even for forest soils with a non-negligible rut depth or rolling resistance prediction was developed root and gravel content. by using hydrological modelling to account for changing weather conditions. The map had a resolution of 16 × 16 m and a wide range of open-source spatial data was included, Trafficability Maps Several trafficability maps have been e.g. soil texture, a digital elevation model and stand charac- developed and tested in the field in the recent years. In Fin - teristics from the Finnish forest inventory, as well as trans- land, a static trafficability map was developed by Arbonaut ported mass through a cell. In the same study, harvester OY and tested by Kankare et al. [46]. The model was based CAN (Controlled Area Network)-bus data was pointed out on topographic wetness index (TWI), amount of vegeta- as a useful tool to measure rolling resistance coefficient in tion, ground water height and ditch depth. The main point the field [ 51]. was to provide information about in which season different areas may be harvested with standard machinery (harvesters Predicting Trafficability Based on Field Measurements Cone and forwarders) with a 16 m × 16 m resolution. When the penetrometers are popular and simple tools for measure- maps were used in thinning operations, ~ 70% of the evalu- ments of bearing capacity in soil disturbance research, ated stands were harvested without causing any damage if where Cone index is the penetrometer resistance at a given they were harvested in the correct season; the correspond- depth (average or maximum). Allman et al. [52] found that ing percentage for incorrect timing was ~ 40%. In Western Cone index of the upper 30 cm of the mineral soil was a Montana (US), Reeves et al. [47] developed a model based weak predictor of soil disturbance (rutting and soil com- on landscape characteristics and the season of harvest that paction) in the temperate forests in the mountains of central with local calibration can be used for prediction of detri- Europe. There was a weak relationship between maximal rut mental soil disturbance and help adapt management strate- depth and cone index for the eight harvested forest stands, gies. Aspect, slope, land type and the interaction between but not between cone index and mean rut depth [52]. In a harvest season (winter or non-winter) and land type were study in Croatia, both cone index and shear strength were significant variables in the model. Another decision support found to following the same pattern over a year as the volu- tool for predicting forest harvesting impact on soil properties metric water content (VWC) measured by a portable TDR was developed by Shabani et al. [48] generating maps with (time-domain reflectometry); high VWC resulted in low 1 3 26 Current Forestry Reports (2022) 8:20–37 cone index and shear strength and vise verse [53]. All three Operational Methods parameters were related to precipitation and temperature and their fluctuations over the year. There are several means by which soil compaction and rut- Different soil properties can be measured and available ting can be minimized within an operational context. At the inventory data used for predicting trafficability [ 54–56]. forefront, this includes improved planning and choice of har- Uusitalo et  al. [54] presented a model for predicting rut vesting method. In the literature there are examples of fol- depth by an 8-wheeled forwarder in fine-grained boreal for - low-up techniques and methods to counteract soil compac- est soil. The best predictors were VWC, the thickness of tion by site preparation to enhance seedling growth. There humus layer, cumulative mass of machine passes, and bulk are two main harvesting methods dominating; cut-to-length density. For a forwarder on fine grained soil, rut depth was (CTL) and tree length (TL). The CTL method is normally found to be related to total overdriven mass, VWC, cone performed with a harvester-forwarder system, while the TL penetration resistance and the harvester rut depth [56]. Five method usually includes felling (by machine or manual) and different models were tested, and the authors conclude that skidders of different types. These differences can be consid - the model including harvester ruth depth and cumulative ered when evaluating the results. Furthermore, differences overdriven mass is a good predictor of rut depth. The impact in management strategies, final felling or thinnings could of the harvester on the soil was a good predicter of the for- explain some of the results since transported volume per warder rut formation also on mid-grained soils [55]. Here, area differs. the number of machine passes, VWC in the mineral soil and depth of the organic layer was controlling factors for Reduced Compaction by Improved Planning The way har- rut formation. vesting operations are planned could have a large impact on soil damages. Improved planning could be done by a Route‑Planning A model called “Direct Skid Trail Pattern better distribution of machine operating trails [59], mini- (DSTP) has been developed for planning skidding opera- mizing driving by optimizing distances between harvested tions with farm tractors in Turkey [57]. The aims were to trees [60] or establish permanent designated trails [61]. As minimize skidding time, achieve a high extraction productiv- presented in the results of the terrain-related factors, it is ity and optimize the skid trail pattern in order to minimize demonstrated that the direction uphill or downhill could spatial soil compaction and soil loss. Working time perfor- affect soil disturbances. There are indications that driving mance/productivity increased by 17% when the “Direct Skid downhill reduces the negative impacts on the soil [28, 62]. Trail Pattern”-model was used compared to not using the The extent of machine turning can also influence the severity model. At the same time, the average length of skid trails per of soil disturbances, particularly when the change in direc- hectare decreased by 33%, and thus the spatial area affected tion is pronounced [63]. Driving distances within a harvest by soil compaction and the potential soil loss decreased site could also have an impact on the extent and severity of accordantly [57]. compaction or rather, that all traffic is funneled to landing Picchio et al. [58] demonstrated a method for strip road sites leads to higher soil compaction at the perimeters of planning, where a digital elevation model (DEM) of 2 m res- harvest sites [64]. olution was used, together with data on no-go areas within the forest stands. Driving steeper than 45% forward-back- Reduced Compaction by Logging Methods Literature con- wards or steeper than 25% sideways were not allowed (based cerning reduced compaction by harvesting methods are quite on the tipping risk of the machine used). From each position diverse. A study from Poland reported of less damages on of the forwarder, it was assumed that logs up to 12 m from remaining trees and soil following CTL as compared to TL the center of the strip-road could be reached. The harvested [65 ]. Similar results were obtained from a study in Maine area was divided into pixels, based on the total harvested (USA) were the harvester-forwarder system had the low- mass for the entire harvested area, where each pixel was one est impact on soil bulk density compared to skidding with forwarder load (assuming that the harvested mass was evenly tractor, bulldozer, or skidder [66]. This is in contrast to a spread over the clearcut). The strip road pattern and routes study by Dudakova et al. [67] in Slovakia where a harvester- were optimized with various GIS-tools to minimize surface forwarder system caused the largest damages. Cambi et al. impact of the operation. Finally, the routes were transferred [68] used a photogrammetry method to compare effect of to the machine and followed by the operator. When com- forwarder traffic vs. skidder traffic and concluded that the paring the GIS-planned routes with the harvester strip road forwarder caused more damages to the soil. In a Turkish pattern, the area of impacted soil was reduced between 50 study by Eroğlu et al. [69], four different extraction meth - and 70% (three different stands). ods were compared: skyline, ground skidding by manpower, skidders and a chute system. The results imply that both ground-based system had an impact on soil permeability, 1 3 Current Forestry Reports (2022) 8:20–37 27 bulk density and soil water balance. Marchi et al. [70] meas- it is fundamental to choose appropriate machines consider- ured the difference between two carrying systems compared ing local conditions in terms of logistics and soil properties to two winching system and concluded that dragging the [79–81]. Several studies investigated the use of machines logs on the ground caused more severe disturbances. applying different technological solutions to mitigate the negative effects on soil. Coppice Forestry Coppice or short rotation forestry is a common forest management system in the Mediterranean Technical Features area, where about 23 million ha are managed as coppice [71]. The coppice system is characterized by short rotation, The reduction of ground pressure exerted to the soil is key 6–10 years between cutting meaning that the time for soil in minimizing soil compaction and rutting [15]. For this recovery is very short between operations. Spinelli et al. [72] reason, several strategies have been tested and reported in published an extensive review covering 65 stands in central the literature. The addition of steel flexible tracks (SFT), and Southern Italy and Laschi et al. [22] made a thorough created from steel cross-members joined by chain links Life Cycle Analysis (LCA) and concluded that the extraction that span the entire length of a bogie axle, are common for phase had the largest impact on the end result. Venanzi et al. wheeled machines as they contribute to reduce soil com- [71, 73] compared two skidder systems with light weight paction in many conditions by increasing the contact area machines and a harvester/forwarder system and reported between machines and soil surface [82–84]. In this context, some soil recovery already one year after operations. In Labelle and Jaeger [85] investigated the load distribution on contrast, Naghdi et al. [74] found no sign of recovery dur- a load test platform using a forwarder in different conditions ing the first year after skidding. Paya et al. [ 71] found that (loaded/unloaded and with/without SFT). They assessed following skidding operations, microporosity was reduced for the first time the dynamic load distribution below SFT, by 62.5 and 53.8% for 0–10 cm and 10–20 cm, respectively, obtaining 30% lower values of dynamic peak loads mounting in comparison to unaffected soil surfaces. SFT than without SFT. In another study, Haas et al. [82] compared three configu - Site Preparation In four cases of the articles in this review, rations of tires – 940 mm width, 710 mm with and without site preparation has been reported to counteract soil com- steel flexible tracks (SFT)- on an approx. 30 000 kg loaded paction. Silveira França et al. [75] showed that planting in forwarder to assess the effects of rutting. Results indicated borehole through the compacted layer improved seedling that deeper ruts were caused by narrower tires and also growth. Following a wind throw event on a wet soil in South higher lateral soil displacement, causing more consistent Carolina, long-term effects of different types of site prepara - bulges than wider tires. The use of wider tires as mitiga- tion were studied [76, 77]. The result showed that so-called tion strategy for soil compaction has also been confirmed by bedding (i.e., creating a string of soil on top of the soil sur- Cudzik et al. [83] and Solgi et al. [86], which found a posi- face for better aeration at wet sites) led to improved seedling tive effect in terms of tractive efficiency [ 83] and soil physi- growth. Long-term studies are rare, but Cerise et al. [78] cal properties [86]. According to Starke et al. [84], reduc- sampled a 45-year old felling where different types of site ing tire inflation pressure of a ten-wheeled forwarder caused preparation had been tested and could not detect any residual lower rut depth. Moreover, authors evidenced the potential effects. positive effects of an added bogie to reduce soil compaction, as suggested also by Solgi et al. [86], especially on soft soil Machine Modifications conditions [84]. An interesting improvement on forwarders to reduce rutting has been tested in Sweden by applying a Twenty-seven articles investigated the effects of various pendulum arm technology to a 6-wheel forwarder. It has machines operating in different conditions on their impacts been compared with a traditional 8-wheel forwarder; results on soils and identifying possible mitigation strategies linked of rutting measurements were not better than traditional to machine modic fi ations. In fact, the use of machines in for - technology, mainly due to the lower number of wheels, but est operations is the direct cause of impacts on soil. For this the good potential of this technology was reported [87]. Fur- reason, many studies investigated different factors to find thermore, this potential was partially confirmed in another solutions to reduce the impacts, mainly in relation with soil study comparing different forwarders equipped with stand - compaction. In this context, machine size, traction type, tire ard wheels, SFT, rubber tracks, and pendulum arms. In fact, size, axle load and the total number of passes (traffic) are the pendulum arm technology obtained promising results in most important variables affecting soil compaction; the role terms of rutting, but rubber tracks obtained the most encour- and the effects of these factors can be increased or reduced aging results. In fact, rubber tracks used under loaded con- depending on soil properties. In particular, slope, soil water ditions were able to lower rut depth by up to three times content and soil texture are the most crucial. In this context, as compared to conventional tracks [88], highlighting the 1 3 28 Current Forestry Reports (2022) 8:20–37 importance to distribute the applied load. In this context, the difference was identified as a poorer load distribution and response of tracks in relation to wheels of crawler tractors higher ground pressure of the tractor. Other studies obtained on soils exposed to different conditions of slope and traffic, different results, as reported in Solgi et al. [ 97], comparing caused significantly lower increases of bulk density and rut - skidding operations with a light farm tractor (3 100 kg) and ting [79, 89–91]. In this sense, similar results were obtained a skidder (11 500 kg) in clay loam soil. In this case study, by Cambi et al. [90] comparing the effects of a tracked and a the negative effects of traffic on soil properties in terms of wheeled agricultural forest tractor adapted for use in forestry bulk density, porosity and rut depth after the same number on forest soil physical properties (bulk density, penetration of passes were higher for skidder than tractor [97]. Similar resistance, porosity and shear resistance) at different soil results and conclusions are reported in Nikooy et al. [99]. In water contents. general, the results obtained comparing different machines or solutions in terms of impacts on soils are strictly related Solutions for Steep Terrains to local conditions; this is also valid for comparing different harvesting systems as reported by Venanzi et al. [71], which Slope is one of the most important factors affecting the obtained lower soil impacts by winching in comparison with impacts of forest operations on soils. This is one of the rea- forwarding in coppice. sons why, together with productivity and safety improve- ment, cable-assisted machines have been developed and increasingly used in the last years, and why this technology Machines Use is considered as “one of the major innovations in steep ter- •• rain harvesting in the twenty-first century” [ 92 ]. In fact, Beyond the above-mentioned technical aspects, a strong thanks to this technology, slip during machine traffic can be influence on the effective impacts on soil derives from the reduced [93], together with reducing maximum ground pres- proper or unproper use of machines, which is influenced •• sure [92 , 94]. Moreover, Garren et al. [94] in an experi- by the workers’ experience and the operational conditions. mental area in Brazil, found similar impacts of bulk density In this sense, several authors have reported a series of rec- for both forwarding on gentle slope and cable-assisted for- ommendations to mitigate impacts on soil physical proper- warding on steep slope [94]. Furthermore, it could have pos- ties. Many authors agree on the fundamental need to have •• itive effects also on flat terrains and soft soils [ 92 ]. Finally, a correct and rational planning of forest roads and machine Green et al. [93] reported that cable-assisted machines main- operating trails in particular [38, 87, 96, 100]. On the other tain consistent travel paths concentrating the impacts on a hand, some authors recommend to reduce the number of smaller surface than non-tethered ones, confirming also a trails in order to concentrate impacts on limited surfaces lower effect in terms of bulk density increase after traffic. [38], while others suggest to limit the number of passes in Nevertheless, comparing tethered equipment and traditional a single trail recommending the creation of more trails to systems for felling and extraction, Chase et al. [95] observed distribute soil compaction in the forest [100]. Solgi et al. lower soil disturbance and stream-adjacent disturbance [86] included both options suggesting to limit the number applying motor-manual felling and extraction by conven- of passes in a trail up to 15, or to create high-traffic areas. tional cable yarder than using cable-assisted machines. Uusitalo et al. [101] suggested to create wider trails in peat- lands to avoid to pass with wheels or tracks in the same area, Machine Choice thus distributing the traffic on a larger path. In any case, machines should avoid operating outside the planned trails Regarding machines, another important factor to miti- [102]. It is important considering that the negative effects gate the impacts on soil properties is strictly related with of traffic in terms of increased bulk density are extended the importance in choosing the most suitable machine for up to 1 m far from tracks, enlarging the total damaged for- specific worksite conditions [ 79, 80, 96, 97]. In this con- est surface [15]. Moreover, trails must be maintained and text, the debate on the best choice between traditional or restored to minimize erosion risks [79] and they should be mechanized harvesting systems is very active, where “tra- used in dry conditions [15, 80, 96, 97, 102]. When it is not ditional” means low mechanized systems including the use possible and work is necessary in wet conditions, the use of of chainsaw for felling and processing, and farm tractors tracked tractors is recommended and preferred to wheeled adapted for forestry for bunching and extraction [97–99]. [90]. However, to evaluate if soil water content allows forest In some cases, mechanization is considered better than tra- operations, Allman et al. [103] suggest to test if Atterberg ditional systems in terms of impacts on soils; Rejšek et al. plasticity limits of soil are exceeded or not. [98] applying a dynamic penetration test obtained lower effects operating with a three-axle harvester and four-axle forwarder than with a universal wheeled tractor. The main 1 3 Current Forestry Reports (2022) 8:20–37 29 can also mitigate the increase of soil bulk density during Use of Amendments as Mitigation Techniques ground-based machine traffic [ 110]. Specifically, Labelle et al. [105], Solgi et al. [108], Agherkakli et al. [111], and In total, 15 articles dealt directly with amendments that can be placed on the soil surface during or after harvesting oper- Matangaran [112] all reported a reduction in the compac- tion rate (difference between pre- and post-impact soil bulk ations to act as a protective layer against machine-induced soil disturbances (Table 2). From these articles, two main density) as brush amount increased. However, this finding was not supported by all studies as Parkhurst et al. [107] groups of amendments emerged: brush (12 articles) and mulch (3 articles). These amendments were tested to see and Hashimoto et al. [114] were not able to report any clear trends as to the ability of brush mats to reduce soil bulk how they could impact load distribution, bulk density, rut depth, penetration resistance, and total porosity following density increases as compared to a no brush scenario. Since the compaction rate is often linked to the initial (pre-harvest) ground-based machine traffic. Brush is defined as harvesting debris (treetops, branches, and foliage) that originates from state of the soil, the concept of relative bulk density has also been applied to obtain a qualitative assessment of the the processing phase and is placed on machine operating trails in order mitigate the effect of harvesting and forward - disturbance [15, 105]. In this context, relative bulk density is referred to as the ratio between field bulk density and ing machines on soils as they are being operated in a harvest block. The use of brush is more commonly linked to cut-to- maximum bulk density achieved by the standard Proctor test. A relative bulk density threshold of 0.8 has been linked to length operations where harvesting debris remains in the stand. Once placed on a machine operating trail, harvest- reduced growth of Norway spruce (Picea abies Karst) [21]. Using this relative bulk density concept, Labelle et al. [105] ing debris forms a brush mat. When a machine is in direct contact with a brush mat, the friction between the pieces reported that 40.5%, 17.9%, 14.3%, 15.5%, and 3.6% of soil relative bulk density measurements covered with 0, 5, 10, constituting the brush mat increases and creates a reinforced –2 surface that can expand the contact area between a machine’s 15, and 20 kg  m of brush, respectively, exceeded the 0.8 threshold when performing operations with a Timberjack running gear and the soil surface [10]. The other main group of amendment, referred to as mulch, includes sawdust, straw 1110D 8-wheel forwarder (32 860 kg loaded) on silty soils. and litter. Unlike brush, which is a direct raw biproduct of forest operations, mulch types are normally brought to the Rut Depth and Penetration Resistance As the distance between the point of impact (below the machine running harvest site and originate from a secondary transformation. gear) and the soil surface increases through the use of brush mats, rut depth caused by machine traffic can also be reduced Brush [104, 106, 108, 111–113]. In a study performed in eastern Canada with a 35 800 kg loaded 8-wheel forwarder, brush As bioenergy markets are expanding, the competition −2 between using harvesting debris as a soil protective layer mats of 5, 10, 15, and 20 kg  m offered reductions of indent areas (depression of the soil below the pre-impact level) of during forest operations or for bioenergy operations is on the rise [15]. Finding a certain equilibrium between the two 0.0, 14.3, 71.4, and 90.5%, respectively, as compared to the no brush treatment [106]. Moreover, brush mats of 15 and uses remains complicated and is normally a function of bio-  −2 mass markets and a site’s susceptibility to disturbances. The 20 kg  m were able to statistically reduce rut depth as com- pared to when the forwarder was driven directly on the for- quantity and quality of brush available for soil protection is directly related to site quality, stand characteristics (species, est floor, a finding that was also supported by Ilintsev et al. [104] when using an 8-wheel John Deere 1210 (31 080 kg age, diameter at breast height (dbh), and height), silvicultural treatment, and degree of tree removal [117]. loaded). On highly susceptible soils (clayey silts), research by Matangaran et al. [112] showed that average rut depth Load Distribution and Bulk Density Brush mats are able to created by a Valmet 860.1 forwarder weighing 29 900 kg decreased from 24 cm down to 0 cm when using 1 m thick distribute applied loads to a greater area below the mats and thus decrease peak pressures exerted to the soil [10, 109]. brush mats composed of black wattle (Acacia mangium Willd.) as compared to no brush. In a controlled test, Labelle and Jaeger [10] demonstrated −2 that brush mats of ≥ 20 kg  m were able to statistically In addition to soil bulk density, penetration resistance measured pre and post harvest can also be used to gain fur- lower peak pressures exerted below an 8-wheel forwarder (30 180 kg loaded) as compared to a no brush scenario and ther insight soil disturbances. According to Labelle et al. [105] who performed field trials with a Timberjack 1110D that these thicker mats offered better protection against repeated forwarder passes as compared to lighter brush mats 8-wheel forwarder weighing 32  980  kg, soil penetration −2 values > 3.0 MPa represented 23.7%, 15.0%, 9.4%, 4.6%, (< 20 kg  m ). Those tests were performed on a so-called load test platform equipped with high-capacity load cells. and 0.7% of all post-forwarding test plots that were covered –2 by 0, 5, 10, 15, and 20 kg  m of brush, respectively. The Beyond their capabilities of load distribution, brush mats 1 3 30 Current Forestry Reports (2022) 8:20–37 1 3 Table 2 Summary of studies focused on the use of amendments for soil mitigation Amendment type Ref Country Machine Mass (kg) Amendment amount Soil properties measured Key findings −2 Brush [104] Russia John Deere 1210 8-wheel forwarder 31 080 (loaded) average of 12 kg  m RD Exponential regression explains the relation- ship between rut depth and amount of brush −2 −2 [105] Canada Timberjack 1110D 8-wheel forwarder 32 860 (loaded) 5, 10, 15, 20 kg  m BD, PR Recommending brush mats of 15–20 kg  m to statistically reduce soil density increase −2 [106] Canada Logset 8F 8-wheel forwarder 35 800 (loaded) 5, 10, 15, 20 kg  m RD Rut depth significantly decreased when using −2 15–20 kg  m brush mats [107] USA John Deere 450E bulldozer / Caterpillar 8 606 and 21 192 (both loaded) 1 m thickness BD, TP No clear link reported between the influence 525D wheeled grapple skidder of brush for mitigating bulk density increases −2 −2 [108] Iran Timberjack 450C 4-wheel cable skidder 10 257 (unloaded)10, 20 kg  m BD, RD20 kg  m brush mats significantly reduced bulk density increases as compared to no brush and both tested brush amounts statistically reduced rut depth −2 [109] Canada Not relevant Not relevant 10, 20, 30, 40 kg  m Microstrains below brush Softwood brush showed a slightly better capacity at distributing loads as compared to hardwood mats −2 [110] Germany Timberjack 1110 8-wheel forwarder 26 000 (loaded) average of 27 kg  m BD, TP Increases in soil bulk density were significantly higher in the upper 10 cm following opera- tions on brush as compared to no brush −2 [111] Iran LTT-100A steel-tracked skidder 11 200 (unloaded)7.5, 17.5 kg  m BD, RD Both tested brush amounts statistically lowered rut depth as compared to no brush. Benefits of brush to mitigate compac- tion was limited to 5 machine passes and following passes were only helpful to reduce rutting −2 −2 [10] Canada Timbo TF820D 8-wheel forwarder 30 180 (loaded) 5, 10, 15, 20, 25, 30 kg  m Peak loads below brush Recommending brush mats of 15–20 kg  m [112] Indonesia Valmet 860.1 8-wheel forwarder 29 900 (loaded) 1 m thickness BD, RD Soil compaction rates were reduced by half with the use of brush as compared to no brush. Ruts were not present with brush and averaged 24 cm without brush −2 [113] Russia Ponsse Elk 6-wheel forwarder and John 31 800 / 30 500 (both loaded)15 kg  m BD, RD Brush constrained rut formation after all Deere 1410 8-wheel forwarder forwarder passes and was deemed neces- sary for environmentally sensitive wood harvesting on soft soils −2 [114] Japan Komatsu PC120 excavator with GC-40CA 13 8005, 15 kg  m BD, PR No clear link reported between the influence feller-buncher head of brush for mitigating bulk density and penetration resistance increases −2 −2 Mulch [115] Iran TAF E655 wheeled skidder 6 800 (unloaded)1.27 kg  m straw and 1.67 kg  m BD, PR Three years after mulch application, bulk litter density recovery values were significantly higher when using straw as opposed to litter mulch −2 [116] Iran Timberjack 450C 4-wheel cable skidder 10 300 (unloaded)3.65 kg  m straw BD, TP, RD, PR On trails with a slope of 10%, straw mulch allowed for faster recovery of bulk density, rut depth and penetration resistance as compared to untreated trails −2 −2 [108] Iran Timberjack 450C 4-wheel cable skidder 10 257 (unloaded)10, 20 kg  m sawdust BD, RD20 kg  m mats are significantly more effec- tive in reducing adverse effects than lighter −2 10 kg  m mats BD bulk density, PR penetration resistance, TP total porosity, RD rut depth Current Forestry Reports (2022) 8:20–37 31 penetration resistance threshold of 3.0 MPa was used since soil compaction and rut depth were significantly higher in it has been linked interrupted root growth [118]. moist soils as compared to drier soils [11, 17, 33]. In many regions, ground-based logging operations were conducted on frozen soils, a natural barrier that greatly reduces soil Mulch disturbances [2]. However, in changing climate that triggers highly variable weather conditions, forest operations can be Mulch has been used as a post-impact amendment to try presented with shortened windows of operations, particu- and accelerate the rehabilitation of certain soil physical larly in sensitive areas of high water content [1, 104, 119]. properties. Jourgholami et  al. [115] tested the influence In fact, studies revealed that fine-textured soils coupled with of straw and litter mulches applied to the machine operat- high water content are easily prone to soil compaction and −2 −2 ing trail at rates of 1.27 kg  m and 1.67 kg  m , respec- rutting [23, 32, 39]. Forest soils with low intrinsic bulk den- tively, on the recovery of soil bulk density and penetration sity, well-structured, and high organic matter content might resistance measured three years after mechanized opera- be more vulnerable to compaction and rutting, which appro- tions. Results indicate that both bulk density and penetra- priate measures should be applied to prevent the adverse tion resistance values recovered at a faster rate with the use effects on forest soil. of litter mulch as compared to straw mulch but that both mulch types improved recovery as compared to the uncov- Operational Planning ered plots. Jourgholami et al. [116] reported that applying −2 sawdust mulch (3.65 kg  m ) allowed for a faster recovery The different theoretical mathematical models, described measured six years after harvesting operations of soil bulk above, need to be incorporated in a user-friendly decision density (8%), rut depth (13%) and penetration resistance support tool to be of use for operational planning in forestry. (19%) as compared to untreated trails. It is important to note The static and dynamic trafficability maps summarized in that these findings are for trails with a longitudinal gradient this paper appear to be useful tools for operational planning. of 10%. In a study performed in Iran with a 4-wheel skidder However, their use may be limited to the region/country in operated on a silty loam, Solgi et al. [108] only reported which they were developed, as they are based on conditions statistical differences in soil bulk density when using the and data availability in that region. As pointed out by some −2 thicker 20 kg  m sawdust mat as compared to unprotected studies included in this review [55, 56], the rut depth of the soil but statistical differences were noted when using the harvester is a useful predictor of where more severe rutting −2 thicker 20 kg  m sawdust mats. is expected by the forwarder. There are several techniques of directly measuring the forwarder rut depth, but they all require additional sensors to be mounted at the rear of the Discussion and Recommendations harvester. A more cost-efficient method could be to use harvester CAN-bus data (i.e., data on the power used for Terrain‑Related Factors driving) to predict trafficability of the forwarder, as tried by Salmivaara et al. [51]. This technique has been further Terrain steepness was an important factor that influences tested by Ala-Ilomäki et al. [120 ] and they found that the the trafficability of ground-based machines on forest soils CAN-bus data was efficient for mapping site trafficability. and has a direct impact on changes in soil physical proper- In addition, assessments of harvester rolling resistance by ties (i.e., bulk density, porosity, perpetration resistance, rut using CAN-bus data has the potential to be used in a larger depth) [23, 31, 37]. Both unbalanced load distribution and scale (Big Data), as indirect power recording is possible in wheel slippage on the soil surface are the main contributors all modern harvesters and the cost of automatically creating to excessive soil compaction on steeper terrains [34, 36]. trafficability maps is negligible, according to Ala-Ilomäki Studies highlighted that ground-based harvesting operations et al. [120 ]. should be planned whenever possible on slopes less than A couple of tools for route-planning of an area to be 20% with downhill directions [28, 30, 36, 38]. By increasing harvested were within the search string of this review [57, the slope of machine operating trails, environmental impacts 58]. However, there are other tools with high potential for on soils appeared to be intensified, thereafter; several studies efficient and gentle route-planning, where less soil distur - concluded that a critical slope can be appointed coupled with bance could be an important biproduct. Rönnqvist et  al. soil texture and water content conditions [2, 31, 33]. [121] demonstrated a decision support tool (BestWay) for Changes in soil water content during the year by seasonal primary extraction routes to minimize forwarding distance alterations and precipitation has a significant effect on soil while avoiding steep terrain and wet areas. The input data vulnerability to compaction and rut formation, especially in the optimization model were harvesting block perimeter, in fine-textured soils [ 9, 13, 30]. Studies demonstrated that forest volume density (from LiDAR-data), a digital terrain 1 3 32 Current Forestry Reports (2022) 8:20–37 model (2 m resolution), a depth-to-water map and no-go of brush on machine operating trails, at least not to the areas (e.g., historical or nature conservation areas). The suggested extent, but rather try to reach the recommended −2 BestWay tool has been developed to also suggest landings targets (15–20 kg  m ) on sensitive areas (depressions, based on the forest road network, closeness to crossings, moist soils, fine-textured soils, high traffic areas, etc.) power lines, streams, and too steep areas for storage [122]. where soils are most susceptible. Most brush studies per- If wood can be extracted through shorter distances and trans- formed in a context of soil protection presented brush −2 port times and at the same time with less impact on soils and amounts in kg m . This unit is practical for scientific waters, there is a high potential for acceptance and a smooth experiments but remains quite cumbersome to translate implementation in operational forestry. into an operational context. To circumvent this, Labelle −2 et al. [10] determined that kg m brush amounts can be Machine Modifications converted to cm thickness by applying a factor of 2 or 2.5, −2 hence, a 20 kg  m brush mat would approx. equal a 40 to The analysis of the wide existing literature in terms of 50 cm loose thickness. impact on soils due to machine traffic highlighted how the Brush generally provided the highest protection against different aspects are always directly or indirectly related soil bulk density increases and rut depth as compared to with the pressure exerted on the ground. In fact, the mitiga- the other reported mulch types. This is particularly the tion techniques reported, such as wider tires, reduced tire case when machine traffic is low. When brush is exposed inflation pressure, additional axles and wheels, steel flexible to repeated machine passes, it compacts and a high propor- tracks, etc. are all systems aimed at reducing ground pres- tion of branches are broken or sheared, which can in turn sure and consequently the negative effects on soil physi - decrease the overall strength of the mat [10]. Despite this cal properties. As reported, many factors affect the perfor - lowered strength, Labelle and Jaeger [10] reported that −2 mances of the existing solutions limiting impacts on soil. thicker brush mats of 15 and 20 kg  m still offered ben - In particular, soil texture, soil bearing capacity and terrain eficial load distributing capabilities after 12 loaded passes. slope have to be considered as fundamental to choose the Another advantage of brush is that it is readily available at best technical solution. In this context, the right option is the harvest site, especially when no bioenergy markets are not certain, even if some solutions often resulted better than developed in the proximity to the harvest area. others. In general, harvester-forwarder are considered the least impactful machine-based forest operations [2]. Never- theless, conflicting results emerged from some studies that General Management Recommendations compared traditional tractors with highly mechanized sys- and Strategies tems. In many cases, using traditional tractors resulted in lower forest soils disturbances than with highly mechanized Pre ‑har vest Operational planning of forest operations systems. This result could be attributed to the fact that com- is the key to avoiding severe soil disturbances, including parisons were made in terms of similar operational condition which forest units to be cut in which season, which machine and comparing the same number of passes. To gain further resources to be used (including machine modifications), and insight, it would be interesting to compare the impacts on the planning of landings, main extraction routes (machine soil after the same wood quantity processed and transported operating trails), no-go areas, and amendments to be used, by the two machines (and consequently a different number such as portable bridges and brush. Following recommenda- of passes). Moreover, it would be useful to investigate if tions should be considered during planning: workers’ level of both training and experience can affect the impacts on soil during operations. This is demonstrated in Adapting the forest practice to local conditions and spe- terms of productivity [123], but not in terms of efficiency to cific site characteristics with different soil types, hydrol - reduce impacts on soil, even if remarked in some studies as ogy, and slope. a factor to be considered [2, 124]. Planning landings and forest road network in steep terrain in such a way that the timber extraction is done in down- Use of Amendments as Mitigation Techniques hill directions; upward movement of untethered loaded machines should be prevented as far as possible. The frequency of partial harvests is on the rise as more Limiting logging operations to gentle slopes, particu- forest operations are performed under close-to-nature/ larly < 25% appeared to be an effective way to minimize continuous cover forestry [125, 126]. During such opera- the degree and extent of soil compaction and rutting. tions, it might become more difficult to reach the suggested Lowering the ground pressure of the machines by using brush amounts for adequate soil protection [117]. How- larger tires, reduced inflation pressure, additional axles ever, it is not necessary to aim for a uniform distribution and wheels, and/or bogie tracks. 1 3 Current Forestry Reports (2022) 8:20–37 33 Limiting logging operations on fine-textured soils to dry mitigation techniques. The studies reviewed were wide periods or frozen soil in addition to minimizing ground ranging, both in terms of research scope and geographical pressure. location. The latter undoubtably influences the direction of Use route-planning tools based on various geo-data for the research through local policies and operational guide- finding potential landings and effective wood extraction lines. Below is a list of selected research opportunities that routes to increase productivity (by shortening off-road either remain untapped or offer possibilities of expanded transports) at the same time as soil disturbances are mini- research. mized. • • Plan the operation beforehand as regards soil conditions Digital forestry provides almost endless possibilities of and remaining trees to avoid turning. research. Now that trafficability maps are available, the challenge will be shifted to providing dynamic infor- During harvest One of the most pivotal issue to minimize mation to machine operators on where to travel, under soil compaction and rutting is to prohibit machine entrance which loading condition, and at which traffic frequency on wet soils. to minimize soil physical disturbances. A lingering research question remains regarding Using real time mapping of sensitive soil based on local the contrasting concepts of reducing the number weather forecasts could be useful for help avoiding oper- of machine operating trails as much as possible via ations in areas where soil disturbances could be severe. increased trail spacing, thus further concentrating Measuring soil water content at the beginning of forest machine-induced disturbances to a reduced surface or operations with some field methods could help machine distributing the stress of machine passes over a larger operators to take decisions to concentrate the routes area of a harvest site. This question is not trivial as the where high volume is extracted to drier areas and of answer might be influenced by soil bearing capacity, increased use of amendments such as brush, mulch, cor- operational considerations and machine use. duroy mats in more sensitive areas. The presence of stones and boulders can increase soil With the increase of load capacity on new forwarder bearing capacity but can also cause shifts in machine models, improved load distribution becomes paramount. weight distribution if they are large enough, thereby In this context, the use of harvesting debris (brush mats) increasing the exerted loads to certain wheels. We obtained from the processing phase, remains one of the could not find studies that addressed this dual influ - most beneficial methods of distributing applied loads. ence. −2 • • Applying between 15 and 20 kg  m of brush on machine Consider new soil mitigation techniques that are in line operating trails to reduce soil bulk density increases and with the effects of changing climate (increased calami - rut formation. ties, insect outbreaks, draughts, etc.). For instance, a better understanding of how different mitigation tech - Post harvest It is advisable to avoid rutting and soil compac- niques can be used simultaneously. Furthermore, the tion in the first place, but given that severe soil disturbances competition for use of brush between soil protection in have occurred, there are some ways to try to mitigate the forest stands and bioenergy operations as a source of effect or speed up the recovery rate: biofuel should be studied further. Can an equilibrium be reached between the two uses? The answer probably The usage of mulch types as a biproduct of a secondary rests on the status of biomass markets and the need to transformation can help to accelerate the recovery of soil assign a value to the protection of forest soil. properties post harvest. One caveat is that is does require Most studies have focused on quantifying one or two material to be brought to a harvest site and spread to factors causing soil disturbances. There is a need for machine operating trails post harvest, thus adding costs. having a multi-disciplinary approach to tackle the Mechanical site preparation could be used for loosening complex problems associated with machine/soil/plant the topsoil after soil compaction and also backfill wheel interactions. Certain relationships between mitigation ruts with the use of excavators. Such methods are nor- techniques and their performance can only be fully mally applied in areas of high population (e.g., frequently understood through holistic projects. accessed forest roads, areas of high visibility, etc.). Acknowledgements The authors wish to thank the Université Laval Further Research and Skogforsk for covering the open-access fees. This work was con- ducted within IUFRO division 3.05. Despite the wealth of published literature, it is clear that extensive research is still required in the field of soil 1 3 34 Current Forestry Reports (2022) 8:20–37 10. Labelle ER, Jaeger D. Quantifying the use of brush mats in Declarations reducing forwarder peak loads and surface contact pressures. Croat J For Eng. 2012;33:249–74. Conflict of Interest The authors declare that they have no conflict of 11. Jourgholami M, Majnounian B. Effects of wheeled cable skidder interest. on rut formation in skid trail - a case study in Hyrcanian forest. J Forestry Res. 2011;22:465–9. Human and Animal Rights and Informed Consent This article does not 12. Naghdi R, Solgi A, Zenner EK, Tsioras PA, Nikooy M. Soil dis- contain any studies with human or animal subjects performed by any turbance caused by ground-based skidding at different soil mois - of the authors. ture conditions in Northern Iran. Int J For Eng. 2016;27:169–78. 13. Allman M, Jankovský M, Messingerová V, Allmanová Z. Soil moisture content as a predictor of soil disturbance caused by Open Access This article is licensed under a Creative Commons Attri- wheeled forest harvesting machines on soils of the Western Car- bution 4.0 International License, which permits use, sharing, adapta- pathians. J For Res. 2017;28:283–9. tion, distribution and reproduction in any medium or format, as long 14. Jourgholami M, Ghassemi T, Labelle ER. Soil physio-chemical as you give appropriate credit to the original author(s) and the source, and biological indicators to evaluate the restoration of compacted provide a link to the Creative Commons licence, and indicate if changes soil following reforestation. Ecol Indic. 2019;101:102–10. were made. The images or other third party material in this article are 15. Labelle ER, Jaeger D. Soil compaction caused by cut-to-length included in the article's Creative Commons licence, unless indicated forest operations and possible short-term natural rehabilitation otherwise in a credit line to the material. If material is not included in of soil density. Soil Sci Soc Am J. 2011;75:2314–29. the article's Creative Commons licence and your intended use is not 16. Naghdi R, Solgi A, Zenner EK, Najafi A, Salehi A, Nikooy permitted by statutory regulation or exceeds the permitted use, you will M. Compaction of forest soils with heavy logging machinery. need to obtain permission directly from the copyright holder. To view a Silva Balc. 2017;18:25–39. copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . 17. Hansson L, Šimůnek J, Ring E, Bishop K, Gärdenäs AI. Soil compaction effects on root-zone hydrology and vegetation in boreal forest clearcuts. Soil Sci Soc Am J. 2019;83:105–15. 18. Jourgholami M, Fathi K, Labelle ER. 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Journal

Current Forestry Reports – Springer Journals

Published: Mar 1, 2022

Keywords: Harvesting; Planning; Rutting; Soil compaction; Mitigation techniques; Sustainability

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