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Quantifying the Effect Size of Management Actions on Aboveground Carbon Stocks in Forest Plantations

Quantifying the Effect Size of Management Actions on Aboveground Carbon Stocks in Forest Plantations Purpose of the Review Improved forest management is a promising avenue for climate change mitigation. However, we lack synthetic understanding of how different management actions impact aboveground carbon stocks, particularly at scales rel- evant for designing and implementing forest-based climate solutions. Here, we quantitatively assess and review the impacts of three common practices—application of inorganic NPK fertilizer, interplanting with N-fixing species, and thinning—on aboveground carbon stocks in plantation forests. Recent Findings Site-level empirical studies show both positive and negative effects of inorganic fertilization, interplant- ing, and thinning on aboveground carbon stocks in plantation forests. Recent findings and the results of our analysis suggest that these effects are heavily moderated by factors such as species selection, precipitation, time since practice, soil moisture regime, and previous land use. Interplanting of N-fixing crops initially has no effect on carbon storage in main tree crops, but the effect becomes positive in older stands. Conversely, the application of NPK fertilizers increases aboveground carbon stocks, though the effect lessens with time. Moreover, increases in aboveground carbon stocks may be partially or completely offset by emissions from the application of inorganic fertilizer. Thinning results in a strong reduction of aboveground carbon stocks, though the effect lessens with time. Summary Management practices tend to have strong directional effects on aboveground carbon stocks in plantation forests but are moderated by site-specific management, climatic, and edaphic factors. The effect sizes quantified in our meta-analysis can serve as benchmarks for the design and scoping of improved forest management projects as forest-based climate solutions. Overall, management actions can enhance the climate mitigation potential of plantation forests, if performed with sufficient attention to the nuances of local conditions. Keywords Natural climate solutions · Improved forest management · Carbon · Fertilization · Thinning · Forest plantation * Cyril H. Melikov Environmental Defense Fund, New York, NY, USA cyril_melikov@berkeley.edu Department of Environmental Science, Policy Jacob J. Bukoski and Management, University of California, Berkeley, CA, jbukoski@berkeley.edu USA Susan C. Cook-Patton Moore Center for Science, Conservation International, susan.cook-patton@tnc.org Arlington, VA, USA Hongyi Ban Department of Forest Ecosystems and Society, Oregon State stellaban@berkeley.edu University, Corvallis, OR, USA Jessica L. Chen The Nature Conservancy, Arlington, VA, USA jessicaliuchen@berkeley.edu Carbon Direct Inc, New York, NY, USA Matthew D. Potts mdpotts@berkeley.edu Vol.:(0123456789) 1 3 Current Forestry Reports geographies [32] or specific tree species [16]. Further - Introduction more, these prior large-scale meta-analyses are now more than 10 years old, and to the best of our knowledge, none Mitigating the most damaging effects of climate change has examined moderators of the fertilizing effect nor inves- requires concerted and urgent action within this decade tigated how the effect size might vary with site and stand in both the energy and land sectors [1]. Recently, stud- characteristics. ies have highlighted the potential of tree cover restoration Similarly, the effects of thinning stand biodiversity [33, for mitigating climate change, in places where formerly 34], soil carbon stocks [35, 36], soil microbial carbon [37], forested landscapes have been lost or severely degraded and even drought-related tree stress [38] have been studied [2–6]. Despite momentum to restore tree cover, improv- at site-level scales. However, only a handful of studies have ing the management of existing forests may represent a synthesized the changes in plantation aboveground carbon more cost-effective and rapidly deployable natural climate stocks after thinning operations in large-scale meta-analyses solution, and could sequester 0.1 to 2.3 PgCO e per year • •• [39–41], and most of the previous investigations were plot- [1, 5, 7 , 9 ]. Given limited resources and the urgency of scale empirical studies [42–44]. Similar to fertilization treat- climate change, identifying near-term management actions ments, few studies have investigated how thinning effects that can maximize carbon stocks in managed forest stands change with site characteristics, with most of the past efforts is of paramount importance. focusing on the impacts of stand age and time since treat- In natural forests, multiple studies have focused on ment on the treatment’s effect size [39, 40]. how improved forest management practices, such as The global effects of these silvicultural treatments as well reduced-impact logging and liana control, can increase as the contexts under which they deliver the most carbon aboveground carbon stocks, relative to standard forestry • benefits still need to be documented. Our work builds on practices [5, 8 , 10–12]. In planted forests, studies sug- these previous empirical studies and meta-analyses. It aims gest that extending rotations to a biological rather than to improve understanding of how prominent silvicultural economical optimum can substantially increase time-aver- practices impact aboveground carbon stocks in plantations aged carbon stocks [5, 12]. Nonetheless, forestry practi- globally. To do so, we systematically reviewed and compiled tioners commonly perform additional operations in planted aboveground carbon measurements from interplanted, ferti- stands such as fertilization and stand density management •• lized, and thinned tree plantations distributed across six con- that directly influence plantation carbon stocks [13 , tinents and 18 countries. Using this dataset, we then quan- •• 15 ]. Here, we consider the effects of two fertilization tified (i) how each treatment affected aboveground carbon practice-intercropping of nitrogen (N)-fixing species and stocks and (ii) how the effect size of each treatment varied application of inorganic nitrogen, phosphorus, potassium with different environmental and management factors. In (NPK) fertilizer-and one stand density management oper- doing so, we provide insights on how fertilization and thin- ation (thinning) on aboveground carbon stocks in forest ning treatments can be improved to promote carbon stocks plantations. in planted forests. While the use of NPK fertilizers remains the most com- •• mon fertilization practice [15 ], interplanting of N-fixing plants as an alternative method has become increasingly • Methods prominent in mixed species plantations [16, 17, 18 , 19]. In addition to providing N-fixing benefits, planting of mul- Literature Search and Data Collection tiple species may confer additional ecological benefits, such as habitat for biodiversity and improved soil fertility •• • We conducted this meta-analysis using a recently pub- [15 , 18 , 20, 21]. Furthermore, the use of conventional lished global database that compiles 4756 measurements inorganic NPK fertilizers has potentially adverse environ- of aboveground live tree carbon stocks in timber planta- mental impacts from improper and over-use, including tions, collected from 829 distinct sites across 278 studies declines in soil fertility, elevated groundwater and sur- •• [45 ]. We subset this dataset to the 654 measurements face water pollution, and increased GHG emissions from of aboveground live tree biomass in timber plantations fertilizer production and use [22, 23]. across 45 studies, 56 distinct sites, and 19 tree genera The empirical effects of these fertilization techniques that included relevant management details. Full details on aboveground carbon stocks in plantations has been well on the monoculture plantation database compilation and documented in multiple locations [24–29]. However, the data standardization processes are described in Bukoski magnitude of the effect has been rarely assessed at large •• et al. [45 ]. In addition, we included 97 measurements of scales [17, 30, 31] relevant for the design of forest-based aboveground live trees biomass collected from one large climate solutions, with prior efforts constrained to regional compilation of aboveground carbon stocks in planted 1 3 Current Forestry Reports •• forests [46 ] that was found at a later date. We elected Standardization of Treatment Eec ff t Using Natural to include it as it substantially increased our sample size. Log of the Response Ratio In total, we collected 751 measurements of aboveground live tree biomass across 68 studies (Table S1.1, S1.2, and For our meta-analysis, we used the natural log of the S1.3), 80 distinct sites, and 19 tree genera (Fig. 1). response ratio metric (lnRR) to standardize the effect of For each of the three dominant silvicultural treat- treatments on aboveground carbon across studies. Here, ments—intercropping of N-fixing species, application of lnRR reflects the change in aboveground carbon induced inorganic NPK fertilizer, and stand density management by each of our three treatments—interplanting of N-fixing (i.e., thinning), we selected studies from our monoculture species, application of NPK fertilizers, and thinning. We plantation database that had both (i) measurements of calculated lnRR using Eq. (1): aboveground biomass from plots in which the treatment Xt had been applied and (ii) measurements of aboveground lnRR = ln = lnXt − lnXc (1) biomass from control plots (without the treatment of inter- Xc est). Only studies with imposed, replicated treatments at In Eq. (1), Xt and Xc are mean aboveground carbon in the one or more sites were included in our dataset. Multiple paired treatment and control, respectively. A lnRR value of comparisons within a single study (e.g., comparing dif- 0 means the treatment did not induce any change in carbon ferent thinning intensities to a single unthinned control) compared to the control, while a positive value indicates were considered as distinct within-study observations, and the treatment had a positive effect on aboveground carbon, in that case, treatment values were compared to the same and a negative value indicates a decrease in aboveground control value. In total, there were 43 studies comprising carbon. To account for variation in sampling effort between 197 comparisons for intercropping of N-fixing plants, 17 studies, we weighted the effect sizes by the inverse of the studies comprising 164 comparisons for NPK fertiliza- sample variance for each response ratio. We calculated the tion, and 8 studies comprising 62 comparisons for thin- sample variance using the standard error and number of ning (the included studies are in Tables S1.1 & Table S1.2 replicates reported for each study [48]. When studies did & Table  S1.3). Within the intercropped plots, we con- not report the standard deviations associated with above- servatively accounted for aboveground biomass of only ground carbon measurements, we followed the methodol- the main tree crops to assess the fertilization effect of the ogy of Lajeunesse [49] and Koricheva [50]. This consisted interplanted N-fixing plants on the latter. Before running of imputing the missing standard deviations by calculating the analysis, we converted aboveground live tree biomass the median coefficient of variation (ratio of standard devia- measurements to aboveground carbon using a 0.47 default tion and mean) for each group (treatment or control) from conversion factor [47]. Fig. 1 Map of locations of all sites included in the meta-anal- ysis of intercropping N-fixing plants, NPK fertilization, and thinning effects on planted for - ests aboveground carbon stocks 1 3 Current Forestry Reports studies that reported both means and standard deviations. biological, and human factors, but was also limited by data We then multiplied the median coefficient of variation by availability (Table 1). Except soil moisture regime, all mod- the reported mean for either treatment or control groups for erator variables were recorded using information reported which standard deviations were missing. We performed a in the studies themselves. To obtain soil moisture regimes, sensitivity analysis to test for any potential effects of these we intersected the geographic coordinates of each site with assumptions on our results (see Supplementary Informa- a map of global soil moisture regimes developed by USDA- tion), which revealed that almost all results were robust to NRCS [55]. This map of global soil moisture regimes was this approach, but we note where results were sensitive to the built using data taken from more than 22,000 climatic sta- imputed standard deviations. Finally, we used funnel plots tions distributed around the world [55]. Soil moisture regime to confirm the absence of publication bias [51]. Response data were interpolated and rasterized on a 2-min grid cell ratios were calculated using the {metaphor} package [52] in [55]. Program R v.1.4.1103 [53]. Moderators were tested individually with separate mod- els, such that the influence of a moderator on the effect size Testing of Moderator Eec ff ts and Mixed‑Eec ff ts was determined using all available data for that particular Approach moderator variable. Prior to analysis, we dropped catego- ries of moderators for which we had only one observation. We inserted several moderators (variables that influence the To account for the non-independence of multiple within- strength and/or form of a relationship between a predictor study observations, we inserted publication-level random and a dependent variable [54]) into the mixed-effects model effects into each of our models [50]. We fitted all models to assess how the treatment effect size varied with other using restricted maximum likelihood estimation [50]. We factors hypothesized to influence aboveground carbon (e.g., determined the statistical significance of each moderator genus or soil moisture regime). Our selection of moderator variable using an omnibus test of all model coefficients variables included both categorical and continuous modera- (p-value < 0.05) [52]. Prior to reporting our final results, we lnRR tors and sought to account for an array of environmental, back-transformed (e ) the mean log response ratios and Table 1 Moderator variables inserted in the mixed-effects meta-analytic models for each treatment Treatments Moderator Moderator type Categories/units Interplanting N-fixing plants Previous land use Categorical Cropland, natural forest, plantation Tree genus Categorical Alnus, Anacardium, Casuarina, Eucalyptus, Hymeronima, Pachira, Pinus, Populus, Pseudotsuga Soil moisture regime Categorical Perudic, udic, ustic, xeric Intercropped plant genus Categorical Acacia, Albizia, Alnus, Dalbergia, Enterolobium, Hippophae, Leu- caena, Lupinus, Paraserianthes, Robinia, Salix Wood type Categorical Hardwood, softwood Experimental design Categorical Additive, replacement Stand age Continuous Years Mean annual precipitation Continuous mm/year Inorganic NPK fertilization Previous land use Categorical Cropland, fire, natural forest, plantation Tree genus Categorical Ailanthus, Eucalyptus, Macaranga, Picea, Pinus Soil moisture regime Categorical Perudic, udic, ustic, xeric Wood Type Categorical Hardwood, softwood Stand age Continuous Years Time since fertilization Continuous Years Application method Categorical Continuous, pulse Mean annual precipitation Continuous mm/year Thinning Previous land use Categorical Cropland, natural forest, plantation Soil moisture regime Categorical Perudic, udic, ustic, xeric Wood Type Categorical Hardwood, softwood Tree genus Categorical Acacia, Cunninghamia, Eucalyptus, Pinus Time since thinning Continuous Years Mean annual precipitation Continuous mm/year Basal area removed Continuous Percent 1 3 Current Forestry Reports their 95% confidence intervals and converted these values fertilizer emissions and examined how this effect varied with to percent change relative to the control. the amount of N fertilizer applied using a linear regression Depending on data availability, we also tested how the model. Finally, we estimated the median net carbon balance −1 effect size varied with either tree age or time since treatment of fertilized stands (expressed MgCOe ha ) across all stud- using linear mixed-effects models. In all the interplanting ies included for the NPK fertilization treatment. treatment studies, N-fixing crops were planted at the same time as the main tree crop, and we therefore used stand age as our time variable. To limit the potential effects of Results sparse data at older ages, we dropped measurements above 20 years old before performing this regression. In total, 2 Our results showed effects on aboveground carbon that were data points at 21 and 28 years old were dropped prior to this (i) strongly positive and statistically significant for N-fixing linear regression analysis. Of the inorganic fertilizer stud- species, (ii) strongly positive and statistically significant for ies, 9 of 18 continuously applied inorganic fertilizer over the use of NPK fertilizer, and (iii) negative and statistically the course of the study period and we used stand age as our significant for thinning. Further, our incorporation of mod- time variable. However, 10 of 18 inorganic fertilizer treat- erator variables provided additional nuance on how these ment studies reported “time since treatment” data. For these silvicultural practices impact aboveground carbon in plan- observations, we also used mixed-effects regression models tations. We provide additional details on each of the three to test how the effect size varied with time since treatment treatments below. in inorganically fertilized plots (Table 1). Limited data were available for older times since NPK fertilization. To limit Interplanting of N‑Fixing Species the potential effects of sparse data at older times since treat- ment, we dropped measurements above a time since treat- Overall, intercropping of N-fixing plants in monocul- ment of 5 years before running this regression. In total, two ture stands had a significantly positive effect (approxi - data points at 21 years old were dropped prior to this lin- mately + 20%) on aboveground carbon of the primary spe- ear regression analysis. For our thinning observations, we cies (Fig. 2) (p-value = 0.0031). All moderator variables assessed how the effect size varied with time since treatment were found to have significant effects on the relationship in thinned plots as well (Table 1). We used all the time since between interplanted N-fixing crops and aboveground thinning data to perform the regression analysis. Finally, we carbon (Fig.  2). We found that “soil moisture regime” evaluated whether annual mean precipitation had an influ- had a strong influence on the magnitude of the effect size ence on the effect size of all three treatments using the same (p < 0.001) (Fig.  2). Interplanting of plantations grow- regression model type (Table 1). ing on moister soils significantly increased aboveground carbon (+ 52% for perudic and + 42% for ustic soils), Greenhouse Gas Emissions from Inorganic Fertilizer but not in places with drier xeric soil moisture regimes Application (Table S5.1). Furthermore, N-fixing companion crops sig- nificantly increased the aboveground carbon stocks of the For each aboveground carbon measurement reported in NPK main tree crop when plantations occurred on former crop- fertilization studies, we calculated the business as usual lands (+ 67%, p < 0.0001) (Fig. 2), rather than locations (BAU) associated amount of nitrous oxide (N O) emitted with tree cover. The genus of the main tree crop also influ- (expressed in CO e) for each ton of synthetic nitrogen (N) enced the magnitude of the effect size (p < 0.001). Specifi- fertilizer applied, both in-field and upstream from fertilizer cally, establishing Eucalyptus trees with N-fixing compan- manufacturing itself [5]. We used an emissions factor of ion crops increased the latter aboveground carbon stocks 2.54% for N fertilizer (11.9 MgCO e per MgN applied) for by + 25%, whereas other genera did not show a significant in-field emissions and an upstream emissions factor of about effect. Additionally, the genus of the intercropped species 4 kgCO e per kgN produced [5]. We extracted tonnage of had a significant impact (p -value < 0.001) on the effect N fertilizer directly from the studies included in this meta- size as well (Fig. 2). Intercropping with Leucaena, Albizia, analysis (Table S1.2). When the amount of N fertilizer used Enterolobium, and Hippophae induced a 52%, 80%, 87%, was reported as kilograms per tree, we converted to kilo- and 113% increase in the aboveground carbon of the main grams per hectare using the reported stand density value. tree crop, respectively, whereas other intercropping gen- We then compared the additional aboveground carbon gain era did not have a significant effect (Fig.  2; Table S5.1). induced by NPK fertilization (i.e., the difference in above- The type of wood of the main tree species also moderated ground carbon between the treated and control plots) with the effect of the treatment (p -value = 0.0036). Hardwoods emissions from N fertilizers use and manufacturing. We reacted positively to interplanted N-fixing crops (+ 25% in calculated the difference between additional carbon and N aboveground carbon stocks) (Table S5.1). Finally, the type 1 3 Current Forestry Reports Fig. 2 Meta-analysis results of the change in aboveground carbon of plantation trees in response to the interplanting of N-fixing plants. Error bars represent the 95% confidence intervals. Omnibus tests of significance for moderator variables are shown on the right side (NS means “not sig- nificant”). Results for the “Tree genus” moderator are only pro- vided for genera with more than 5 observations (see Table S5.1). Results for the “Intercropped Genus” moderator are only provided for genera with significant effects or for genera with more than 5 observations (see Table S5.1). The number of observations in each category is shown in parenthesis of experimental design influenced the magnitude of effect Inorganic NPK Fertilization size (p-value < 0.001) (Fig.  2). The use of interplanted N-fixing crops appeared to be more beneficial when an We found that inorganic NPK fertilizers signifi - additive design was adopted (+ 72%) than a replacement cantly increased aboveground carbon by 44.5% over- one (+ 16%) (Table S5.1). all (p-value < 0.001) (Fig.  4). Of the individual studies However, our results also suggest that the effect of inter - included in the meta-analysis, most reported a significant cropping N-fixing species on aboveground carbon varied positive effect of NPK fertilization on aboveground car - with stand age. By regressing ln(RR) on stand age, we iden- bon, whereas six studies found a non-significant effect. tified a significant positive association with the treatment Similarly to interplanting of N-fixing species, all mod- effect size (p -value < 0.0001) (Fig. 3). For every additional erator variables were found to have significant effects on year that a stand was allowed to grow, the influence of the relationship between NPK fertilizer and aboveground intercropping N-fixing species on plantation aboveground carbon. carbon was increased by 3.6% (Fig. 3). Intercropping ini- We found that “tree genus” significantly influenced tially decreased aboveground carbon in the main tree crop, the change in aboveground carbon attributed to inor- although not significantly, but its effect became positive ganic NPK fertilization (Fig.  4) (p-value < 0.001). The when the stand was 3 years old and steadily increased over aboveground carbon of treated Pinus and Eucalyp- time from then on t (+ 84% at 20 years old) (Fig. 3). tus plantations were 51% (p-value < 0.001) and 37% Finally, we did not identify a significant effect of mean (p-value < 0.001) higher in fertilized plots compared to annual precipitation, the other continuous variable in our the controls, respectively (Fig. 4; Table S5.2). In addition, model, on the magnitude of the effect size (p -value = 0.0872; both hardwoods and softwoods responded positively to Table S5.1). the treatment; however, NPK fertilization had a slightly 1 3 Current Forestry Reports Fig. 3 Change in the effect size of interplanting N-fixing plants as a function of stand age. The significance of the regression is indicated by the p-value in the upper right as well as the inter- cept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval greater effect on aboveground carbon in hardwood (+ 46%) treatment, the effect size of inorganic fertilization on carbon (p-value < 0.001) relative to softwood plantations (+ 43%) stocks decreased by 6.66% (Fig. 5). (p-value < 0.001) (Fig. 4; Table S5.2). Mean annual precipitation also had a significant nega- Soil moisture regime also had a strong influence on the tive effect (p -value < 0.001) on the magnitude of the change magnitude of the effect size (p -value < 0.001) (Fig. 4). NPK in aboveground carbon (Figure S3.1). For every additional fertilization of plantations growing on moister soils signifi- millimeter of precipitation received per year, the treatment cantly increased aboveground carbon (+ 82% for perudic effect size was reduced by 0.03% (Figure S3.1). and + 42% for udic soils), but not in places with xeric and Once we accounted for in-field and upstream fertilizer ustic soil moisture regimes (Table S5.2). It is worth not- emissions, we found a negative median net carbon bal- −1 ing that the influence on the effect size of the perudic soil ance of fertilized stands across studies (− 2 MgCO e.ha ). moisture regime might be overestimated as only one study In other words, fertilized stands were net emitters across reported measurements for that soil moisture regime (Fig. 4). studies (Figure S3.2), and the climate mitigation benefit of Furthermore, the methodology used to apply NPK fertiliz- fertilizer declined with the amount of N fertilizer applied ers significantly influenced the magnitude of the fertilizing (p-value < 0.001). The climate benefit became null at 0.55 effect (p -value < 0.001) (Fig. 4). The effect size appeared MgN or greater applied per hectare (Fig. 6). higher when fertilizers are applied episodically (+ 53%) rather than continuously (+ 36%) (Table S5.2). The type Thinning of previous land use also strongly influenced the treatment effect size and explained part of the heterogeneity in effect Across the different studies, thinning decreased stand- size across studies (Fig. 4) (p-value < 0.001). Specifically, ing aboveground carbon by about 34% (p-value < 0.001) the use of inorganic fertilizers appeared to be more benefi- (Fig. 7). We found soil moisture regime to have a signifi- cial on lands that were previously plantations (+ 44%) or cant impact on the percent change in carbon from thinning natural forests (+ 76%), than those cleared by fire or previ- (p-value < 0.001) (Fig. 7). When thinning was conducted ously used for croplands (Table S5.2). on xeric soils, carbon levels decreased significantly by 59% We found a negative, although not significant (p-value < 0.001) (Table  S5.3). We found type of previous (p-value = 0.5), association between stand age and the mag- land use to also have a signic fi ant inu fl ence on the magnitude nitude of the effect size in fertilized stands (Table S5.2). of the effect size (p -value < 0.001) (Fig. 7). Stands grown However, when we used time since treatment rather than on former agricultural lands had aboveground carbon lev- stand age, we found that the benefit of inorganic fertilizers els that were 55% lower (p-value < 0.001) in thinned stands on aboveground carbon significantly decline through time compared to controls (Table  S5.3). When thinning was (p-value = 0.008) (Fig. 5). For every additional year since the executed in stands that were established on former natural 1 3 Current Forestry Reports Fig. 4 Meta-analysis results of the change in aboveground carbon moderator variables are shown on the right side (NS means “not sig- of plantation trees in response to NPK fertilization error bars repre- nificant”). The number of observations in each category is shown in sent the 95% confidence intervals. Omnibus tests of significance for parenthesis forestlands, the mean decrease in aboveground carbon was Genus of tree did not significantly influence the magnitude 44% (p-value = 0.0026) (Table S5.3). Finally, we also found of the effect size. that the type of wood influenced the magnitude of the effect When we inserted the “time since treatment” continu- size (p-value < 0.001) (Fig. 7). Both hardwood and softwood ous moderator variable in the model, we found that as time stands experienced large reductions in aboveground carbon since thinning increased, the negative effect of thinning on due to thinning operations, which averaged a − 33% change carbon lessened (p-value < 0.001) (Fig. 8). For every addi- in aboveground carbon from thinning for both wood types. tional year since the treatment, the effect size of thinning on 1 3 Current Forestry Reports Fig. 5 Change in the fertilizing effect size of NPK fertilizers as a function of time since treatment. The significance of the regression is indicated by the p-value in the upper right as well as the intercept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval Fig. 6 Change in the net carbon balance of fertilized stands as a function of the amount of N applied. The significance of the regression is indicated by the p-value in the upper right as well as the intercept and slope values, and the coefficient of determination. The area shaded in blue around the regression lines indicate the 95% confi- dence interval carbon stocks decreased by 4.6% (Fig. 8), suggesting that (p < 0.001) (Table S5.3). For every additional percent of basal after 14 years the thinning effect disappears. area removed, the effect of thinning on carbon stocks was Mean annual precipitation also had a significant negative increased by 1.3% (Table S5.3). effect (p -value = 0.04) on the magnitude of the change in aboveground carbon (Figure S3.3). For every additional mil- limeter of precipitation received per year, the effect of thin- ning on carbon stocks was reduced by 0.03% (Figure S3.3). Finally, we found that as the percent of basal area removed increased, the negative effect size of thinning increased 1 3 Current Forestry Reports Fig. 7 Meta-analysis results of the change in aboveground carbon of plantations from thinning. Error bars represent the 95% confidence intervals. Omnibus tests of significance for moderator variables are shown on the right side (NS means “not significant”). The number of observations in each category is shown in parenthesis Fig. 8 Change in the effect size of thinning operations as a func- tion of time since thinning. The significance of the regression is indicated by the p-value in the lower right as well as the inter- cept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval 1 3 Current Forestry Reports •• may emerge soon after their establishment [56, 65 ]. Discussion Conversely, when the N-fixing species is established a few years after the main crop, the former tends to compete with Enhanced aboveground carbon sequestration in plantations the latter to a much lesser degree for light and water [56, via improved stand management is a prominent strategy •• 65 , 66, 67]. As a result, complementarity between the for mitigating climate change. Our study quantified the crops increases and the main crop can allocate a greater effect size of three dominant silvicultural practices—inter - proportion of aboveground growth using the additional N cropping N-fixing species, fertilization with inorganic •• provided by the companion crop [65 ]. NPK, and stand density management via thinning—on Our analysis also showed that several environmental, aboveground carbon stocks in plantations. We found that biological, and human factors influence the magnitude of the magnitude of the effect size often depended on context response of the major crops to interplanting of N-fixing spe- and below we discuss the dominant drivers of variation cies. The main tree crop benefited more from the treatment and implications for landowners and land managers. when it was performed on wetter soils. Soil moisture signifi- cantly affects N-fixation by controlling nodulation and nitro- genase activity [68]. Moist soils promote nodulation and Interplanting of N‑Fixing Plants nitrogenase activity while drier soils reduce the number of nodules produced and inhibit nitrogenase activity, resulting We found that only some intercropping N-fixing species in very low rates of nitrogen fixation [68]. Wetter soils also may be beneficial (in our case Albizia, Enterolobium, Hip- promote the growth of companion crops themselves, which pophae, and Leucaena species) and/or primarily when the allows for more N-fixation and thus increases the amount of crop trees are mature. The species-specific effects may available nitrogen in the soil [68]. be mediated by compatibility of growth patterns [56, 57]. Furthermore, interplanting N-fixing crops was most bene- Growth compatibility can occur through at least three ficial when it was performed in plantations growing on lands mechanisms: reduced competition for light via canopy that previously hosted agricultural crops, compared to those stratification and crown complementarity, reduced com- previously under tree cover. Agricultural lands often expe- petition for nutrients and water via root stratification, and rience soil impoverishment, especially when intense crop- direct or indirect growth facilitation [16, 58, 59]. All three ping techniques are used [69]. Low nutrient concentration, mechanisms may explain our results. We found that inter- especially for N, appears to favor complementarity between cropping with shade tolerant or moderately shade toler- •• the main tree crop and its N-fixing companion crop [65 ]. ant species (e.g., Leucaena spp., Enterolobium spp.) had Nonetheless, since soil impoverishment also depends on the greater effects on aboveground carbon relative to shade intensity of the land use, which was not captured here, this intolerant species (e.g., Alnus spp.). Root stratification result should be interpreted with caution. couples trees with deep taproots (e.g., Eucalyptus spp.) that can acquire nutrients and water from lower soil hori- Inorganic NPK Fertilization zons with species with shallow extensive horizontal root systems (e.g., Acacia spp.) [60]. Interplanting N-fixing The use of NPK fertilizers significantly increased the above- plants may also confer benefits through chemical root ground carbon stocks of plantations, which aligns with the stratification, which may facilitate mycorrhizal relation- findings of others [70, 71]. However, once we accounted ships between intercropped species and the main tree for the CO e emissions resulting from inorganic N fertilizer crops [16]. Furthermore, although we did not directly test manufacturing and use, we found that higher levels of ferti- these interactions, interplanting N-fixing legumes facili- lizer application could negate and/or overwhelm the increase tate nutrient access for plantation trees by decreasing soil in aboveground carbon stocks induced by fertilization. This pH which promotes soil weathering and therefore general • highlights that the climate mitigation potential of fertilized nutrient availability [61 ]. Lastly, companion crops may stands could be substantially overestimated if fertilizer emis- inhibit the growth of competitors via allelopathic effects sions are not taken into consideration. This also emphasizes and/or eliminate local pathogens [58, 62]. that inorganic fertilizers have a greenhouse gas cost that Our results also showed that interplanting N-fixing can exceed their carbon benefit, particularly when they are plants become more beneficial through time. Early growth applied in large amounts [72]. To maximize the climate ben- incompatibility and poor establishment of nitrogen-fixing efits of using synthetic fertilizers in planted forests, a holis- companion crops could explain the negative or absent tic accounting of greenhouse gas emissions associated with effect of intercropping early in the rotation [63, 64]. When producing and applying fertilizers is therefore needed [73]. crops are planted simultaneously, intense competition We found that the benefit of NPK fertilization decreased between them for resources such as light water or nutrients with time. This phenomenon has been noticed in previous 1 3 Current Forestry Reports studies and reviews [14, 74, 75] and several factors are and Saarsalmi and Mälkönen [85] found similar results and believed to explain this trend. During the initial establish- attributed it to higher nutrient-use efficiency under pulse ment phase, young seedlings generally do not have well fertilization. Furthermore, several studies have stressed that developed root systems and providing easily accessible inor- continuous application of N favors faster N saturation, which ganic forms of nutrients can increase growth [18 , 74]. Over reduces the positive effects of inorganic N-based fertilizers time, unfertilized seedlings develop their root systems and on biomass growth in the long-term [86]. Since N saturation gain access to larger nutrient pools, which could explain the is also believed to promote nitrate leaching from soils [87], decrease in the magnitude of the effect size observed here increase emissions of N O [84], and induce large reduction [32, 74]. Furthermore, plantations trees increasingly rely on in mycorrhizal symbionts [88], our results underline that organic forms of N like glycine relative to inorganic forms careful handling of fertilizer operations is needed to maxi- (e.g., NH & NH ) as they age [76], which could explain mize tree growth while minimizing negative environmental 3 4 the lower effect size of fertilizers later in the rotation. Such side effects. Nonetheless, this difference in growth responses a phenomenon might also apply in the context of the inter- to pulsed and continuous fertilization might also be caused planting treatment discussed before. The effect of fertiliz- by other chemical or physical soil variables (e.g., initial soil ers on biomass carbon growth has also been suggested to fertility, soil pH) which were not captured in this analysis. decrease over time due to increased allocation of resources towards reproduction, which leads to lower nutrient uptake Thinning rates [77]. Our results showed that the benefit of NPK fertilization Our results showed that thinning operations significantly on growth declined as precipitation increased. High precipi- lowered aboveground carbon in plantations, which aligns tation levels increase soil water content and, in some cases, with the findings of others [ 40, 89]. Given that thinning is push it beyond field capacity (i.e., soil becomes saturated) performed to reallocate resources and growing space to tar- [78]. Saturated soils tend to be highly prone to leaching of get trees of primary value and induces a trade-off in loss very mobile nutrients such as N, which reduces their avail- of stand-level carbon, this result was expected. Indeed, •• ability for plant uptake [15 , 78]. Our results also suggest increased growth rates for individual trees post-thinning that plantations growing on soils with udic moisture regimes have been well documented [44, 90, 91]. We also identified experienced the largest increases in aboveground carbon an attenuation of the effect on overall stand aboveground when fertilized. Nutrients are generally more available when carbon with increasing time since thinning (Fig.  8). Our soil moisture levels are optimum and remain adequate for results suggest that at the stand level, the negative effect of most of the year, as is the case for soils with an udic soil thinning on aboveground carbon lasts for approximately a moisture regime [79]. decade before becoming attenuated. Conversely to interplanting, the benefit of NPK fertilizers Furthermore, we found that the negative effect of thinning was higher when they were applied in plantations growing on aboveground carbon might be amplified when performed on lands that previously hosted tree plantations or natural in stands located in dry areas and growing on xeric soils. forests. Harvested lands often experience soil impoverish- Non-thinned trees growing in stands on mesic and wetter ment, especially when the whole tree harvesting method is soils tend have access to more resources such as water and employed [80, 81]. Indeed, harvest promotes nutrient min- nutrients after treatment than the same trees growing on eralization and nitrification, making them more mobile and xeric soils, which could therefore induce a greater growth therefore more subject to leaching [82]. The removal of trees response to thinning operations and a weaker treatment themselves also leads to a decrease in soil nutrient capital effect in the former scenario than in the latter. and can curb the growth of the next trees [83]. Fertilization Thinning operations appear to have a more negative is often used to compensate for the loss of nutrients [83], and effect on stand carbon when they are performed on former poor soil post-harvest may explain the fertilizer benefit we agricultural lands. Nutrient and water availability are gen- observed. However, since this analysis did not account for erally lower in those areas, and fertilization and irrigation the effect of land use intensity on soil impoverishment, this are often needed to ensure afforestation/reforestation suc- result should be interpretated with caution. Overall, these cess [92]. As a result, thinned stands growing on former findings related to site conditions underline the fact that agricultural lands might have access to smaller nutrient response of trees to fertilization is heavily site dependent and water pools that constrain post-thinning growth and and varies with site productivity and quality, and/or intensity therefore induce a larger thinning effect on stand-level of prior land use. aboveground carbon. Furthermore, former agricultural This study also revealed that plantation stands store more lands tend to be more compacted due to heavy machin- aboveground carbon when fertilizer applications are per- ery use [93], which could restrict the common increase in formed in pulses, rather than continuously. Aber et al. [84] resource availability after thinning operations and further 1 3 Current Forestry Reports accentuate the effect on aboveground carbon [ 43]. Overall, Management Implications these results stress that response of stands to thinning is heavily site dependent. Stands on more productive sites This study reveals that the use of N-fixing companion crops will often respond quite differently to the same thinning as a fertilization technique is challenging and primarily treatment than a stand on lower quality sites [94]. depends heavily on growth complementarity between the crop and intercropped species. Knowledge of the silvics of individual species is a critical precursor to ensure success- Caveats and Potential Limitations ful growth benefits, and poorly implemented systems may induce null effects on growth. However, use of intercrop- Our study focuses on aboveground live tree carbon stocks ping as the main fertilization technique can also reduce N O only and did not examine changes in belowground carbon emissions and nutrient volatilization, which are common and soil organic carbon in response to fertilization and issues associated with the application of inorganic NPK fer- thinning. Belowground carbon and soils are substantial tilizers [101]. Factors such as prior land use, soil moisture carbon pools in forest ecosystems. For example, soil car- regime, and method of implementation are also likely to be bon stocks are believed to represent 90% and 50% of the key moderators of the effect of interplanted N-fixing plants total carbon stock in boreal and tropical forests respec- on aboveground carbon. Knowledge of the site characteris- tively [95]. Furthermore, in these forest types, roots are tics and history, as well as interactions between tree species believed to store between 23 and 27% of total tree biomass seem therefore crucial to ensure successful associations. carbon [96]. However, we omitted these pools due to little Inorganic fertilizers, despite being growth catalyzers, can data availability and inconsistent field sampling methods have detrimental effects on the environment which includes across reviewed studies. This decision was further justified water eutrophication via nutrient leaching and runoffs, and by the fact that changes in soil carbon stocks tend to be air pollution via greenhouse gas (GHG) emissions from small relative to changes in the aboveground carbon pool fertilizers manufacturing and nutrient volatilization. Thus, and it takes longer to detect them [97]. Previous studies there are benefits to minimizing additions. Our results show have found positive effects of NPK fertilization and inter - that NPK fertilizers are not universally beneficial and their planting N-fixing plants on soil carbon stocks in forest positive effects, when observed, decline through time. Fur - plantations [86, 98, 99]. Other studies have found neutral thermore, we found that GHG emissions resulting from or negative effects of thinning on soil carbon stocks [ 35, N fertilizer manufacturing and in-field application could 36, 40, 89, 100] and negative effects on belowground car - exceed the aboveground carbon gains induced by their use. bon as well [100]. If these results occur in other forest Our results also stress that factors such as prior land use and plantation management studies, focusing solely on car- soil moisture regime may be key moderators of the effect of bon in aboveground biomass tissues may underestimate fertilizers on aboveground carbon. Our findings emphasize the impact of forest management actions such as fertiliza- therefore the need to apply the right rate of nutrients at the tion on total ecosystem carbon stocks and overestimate it right times and in the right context. Landowners should care- in the context of thinning. Additional empirical data on fully handle fertilization operations to maximize their carbon the impact of these management operations on plantations benefits and minimize their costs. total carbon stocks as well as studies reporting responses Next, our study suggests that the initial decrease in of the different carbon pools are needed. aboveground carbon caused by thinning operations could Data limitations also partly restricted our ability to be compensated over time by the recovery of carbon post- assess the exact timing at which the positive effects of thinning, particularly when coupled with extended rotation inorganic fertilizers on aboveground stand carbon stocks lengths. Indeed, by increasing rotation lengths, landowners disappear. Although the data stopped at 5 years since treat- can further attenuate the effects of thinning on stand carbon ment, if the decline through time continues following the up to a point at which the initial loss of carbon is offset by same path, then we would predict the carbon benefits of the recovery of a productive and well-stocked stand [40, inorganic fertilizers to disappear eight years after treat- 42, 102]. From there, the individual trees in thinned stands ment. Further data on the impact of time on the effect could sequester additional carbon to levels as high or higher size of fertilizers would therefore be valuable to refine our than unthinned stands of the same age [42, 102]. Criti- predictions. Moreover, data were heavily skewed towards cally, the likelihood of recovering unthinned carbon levels younger ages across all of our focal silvicultural treat- post-thinning will depend on the intensity of thinning and ments. Long-term measurements of the effects of silvi- thinning technique employed, with more intensive thinning cultural treatments on plantation forest carbon stocks will practices tending to result in longer or unachievable recov- •• greatly improve our understanding of the climate mitiga- ery of aboveground carbon [15 , 42, 91, 102]. For exam- tion potential of these systems. ple, plots thinned from below may rapidly recover carbon 1 3 Current Forestry Reports Acknowledgements We thank Timothy Bowles and the Potts Group sequestration rates equivalent to control plots, while stands at U.C. Berkeley for valuable feedback on early drafts of this manu- that experienced dominant and crown thinning operations script. Although their comments have greatly improved our study, the may have longer periods to carbon recovery [42]. However, ideas and opinions expressed in our study do not necessarily reflect thinning can also confer important forest structural qualities their own. important for management of forest carbon beyond maximi- Funding We thank the Bezos Earth Fund for supporting C. H. M.’s •• zation of standing aboveground carbon stocks [13 ], such time on this work. Grants from the following institutions to the Nature as conferring resistance to low or medium-intensity wild- Conservancy partially funded S. C. C.-P.’s time on this work: the Chil- fire in fire-prone landscapes and insect/pathogen outbreaks dren’s Investment Fund Foundation, COmON Foundation, the Craig •• and Susan McCaw Foundation, the Doris Duke Charitable Foundation, [13 , 103]. Further research on these co-benefits and the Good Energies Foundation, and the Bezos Earth Fund. coupling of thinning with extended rotations is needed to validate the use of thinning as a forest carbon management Data Availability Data used for the study are published on tool. Zenodo (https:// doi. org/ 10. 5281/ zenodo. 77898 68) Lastly, this meta-analysis underlines the need for for- Declarations estry practitioners to be aware of site conditions, quality, and land use history before conducting thinning operations Conflict of Interest M. D. P. is the Chief Science Officer for Carbon because their effects can vary from site to site, and they tend Direct Inc., a company combining science, technology, and capital to to strongly depend on local conditions. Additional studies deliver quality CO management at scale. M. D. P. is a shareholder in the company and thus stands to benefit financially from forest man- on the influence of soil moisture regimes and previous land agement targeted at climate change mitigation. The remaining authors use on the magnitude of the effect size of thinning on stand declare no competing interests. aboveground carbon are needed to consolidate our results. Overall, our study underscores that selecting the appropri- Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any ate species and treatments for each site is crucial to ensure of the authors. an effective carbon management plan in forest plantations. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long Conclusion as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are Improved forest management has been highlighted as a key included in the article's Creative Commons licence, unless indicated natural climate solution because of its ability to deliver otherwise in a credit line to the material. If material is not included in • • climate benefits within short-time scales [1, 5, 7 , 8 ]. the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will Nonetheless, there is still substantial uncertainty regarding need to obtain permission directly from the copyright holder. To view a the forest practices that would help realize this mitigation copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . potential and the context in which they would deliver the most climate benefits [1 ]. Our study provides additional considerations that help facilitate the use of improved man- References agement practices to increase plantations carbon stocks and hence mitigate climate change. 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Scand J For Res. 2001;16:514–35. 1 3 Current Forestry Reports 106.• Osuri AM, Gopal A, Raman TRS, DeFries R, Cook-Patton SC, the need to diversify current monoculture tree plantations Naeem S. Greater stability of carbon capture in species-rich to make them more resilient and resistant to an uncertain natural forests compared to species-poor plantations. Environ future.) Res Lett. 2020;15:034011. (This study provides evidence of greater stability of carbon stocks in species-rich forests Publisher's Note Springer Nature remains neutral with regard to than in species-poor tree plantations. This study highlights jurisdictional claims in published maps and institutional affiliations. 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Forestry Reports Springer Journals

Quantifying the Effect Size of Management Actions on Aboveground Carbon Stocks in Forest Plantations

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10.1007/s40725-023-00182-5
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Abstract

Purpose of the Review Improved forest management is a promising avenue for climate change mitigation. However, we lack synthetic understanding of how different management actions impact aboveground carbon stocks, particularly at scales rel- evant for designing and implementing forest-based climate solutions. Here, we quantitatively assess and review the impacts of three common practices—application of inorganic NPK fertilizer, interplanting with N-fixing species, and thinning—on aboveground carbon stocks in plantation forests. Recent Findings Site-level empirical studies show both positive and negative effects of inorganic fertilization, interplant- ing, and thinning on aboveground carbon stocks in plantation forests. Recent findings and the results of our analysis suggest that these effects are heavily moderated by factors such as species selection, precipitation, time since practice, soil moisture regime, and previous land use. Interplanting of N-fixing crops initially has no effect on carbon storage in main tree crops, but the effect becomes positive in older stands. Conversely, the application of NPK fertilizers increases aboveground carbon stocks, though the effect lessens with time. Moreover, increases in aboveground carbon stocks may be partially or completely offset by emissions from the application of inorganic fertilizer. Thinning results in a strong reduction of aboveground carbon stocks, though the effect lessens with time. Summary Management practices tend to have strong directional effects on aboveground carbon stocks in plantation forests but are moderated by site-specific management, climatic, and edaphic factors. The effect sizes quantified in our meta-analysis can serve as benchmarks for the design and scoping of improved forest management projects as forest-based climate solutions. Overall, management actions can enhance the climate mitigation potential of plantation forests, if performed with sufficient attention to the nuances of local conditions. Keywords Natural climate solutions · Improved forest management · Carbon · Fertilization · Thinning · Forest plantation * Cyril H. Melikov Environmental Defense Fund, New York, NY, USA cyril_melikov@berkeley.edu Department of Environmental Science, Policy Jacob J. Bukoski and Management, University of California, Berkeley, CA, jbukoski@berkeley.edu USA Susan C. Cook-Patton Moore Center for Science, Conservation International, susan.cook-patton@tnc.org Arlington, VA, USA Hongyi Ban Department of Forest Ecosystems and Society, Oregon State stellaban@berkeley.edu University, Corvallis, OR, USA Jessica L. Chen The Nature Conservancy, Arlington, VA, USA jessicaliuchen@berkeley.edu Carbon Direct Inc, New York, NY, USA Matthew D. Potts mdpotts@berkeley.edu Vol.:(0123456789) 1 3 Current Forestry Reports geographies [32] or specific tree species [16]. Further - Introduction more, these prior large-scale meta-analyses are now more than 10 years old, and to the best of our knowledge, none Mitigating the most damaging effects of climate change has examined moderators of the fertilizing effect nor inves- requires concerted and urgent action within this decade tigated how the effect size might vary with site and stand in both the energy and land sectors [1]. Recently, stud- characteristics. ies have highlighted the potential of tree cover restoration Similarly, the effects of thinning stand biodiversity [33, for mitigating climate change, in places where formerly 34], soil carbon stocks [35, 36], soil microbial carbon [37], forested landscapes have been lost or severely degraded and even drought-related tree stress [38] have been studied [2–6]. Despite momentum to restore tree cover, improv- at site-level scales. However, only a handful of studies have ing the management of existing forests may represent a synthesized the changes in plantation aboveground carbon more cost-effective and rapidly deployable natural climate stocks after thinning operations in large-scale meta-analyses solution, and could sequester 0.1 to 2.3 PgCO e per year • •• [39–41], and most of the previous investigations were plot- [1, 5, 7 , 9 ]. Given limited resources and the urgency of scale empirical studies [42–44]. Similar to fertilization treat- climate change, identifying near-term management actions ments, few studies have investigated how thinning effects that can maximize carbon stocks in managed forest stands change with site characteristics, with most of the past efforts is of paramount importance. focusing on the impacts of stand age and time since treat- In natural forests, multiple studies have focused on ment on the treatment’s effect size [39, 40]. how improved forest management practices, such as The global effects of these silvicultural treatments as well reduced-impact logging and liana control, can increase as the contexts under which they deliver the most carbon aboveground carbon stocks, relative to standard forestry • benefits still need to be documented. Our work builds on practices [5, 8 , 10–12]. In planted forests, studies sug- these previous empirical studies and meta-analyses. It aims gest that extending rotations to a biological rather than to improve understanding of how prominent silvicultural economical optimum can substantially increase time-aver- practices impact aboveground carbon stocks in plantations aged carbon stocks [5, 12]. Nonetheless, forestry practi- globally. To do so, we systematically reviewed and compiled tioners commonly perform additional operations in planted aboveground carbon measurements from interplanted, ferti- stands such as fertilization and stand density management •• lized, and thinned tree plantations distributed across six con- that directly influence plantation carbon stocks [13 , tinents and 18 countries. Using this dataset, we then quan- •• 15 ]. Here, we consider the effects of two fertilization tified (i) how each treatment affected aboveground carbon practice-intercropping of nitrogen (N)-fixing species and stocks and (ii) how the effect size of each treatment varied application of inorganic nitrogen, phosphorus, potassium with different environmental and management factors. In (NPK) fertilizer-and one stand density management oper- doing so, we provide insights on how fertilization and thin- ation (thinning) on aboveground carbon stocks in forest ning treatments can be improved to promote carbon stocks plantations. in planted forests. While the use of NPK fertilizers remains the most com- •• mon fertilization practice [15 ], interplanting of N-fixing plants as an alternative method has become increasingly • Methods prominent in mixed species plantations [16, 17, 18 , 19]. In addition to providing N-fixing benefits, planting of mul- Literature Search and Data Collection tiple species may confer additional ecological benefits, such as habitat for biodiversity and improved soil fertility •• • We conducted this meta-analysis using a recently pub- [15 , 18 , 20, 21]. Furthermore, the use of conventional lished global database that compiles 4756 measurements inorganic NPK fertilizers has potentially adverse environ- of aboveground live tree carbon stocks in timber planta- mental impacts from improper and over-use, including tions, collected from 829 distinct sites across 278 studies declines in soil fertility, elevated groundwater and sur- •• [45 ]. We subset this dataset to the 654 measurements face water pollution, and increased GHG emissions from of aboveground live tree biomass in timber plantations fertilizer production and use [22, 23]. across 45 studies, 56 distinct sites, and 19 tree genera The empirical effects of these fertilization techniques that included relevant management details. Full details on aboveground carbon stocks in plantations has been well on the monoculture plantation database compilation and documented in multiple locations [24–29]. However, the data standardization processes are described in Bukoski magnitude of the effect has been rarely assessed at large •• et al. [45 ]. In addition, we included 97 measurements of scales [17, 30, 31] relevant for the design of forest-based aboveground live trees biomass collected from one large climate solutions, with prior efforts constrained to regional compilation of aboveground carbon stocks in planted 1 3 Current Forestry Reports •• forests [46 ] that was found at a later date. We elected Standardization of Treatment Eec ff t Using Natural to include it as it substantially increased our sample size. Log of the Response Ratio In total, we collected 751 measurements of aboveground live tree biomass across 68 studies (Table S1.1, S1.2, and For our meta-analysis, we used the natural log of the S1.3), 80 distinct sites, and 19 tree genera (Fig. 1). response ratio metric (lnRR) to standardize the effect of For each of the three dominant silvicultural treat- treatments on aboveground carbon across studies. Here, ments—intercropping of N-fixing species, application of lnRR reflects the change in aboveground carbon induced inorganic NPK fertilizer, and stand density management by each of our three treatments—interplanting of N-fixing (i.e., thinning), we selected studies from our monoculture species, application of NPK fertilizers, and thinning. We plantation database that had both (i) measurements of calculated lnRR using Eq. (1): aboveground biomass from plots in which the treatment Xt had been applied and (ii) measurements of aboveground lnRR = ln = lnXt − lnXc (1) biomass from control plots (without the treatment of inter- Xc est). Only studies with imposed, replicated treatments at In Eq. (1), Xt and Xc are mean aboveground carbon in the one or more sites were included in our dataset. Multiple paired treatment and control, respectively. A lnRR value of comparisons within a single study (e.g., comparing dif- 0 means the treatment did not induce any change in carbon ferent thinning intensities to a single unthinned control) compared to the control, while a positive value indicates were considered as distinct within-study observations, and the treatment had a positive effect on aboveground carbon, in that case, treatment values were compared to the same and a negative value indicates a decrease in aboveground control value. In total, there were 43 studies comprising carbon. To account for variation in sampling effort between 197 comparisons for intercropping of N-fixing plants, 17 studies, we weighted the effect sizes by the inverse of the studies comprising 164 comparisons for NPK fertiliza- sample variance for each response ratio. We calculated the tion, and 8 studies comprising 62 comparisons for thin- sample variance using the standard error and number of ning (the included studies are in Tables S1.1 & Table S1.2 replicates reported for each study [48]. When studies did & Table  S1.3). Within the intercropped plots, we con- not report the standard deviations associated with above- servatively accounted for aboveground biomass of only ground carbon measurements, we followed the methodol- the main tree crops to assess the fertilization effect of the ogy of Lajeunesse [49] and Koricheva [50]. This consisted interplanted N-fixing plants on the latter. Before running of imputing the missing standard deviations by calculating the analysis, we converted aboveground live tree biomass the median coefficient of variation (ratio of standard devia- measurements to aboveground carbon using a 0.47 default tion and mean) for each group (treatment or control) from conversion factor [47]. Fig. 1 Map of locations of all sites included in the meta-anal- ysis of intercropping N-fixing plants, NPK fertilization, and thinning effects on planted for - ests aboveground carbon stocks 1 3 Current Forestry Reports studies that reported both means and standard deviations. biological, and human factors, but was also limited by data We then multiplied the median coefficient of variation by availability (Table 1). Except soil moisture regime, all mod- the reported mean for either treatment or control groups for erator variables were recorded using information reported which standard deviations were missing. We performed a in the studies themselves. To obtain soil moisture regimes, sensitivity analysis to test for any potential effects of these we intersected the geographic coordinates of each site with assumptions on our results (see Supplementary Informa- a map of global soil moisture regimes developed by USDA- tion), which revealed that almost all results were robust to NRCS [55]. This map of global soil moisture regimes was this approach, but we note where results were sensitive to the built using data taken from more than 22,000 climatic sta- imputed standard deviations. Finally, we used funnel plots tions distributed around the world [55]. Soil moisture regime to confirm the absence of publication bias [51]. Response data were interpolated and rasterized on a 2-min grid cell ratios were calculated using the {metaphor} package [52] in [55]. Program R v.1.4.1103 [53]. Moderators were tested individually with separate mod- els, such that the influence of a moderator on the effect size Testing of Moderator Eec ff ts and Mixed‑Eec ff ts was determined using all available data for that particular Approach moderator variable. Prior to analysis, we dropped catego- ries of moderators for which we had only one observation. We inserted several moderators (variables that influence the To account for the non-independence of multiple within- strength and/or form of a relationship between a predictor study observations, we inserted publication-level random and a dependent variable [54]) into the mixed-effects model effects into each of our models [50]. We fitted all models to assess how the treatment effect size varied with other using restricted maximum likelihood estimation [50]. We factors hypothesized to influence aboveground carbon (e.g., determined the statistical significance of each moderator genus or soil moisture regime). Our selection of moderator variable using an omnibus test of all model coefficients variables included both categorical and continuous modera- (p-value < 0.05) [52]. Prior to reporting our final results, we lnRR tors and sought to account for an array of environmental, back-transformed (e ) the mean log response ratios and Table 1 Moderator variables inserted in the mixed-effects meta-analytic models for each treatment Treatments Moderator Moderator type Categories/units Interplanting N-fixing plants Previous land use Categorical Cropland, natural forest, plantation Tree genus Categorical Alnus, Anacardium, Casuarina, Eucalyptus, Hymeronima, Pachira, Pinus, Populus, Pseudotsuga Soil moisture regime Categorical Perudic, udic, ustic, xeric Intercropped plant genus Categorical Acacia, Albizia, Alnus, Dalbergia, Enterolobium, Hippophae, Leu- caena, Lupinus, Paraserianthes, Robinia, Salix Wood type Categorical Hardwood, softwood Experimental design Categorical Additive, replacement Stand age Continuous Years Mean annual precipitation Continuous mm/year Inorganic NPK fertilization Previous land use Categorical Cropland, fire, natural forest, plantation Tree genus Categorical Ailanthus, Eucalyptus, Macaranga, Picea, Pinus Soil moisture regime Categorical Perudic, udic, ustic, xeric Wood Type Categorical Hardwood, softwood Stand age Continuous Years Time since fertilization Continuous Years Application method Categorical Continuous, pulse Mean annual precipitation Continuous mm/year Thinning Previous land use Categorical Cropland, natural forest, plantation Soil moisture regime Categorical Perudic, udic, ustic, xeric Wood Type Categorical Hardwood, softwood Tree genus Categorical Acacia, Cunninghamia, Eucalyptus, Pinus Time since thinning Continuous Years Mean annual precipitation Continuous mm/year Basal area removed Continuous Percent 1 3 Current Forestry Reports their 95% confidence intervals and converted these values fertilizer emissions and examined how this effect varied with to percent change relative to the control. the amount of N fertilizer applied using a linear regression Depending on data availability, we also tested how the model. Finally, we estimated the median net carbon balance −1 effect size varied with either tree age or time since treatment of fertilized stands (expressed MgCOe ha ) across all stud- using linear mixed-effects models. In all the interplanting ies included for the NPK fertilization treatment. treatment studies, N-fixing crops were planted at the same time as the main tree crop, and we therefore used stand age as our time variable. To limit the potential effects of Results sparse data at older ages, we dropped measurements above 20 years old before performing this regression. In total, 2 Our results showed effects on aboveground carbon that were data points at 21 and 28 years old were dropped prior to this (i) strongly positive and statistically significant for N-fixing linear regression analysis. Of the inorganic fertilizer stud- species, (ii) strongly positive and statistically significant for ies, 9 of 18 continuously applied inorganic fertilizer over the use of NPK fertilizer, and (iii) negative and statistically the course of the study period and we used stand age as our significant for thinning. Further, our incorporation of mod- time variable. However, 10 of 18 inorganic fertilizer treat- erator variables provided additional nuance on how these ment studies reported “time since treatment” data. For these silvicultural practices impact aboveground carbon in plan- observations, we also used mixed-effects regression models tations. We provide additional details on each of the three to test how the effect size varied with time since treatment treatments below. in inorganically fertilized plots (Table 1). Limited data were available for older times since NPK fertilization. To limit Interplanting of N‑Fixing Species the potential effects of sparse data at older times since treat- ment, we dropped measurements above a time since treat- Overall, intercropping of N-fixing plants in monocul- ment of 5 years before running this regression. In total, two ture stands had a significantly positive effect (approxi - data points at 21 years old were dropped prior to this lin- mately + 20%) on aboveground carbon of the primary spe- ear regression analysis. For our thinning observations, we cies (Fig. 2) (p-value = 0.0031). All moderator variables assessed how the effect size varied with time since treatment were found to have significant effects on the relationship in thinned plots as well (Table 1). We used all the time since between interplanted N-fixing crops and aboveground thinning data to perform the regression analysis. Finally, we carbon (Fig.  2). We found that “soil moisture regime” evaluated whether annual mean precipitation had an influ- had a strong influence on the magnitude of the effect size ence on the effect size of all three treatments using the same (p < 0.001) (Fig.  2). Interplanting of plantations grow- regression model type (Table 1). ing on moister soils significantly increased aboveground carbon (+ 52% for perudic and + 42% for ustic soils), Greenhouse Gas Emissions from Inorganic Fertilizer but not in places with drier xeric soil moisture regimes Application (Table S5.1). Furthermore, N-fixing companion crops sig- nificantly increased the aboveground carbon stocks of the For each aboveground carbon measurement reported in NPK main tree crop when plantations occurred on former crop- fertilization studies, we calculated the business as usual lands (+ 67%, p < 0.0001) (Fig. 2), rather than locations (BAU) associated amount of nitrous oxide (N O) emitted with tree cover. The genus of the main tree crop also influ- (expressed in CO e) for each ton of synthetic nitrogen (N) enced the magnitude of the effect size (p < 0.001). Specifi- fertilizer applied, both in-field and upstream from fertilizer cally, establishing Eucalyptus trees with N-fixing compan- manufacturing itself [5]. We used an emissions factor of ion crops increased the latter aboveground carbon stocks 2.54% for N fertilizer (11.9 MgCO e per MgN applied) for by + 25%, whereas other genera did not show a significant in-field emissions and an upstream emissions factor of about effect. Additionally, the genus of the intercropped species 4 kgCO e per kgN produced [5]. We extracted tonnage of had a significant impact (p -value < 0.001) on the effect N fertilizer directly from the studies included in this meta- size as well (Fig. 2). Intercropping with Leucaena, Albizia, analysis (Table S1.2). When the amount of N fertilizer used Enterolobium, and Hippophae induced a 52%, 80%, 87%, was reported as kilograms per tree, we converted to kilo- and 113% increase in the aboveground carbon of the main grams per hectare using the reported stand density value. tree crop, respectively, whereas other intercropping gen- We then compared the additional aboveground carbon gain era did not have a significant effect (Fig.  2; Table S5.1). induced by NPK fertilization (i.e., the difference in above- The type of wood of the main tree species also moderated ground carbon between the treated and control plots) with the effect of the treatment (p -value = 0.0036). Hardwoods emissions from N fertilizers use and manufacturing. We reacted positively to interplanted N-fixing crops (+ 25% in calculated the difference between additional carbon and N aboveground carbon stocks) (Table S5.1). Finally, the type 1 3 Current Forestry Reports Fig. 2 Meta-analysis results of the change in aboveground carbon of plantation trees in response to the interplanting of N-fixing plants. Error bars represent the 95% confidence intervals. Omnibus tests of significance for moderator variables are shown on the right side (NS means “not sig- nificant”). Results for the “Tree genus” moderator are only pro- vided for genera with more than 5 observations (see Table S5.1). Results for the “Intercropped Genus” moderator are only provided for genera with significant effects or for genera with more than 5 observations (see Table S5.1). The number of observations in each category is shown in parenthesis of experimental design influenced the magnitude of effect Inorganic NPK Fertilization size (p-value < 0.001) (Fig.  2). The use of interplanted N-fixing crops appeared to be more beneficial when an We found that inorganic NPK fertilizers signifi - additive design was adopted (+ 72%) than a replacement cantly increased aboveground carbon by 44.5% over- one (+ 16%) (Table S5.1). all (p-value < 0.001) (Fig.  4). Of the individual studies However, our results also suggest that the effect of inter - included in the meta-analysis, most reported a significant cropping N-fixing species on aboveground carbon varied positive effect of NPK fertilization on aboveground car - with stand age. By regressing ln(RR) on stand age, we iden- bon, whereas six studies found a non-significant effect. tified a significant positive association with the treatment Similarly to interplanting of N-fixing species, all mod- effect size (p -value < 0.0001) (Fig. 3). For every additional erator variables were found to have significant effects on year that a stand was allowed to grow, the influence of the relationship between NPK fertilizer and aboveground intercropping N-fixing species on plantation aboveground carbon. carbon was increased by 3.6% (Fig. 3). Intercropping ini- We found that “tree genus” significantly influenced tially decreased aboveground carbon in the main tree crop, the change in aboveground carbon attributed to inor- although not significantly, but its effect became positive ganic NPK fertilization (Fig.  4) (p-value < 0.001). The when the stand was 3 years old and steadily increased over aboveground carbon of treated Pinus and Eucalyp- time from then on t (+ 84% at 20 years old) (Fig. 3). tus plantations were 51% (p-value < 0.001) and 37% Finally, we did not identify a significant effect of mean (p-value < 0.001) higher in fertilized plots compared to annual precipitation, the other continuous variable in our the controls, respectively (Fig. 4; Table S5.2). In addition, model, on the magnitude of the effect size (p -value = 0.0872; both hardwoods and softwoods responded positively to Table S5.1). the treatment; however, NPK fertilization had a slightly 1 3 Current Forestry Reports Fig. 3 Change in the effect size of interplanting N-fixing plants as a function of stand age. The significance of the regression is indicated by the p-value in the upper right as well as the inter- cept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval greater effect on aboveground carbon in hardwood (+ 46%) treatment, the effect size of inorganic fertilization on carbon (p-value < 0.001) relative to softwood plantations (+ 43%) stocks decreased by 6.66% (Fig. 5). (p-value < 0.001) (Fig. 4; Table S5.2). Mean annual precipitation also had a significant nega- Soil moisture regime also had a strong influence on the tive effect (p -value < 0.001) on the magnitude of the change magnitude of the effect size (p -value < 0.001) (Fig. 4). NPK in aboveground carbon (Figure S3.1). For every additional fertilization of plantations growing on moister soils signifi- millimeter of precipitation received per year, the treatment cantly increased aboveground carbon (+ 82% for perudic effect size was reduced by 0.03% (Figure S3.1). and + 42% for udic soils), but not in places with xeric and Once we accounted for in-field and upstream fertilizer ustic soil moisture regimes (Table S5.2). It is worth not- emissions, we found a negative median net carbon bal- −1 ing that the influence on the effect size of the perudic soil ance of fertilized stands across studies (− 2 MgCO e.ha ). moisture regime might be overestimated as only one study In other words, fertilized stands were net emitters across reported measurements for that soil moisture regime (Fig. 4). studies (Figure S3.2), and the climate mitigation benefit of Furthermore, the methodology used to apply NPK fertiliz- fertilizer declined with the amount of N fertilizer applied ers significantly influenced the magnitude of the fertilizing (p-value < 0.001). The climate benefit became null at 0.55 effect (p -value < 0.001) (Fig. 4). The effect size appeared MgN or greater applied per hectare (Fig. 6). higher when fertilizers are applied episodically (+ 53%) rather than continuously (+ 36%) (Table S5.2). The type Thinning of previous land use also strongly influenced the treatment effect size and explained part of the heterogeneity in effect Across the different studies, thinning decreased stand- size across studies (Fig. 4) (p-value < 0.001). Specifically, ing aboveground carbon by about 34% (p-value < 0.001) the use of inorganic fertilizers appeared to be more benefi- (Fig. 7). We found soil moisture regime to have a signifi- cial on lands that were previously plantations (+ 44%) or cant impact on the percent change in carbon from thinning natural forests (+ 76%), than those cleared by fire or previ- (p-value < 0.001) (Fig. 7). When thinning was conducted ously used for croplands (Table S5.2). on xeric soils, carbon levels decreased significantly by 59% We found a negative, although not significant (p-value < 0.001) (Table  S5.3). We found type of previous (p-value = 0.5), association between stand age and the mag- land use to also have a signic fi ant inu fl ence on the magnitude nitude of the effect size in fertilized stands (Table S5.2). of the effect size (p -value < 0.001) (Fig. 7). Stands grown However, when we used time since treatment rather than on former agricultural lands had aboveground carbon lev- stand age, we found that the benefit of inorganic fertilizers els that were 55% lower (p-value < 0.001) in thinned stands on aboveground carbon significantly decline through time compared to controls (Table  S5.3). When thinning was (p-value = 0.008) (Fig. 5). For every additional year since the executed in stands that were established on former natural 1 3 Current Forestry Reports Fig. 4 Meta-analysis results of the change in aboveground carbon moderator variables are shown on the right side (NS means “not sig- of plantation trees in response to NPK fertilization error bars repre- nificant”). The number of observations in each category is shown in sent the 95% confidence intervals. Omnibus tests of significance for parenthesis forestlands, the mean decrease in aboveground carbon was Genus of tree did not significantly influence the magnitude 44% (p-value = 0.0026) (Table S5.3). Finally, we also found of the effect size. that the type of wood influenced the magnitude of the effect When we inserted the “time since treatment” continu- size (p-value < 0.001) (Fig. 7). Both hardwood and softwood ous moderator variable in the model, we found that as time stands experienced large reductions in aboveground carbon since thinning increased, the negative effect of thinning on due to thinning operations, which averaged a − 33% change carbon lessened (p-value < 0.001) (Fig. 8). For every addi- in aboveground carbon from thinning for both wood types. tional year since the treatment, the effect size of thinning on 1 3 Current Forestry Reports Fig. 5 Change in the fertilizing effect size of NPK fertilizers as a function of time since treatment. The significance of the regression is indicated by the p-value in the upper right as well as the intercept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval Fig. 6 Change in the net carbon balance of fertilized stands as a function of the amount of N applied. The significance of the regression is indicated by the p-value in the upper right as well as the intercept and slope values, and the coefficient of determination. The area shaded in blue around the regression lines indicate the 95% confi- dence interval carbon stocks decreased by 4.6% (Fig. 8), suggesting that (p < 0.001) (Table S5.3). For every additional percent of basal after 14 years the thinning effect disappears. area removed, the effect of thinning on carbon stocks was Mean annual precipitation also had a significant negative increased by 1.3% (Table S5.3). effect (p -value = 0.04) on the magnitude of the change in aboveground carbon (Figure S3.3). For every additional mil- limeter of precipitation received per year, the effect of thin- ning on carbon stocks was reduced by 0.03% (Figure S3.3). Finally, we found that as the percent of basal area removed increased, the negative effect size of thinning increased 1 3 Current Forestry Reports Fig. 7 Meta-analysis results of the change in aboveground carbon of plantations from thinning. Error bars represent the 95% confidence intervals. Omnibus tests of significance for moderator variables are shown on the right side (NS means “not significant”). The number of observations in each category is shown in parenthesis Fig. 8 Change in the effect size of thinning operations as a func- tion of time since thinning. The significance of the regression is indicated by the p-value in the lower right as well as the inter- cept and slope values with their corresponding 95% confidence interval. The area shaded in blue around the regression lines indicate the 95% confidence interval 1 3 Current Forestry Reports •• may emerge soon after their establishment [56, 65 ]. Discussion Conversely, when the N-fixing species is established a few years after the main crop, the former tends to compete with Enhanced aboveground carbon sequestration in plantations the latter to a much lesser degree for light and water [56, via improved stand management is a prominent strategy •• 65 , 66, 67]. As a result, complementarity between the for mitigating climate change. Our study quantified the crops increases and the main crop can allocate a greater effect size of three dominant silvicultural practices—inter - proportion of aboveground growth using the additional N cropping N-fixing species, fertilization with inorganic •• provided by the companion crop [65 ]. NPK, and stand density management via thinning—on Our analysis also showed that several environmental, aboveground carbon stocks in plantations. We found that biological, and human factors influence the magnitude of the magnitude of the effect size often depended on context response of the major crops to interplanting of N-fixing spe- and below we discuss the dominant drivers of variation cies. The main tree crop benefited more from the treatment and implications for landowners and land managers. when it was performed on wetter soils. Soil moisture signifi- cantly affects N-fixation by controlling nodulation and nitro- genase activity [68]. Moist soils promote nodulation and Interplanting of N‑Fixing Plants nitrogenase activity while drier soils reduce the number of nodules produced and inhibit nitrogenase activity, resulting We found that only some intercropping N-fixing species in very low rates of nitrogen fixation [68]. Wetter soils also may be beneficial (in our case Albizia, Enterolobium, Hip- promote the growth of companion crops themselves, which pophae, and Leucaena species) and/or primarily when the allows for more N-fixation and thus increases the amount of crop trees are mature. The species-specific effects may available nitrogen in the soil [68]. be mediated by compatibility of growth patterns [56, 57]. Furthermore, interplanting N-fixing crops was most bene- Growth compatibility can occur through at least three ficial when it was performed in plantations growing on lands mechanisms: reduced competition for light via canopy that previously hosted agricultural crops, compared to those stratification and crown complementarity, reduced com- previously under tree cover. Agricultural lands often expe- petition for nutrients and water via root stratification, and rience soil impoverishment, especially when intense crop- direct or indirect growth facilitation [16, 58, 59]. All three ping techniques are used [69]. Low nutrient concentration, mechanisms may explain our results. We found that inter- especially for N, appears to favor complementarity between cropping with shade tolerant or moderately shade toler- •• the main tree crop and its N-fixing companion crop [65 ]. ant species (e.g., Leucaena spp., Enterolobium spp.) had Nonetheless, since soil impoverishment also depends on the greater effects on aboveground carbon relative to shade intensity of the land use, which was not captured here, this intolerant species (e.g., Alnus spp.). Root stratification result should be interpreted with caution. couples trees with deep taproots (e.g., Eucalyptus spp.) that can acquire nutrients and water from lower soil hori- Inorganic NPK Fertilization zons with species with shallow extensive horizontal root systems (e.g., Acacia spp.) [60]. Interplanting N-fixing The use of NPK fertilizers significantly increased the above- plants may also confer benefits through chemical root ground carbon stocks of plantations, which aligns with the stratification, which may facilitate mycorrhizal relation- findings of others [70, 71]. However, once we accounted ships between intercropped species and the main tree for the CO e emissions resulting from inorganic N fertilizer crops [16]. Furthermore, although we did not directly test manufacturing and use, we found that higher levels of ferti- these interactions, interplanting N-fixing legumes facili- lizer application could negate and/or overwhelm the increase tate nutrient access for plantation trees by decreasing soil in aboveground carbon stocks induced by fertilization. This pH which promotes soil weathering and therefore general • highlights that the climate mitigation potential of fertilized nutrient availability [61 ]. Lastly, companion crops may stands could be substantially overestimated if fertilizer emis- inhibit the growth of competitors via allelopathic effects sions are not taken into consideration. This also emphasizes and/or eliminate local pathogens [58, 62]. that inorganic fertilizers have a greenhouse gas cost that Our results also showed that interplanting N-fixing can exceed their carbon benefit, particularly when they are plants become more beneficial through time. Early growth applied in large amounts [72]. To maximize the climate ben- incompatibility and poor establishment of nitrogen-fixing efits of using synthetic fertilizers in planted forests, a holis- companion crops could explain the negative or absent tic accounting of greenhouse gas emissions associated with effect of intercropping early in the rotation [63, 64]. When producing and applying fertilizers is therefore needed [73]. crops are planted simultaneously, intense competition We found that the benefit of NPK fertilization decreased between them for resources such as light water or nutrients with time. This phenomenon has been noticed in previous 1 3 Current Forestry Reports studies and reviews [14, 74, 75] and several factors are and Saarsalmi and Mälkönen [85] found similar results and believed to explain this trend. During the initial establish- attributed it to higher nutrient-use efficiency under pulse ment phase, young seedlings generally do not have well fertilization. Furthermore, several studies have stressed that developed root systems and providing easily accessible inor- continuous application of N favors faster N saturation, which ganic forms of nutrients can increase growth [18 , 74]. Over reduces the positive effects of inorganic N-based fertilizers time, unfertilized seedlings develop their root systems and on biomass growth in the long-term [86]. Since N saturation gain access to larger nutrient pools, which could explain the is also believed to promote nitrate leaching from soils [87], decrease in the magnitude of the effect size observed here increase emissions of N O [84], and induce large reduction [32, 74]. Furthermore, plantations trees increasingly rely on in mycorrhizal symbionts [88], our results underline that organic forms of N like glycine relative to inorganic forms careful handling of fertilizer operations is needed to maxi- (e.g., NH & NH ) as they age [76], which could explain mize tree growth while minimizing negative environmental 3 4 the lower effect size of fertilizers later in the rotation. Such side effects. Nonetheless, this difference in growth responses a phenomenon might also apply in the context of the inter- to pulsed and continuous fertilization might also be caused planting treatment discussed before. The effect of fertiliz- by other chemical or physical soil variables (e.g., initial soil ers on biomass carbon growth has also been suggested to fertility, soil pH) which were not captured in this analysis. decrease over time due to increased allocation of resources towards reproduction, which leads to lower nutrient uptake Thinning rates [77]. Our results showed that the benefit of NPK fertilization Our results showed that thinning operations significantly on growth declined as precipitation increased. High precipi- lowered aboveground carbon in plantations, which aligns tation levels increase soil water content and, in some cases, with the findings of others [ 40, 89]. Given that thinning is push it beyond field capacity (i.e., soil becomes saturated) performed to reallocate resources and growing space to tar- [78]. Saturated soils tend to be highly prone to leaching of get trees of primary value and induces a trade-off in loss very mobile nutrients such as N, which reduces their avail- of stand-level carbon, this result was expected. Indeed, •• ability for plant uptake [15 , 78]. Our results also suggest increased growth rates for individual trees post-thinning that plantations growing on soils with udic moisture regimes have been well documented [44, 90, 91]. We also identified experienced the largest increases in aboveground carbon an attenuation of the effect on overall stand aboveground when fertilized. Nutrients are generally more available when carbon with increasing time since thinning (Fig.  8). Our soil moisture levels are optimum and remain adequate for results suggest that at the stand level, the negative effect of most of the year, as is the case for soils with an udic soil thinning on aboveground carbon lasts for approximately a moisture regime [79]. decade before becoming attenuated. Conversely to interplanting, the benefit of NPK fertilizers Furthermore, we found that the negative effect of thinning was higher when they were applied in plantations growing on aboveground carbon might be amplified when performed on lands that previously hosted tree plantations or natural in stands located in dry areas and growing on xeric soils. forests. Harvested lands often experience soil impoverish- Non-thinned trees growing in stands on mesic and wetter ment, especially when the whole tree harvesting method is soils tend have access to more resources such as water and employed [80, 81]. Indeed, harvest promotes nutrient min- nutrients after treatment than the same trees growing on eralization and nitrification, making them more mobile and xeric soils, which could therefore induce a greater growth therefore more subject to leaching [82]. The removal of trees response to thinning operations and a weaker treatment themselves also leads to a decrease in soil nutrient capital effect in the former scenario than in the latter. and can curb the growth of the next trees [83]. Fertilization Thinning operations appear to have a more negative is often used to compensate for the loss of nutrients [83], and effect on stand carbon when they are performed on former poor soil post-harvest may explain the fertilizer benefit we agricultural lands. Nutrient and water availability are gen- observed. However, since this analysis did not account for erally lower in those areas, and fertilization and irrigation the effect of land use intensity on soil impoverishment, this are often needed to ensure afforestation/reforestation suc- result should be interpretated with caution. Overall, these cess [92]. As a result, thinned stands growing on former findings related to site conditions underline the fact that agricultural lands might have access to smaller nutrient response of trees to fertilization is heavily site dependent and water pools that constrain post-thinning growth and and varies with site productivity and quality, and/or intensity therefore induce a larger thinning effect on stand-level of prior land use. aboveground carbon. Furthermore, former agricultural This study also revealed that plantation stands store more lands tend to be more compacted due to heavy machin- aboveground carbon when fertilizer applications are per- ery use [93], which could restrict the common increase in formed in pulses, rather than continuously. Aber et al. [84] resource availability after thinning operations and further 1 3 Current Forestry Reports accentuate the effect on aboveground carbon [ 43]. Overall, Management Implications these results stress that response of stands to thinning is heavily site dependent. Stands on more productive sites This study reveals that the use of N-fixing companion crops will often respond quite differently to the same thinning as a fertilization technique is challenging and primarily treatment than a stand on lower quality sites [94]. depends heavily on growth complementarity between the crop and intercropped species. Knowledge of the silvics of individual species is a critical precursor to ensure success- Caveats and Potential Limitations ful growth benefits, and poorly implemented systems may induce null effects on growth. However, use of intercrop- Our study focuses on aboveground live tree carbon stocks ping as the main fertilization technique can also reduce N O only and did not examine changes in belowground carbon emissions and nutrient volatilization, which are common and soil organic carbon in response to fertilization and issues associated with the application of inorganic NPK fer- thinning. Belowground carbon and soils are substantial tilizers [101]. Factors such as prior land use, soil moisture carbon pools in forest ecosystems. For example, soil car- regime, and method of implementation are also likely to be bon stocks are believed to represent 90% and 50% of the key moderators of the effect of interplanted N-fixing plants total carbon stock in boreal and tropical forests respec- on aboveground carbon. Knowledge of the site characteris- tively [95]. Furthermore, in these forest types, roots are tics and history, as well as interactions between tree species believed to store between 23 and 27% of total tree biomass seem therefore crucial to ensure successful associations. carbon [96]. However, we omitted these pools due to little Inorganic fertilizers, despite being growth catalyzers, can data availability and inconsistent field sampling methods have detrimental effects on the environment which includes across reviewed studies. This decision was further justified water eutrophication via nutrient leaching and runoffs, and by the fact that changes in soil carbon stocks tend to be air pollution via greenhouse gas (GHG) emissions from small relative to changes in the aboveground carbon pool fertilizers manufacturing and nutrient volatilization. Thus, and it takes longer to detect them [97]. Previous studies there are benefits to minimizing additions. Our results show have found positive effects of NPK fertilization and inter - that NPK fertilizers are not universally beneficial and their planting N-fixing plants on soil carbon stocks in forest positive effects, when observed, decline through time. Fur - plantations [86, 98, 99]. Other studies have found neutral thermore, we found that GHG emissions resulting from or negative effects of thinning on soil carbon stocks [ 35, N fertilizer manufacturing and in-field application could 36, 40, 89, 100] and negative effects on belowground car - exceed the aboveground carbon gains induced by their use. bon as well [100]. If these results occur in other forest Our results also stress that factors such as prior land use and plantation management studies, focusing solely on car- soil moisture regime may be key moderators of the effect of bon in aboveground biomass tissues may underestimate fertilizers on aboveground carbon. Our findings emphasize the impact of forest management actions such as fertiliza- therefore the need to apply the right rate of nutrients at the tion on total ecosystem carbon stocks and overestimate it right times and in the right context. Landowners should care- in the context of thinning. Additional empirical data on fully handle fertilization operations to maximize their carbon the impact of these management operations on plantations benefits and minimize their costs. total carbon stocks as well as studies reporting responses Next, our study suggests that the initial decrease in of the different carbon pools are needed. aboveground carbon caused by thinning operations could Data limitations also partly restricted our ability to be compensated over time by the recovery of carbon post- assess the exact timing at which the positive effects of thinning, particularly when coupled with extended rotation inorganic fertilizers on aboveground stand carbon stocks lengths. Indeed, by increasing rotation lengths, landowners disappear. Although the data stopped at 5 years since treat- can further attenuate the effects of thinning on stand carbon ment, if the decline through time continues following the up to a point at which the initial loss of carbon is offset by same path, then we would predict the carbon benefits of the recovery of a productive and well-stocked stand [40, inorganic fertilizers to disappear eight years after treat- 42, 102]. From there, the individual trees in thinned stands ment. Further data on the impact of time on the effect could sequester additional carbon to levels as high or higher size of fertilizers would therefore be valuable to refine our than unthinned stands of the same age [42, 102]. Criti- predictions. Moreover, data were heavily skewed towards cally, the likelihood of recovering unthinned carbon levels younger ages across all of our focal silvicultural treat- post-thinning will depend on the intensity of thinning and ments. Long-term measurements of the effects of silvi- thinning technique employed, with more intensive thinning cultural treatments on plantation forest carbon stocks will practices tending to result in longer or unachievable recov- •• greatly improve our understanding of the climate mitiga- ery of aboveground carbon [15 , 42, 91, 102]. For exam- tion potential of these systems. ple, plots thinned from below may rapidly recover carbon 1 3 Current Forestry Reports Acknowledgements We thank Timothy Bowles and the Potts Group sequestration rates equivalent to control plots, while stands at U.C. Berkeley for valuable feedback on early drafts of this manu- that experienced dominant and crown thinning operations script. Although their comments have greatly improved our study, the may have longer periods to carbon recovery [42]. However, ideas and opinions expressed in our study do not necessarily reflect thinning can also confer important forest structural qualities their own. important for management of forest carbon beyond maximi- Funding We thank the Bezos Earth Fund for supporting C. H. M.’s •• zation of standing aboveground carbon stocks [13 ], such time on this work. Grants from the following institutions to the Nature as conferring resistance to low or medium-intensity wild- Conservancy partially funded S. C. C.-P.’s time on this work: the Chil- fire in fire-prone landscapes and insect/pathogen outbreaks dren’s Investment Fund Foundation, COmON Foundation, the Craig •• and Susan McCaw Foundation, the Doris Duke Charitable Foundation, [13 , 103]. Further research on these co-benefits and the Good Energies Foundation, and the Bezos Earth Fund. coupling of thinning with extended rotations is needed to validate the use of thinning as a forest carbon management Data Availability Data used for the study are published on tool. Zenodo (https:// doi. org/ 10. 5281/ zenodo. 77898 68) Lastly, this meta-analysis underlines the need for for- Declarations estry practitioners to be aware of site conditions, quality, and land use history before conducting thinning operations Conflict of Interest M. D. P. is the Chief Science Officer for Carbon because their effects can vary from site to site, and they tend Direct Inc., a company combining science, technology, and capital to to strongly depend on local conditions. Additional studies deliver quality CO management at scale. M. D. P. is a shareholder in the company and thus stands to benefit financially from forest man- on the influence of soil moisture regimes and previous land agement targeted at climate change mitigation. The remaining authors use on the magnitude of the effect size of thinning on stand declare no competing interests. aboveground carbon are needed to consolidate our results. Overall, our study underscores that selecting the appropri- Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any ate species and treatments for each site is crucial to ensure of the authors. an effective carbon management plan in forest plantations. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long Conclusion as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are Improved forest management has been highlighted as a key included in the article's Creative Commons licence, unless indicated natural climate solution because of its ability to deliver otherwise in a credit line to the material. If material is not included in • • climate benefits within short-time scales [1, 5, 7 , 8 ]. the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will Nonetheless, there is still substantial uncertainty regarding need to obtain permission directly from the copyright holder. To view a the forest practices that would help realize this mitigation copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . potential and the context in which they would deliver the most climate benefits [1 ]. Our study provides additional considerations that help facilitate the use of improved man- References agement practices to increase plantations carbon stocks and hence mitigate climate change. By specifying the conditions Papers of particular interest, published recently, have in which fertilization and stand density management tend been highlighted as: to be the most beneficial for carbon storage purposes, this • Of importance study provides additional information to forest practitioners •• Of major importance on how to use them as carbon management tools. Although not all management actions studied here provide substan- 1. IPCC. Climate Change 2022: Mitigation of Climate Change. tial increases in carbon storage over the entire lifetime of Contribution of Working Group III to the Sixth Assessment plantation trees (e.g., interplanting N-fixing trees), they still Report of the Intergovernmental Panel on Climate Change. Cam- bridge University Press, Cambridge, UK and New York. 2022. may be desirable by enhancing monocultures biodiversity 2. Cook-Patton SC, Leavitt SM, Gibbs D, et al. 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Scand J For Res. 2001;16:514–35. 1 3 Current Forestry Reports 106.• Osuri AM, Gopal A, Raman TRS, DeFries R, Cook-Patton SC, the need to diversify current monoculture tree plantations Naeem S. Greater stability of carbon capture in species-rich to make them more resilient and resistant to an uncertain natural forests compared to species-poor plantations. Environ future.) Res Lett. 2020;15:034011. (This study provides evidence of greater stability of carbon stocks in species-rich forests Publisher's Note Springer Nature remains neutral with regard to than in species-poor tree plantations. This study highlights jurisdictional claims in published maps and institutional affiliations. 1 3

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Current Forestry ReportsSpringer Journals

Published: Jun 1, 2023

Keywords: Natural climate solutions; Improved forest management; Carbon; Fertilization; Thinning; Forest plantation

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