Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account

Peter D. McIntosh; James L. Hardcastle; Tobias Klöffel; Martin Moroni; Talitha C. Santini

doi:10.1155/2020/6509659

Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account

McIntosh, Peter D.;Hardcastle, James L.;Klöffel, Tobias;Moroni, Martin;Santini, Talitha C. 2020-07-30 00:00:00 Hindawi International Journal of Forestry Research Volume 2020, Article ID 6509659, 16 pages https://doi.org/10.1155/2020/6509659 Research Article Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account 1 2 3 4 Peter D. McIntosh , James L. Hardcastle, Tobias Klo ¨ﬀel, Martin Moroni, and Talitha C. Santini Forest Practices Authority, 30 Patrick Street, Hobart, TAS 7000, Australia School of Earth and Environmental Sciences, University of Queensland, Brisbane, QLD 4072, Australia Research Department of Ecology and Ecosystem Management, Technical University of Munich, Freising, Germany Private Forests Tasmania, 30 Patrick Street, Hobart, TAS 7000, Australia UWA School of Agriculture and Environment, University of Western Australia, Crawley, WA 6009, Australia Correspondence should be addressed to Peter D. McIntosh; peter.mcintosh@fpa.tas.gov.au Received 11 November 2019; Revised 25 February 2020; Accepted 28 March 2020; Published 30 July 2020 Academic Editor: Kurt Johnsen Copyright © 2020 Peter D. McIntosh et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Small areas of the wetter parts of southeast Australia including Tasmania support high-biomass “wet” eucalypt forests, including “mixed” forests consisting of mature eucalypts up to 100 m high with a rainforest understorey. In Tasmania, mixed forests transition to lower biomass rainforests over time. In the scientiﬁc and public debate on ways to mitigate climate change, these forests have received attention for their ability to store large amounts of carbon (C), but the contribution of soil C stocks to the total C in these two ecosystems has not been systematically researched, and consequently, the potential of wet eucalypt forests to serve as long-term C sinks is uncertain. ,is study compared soil C stocks to 1 m depth at paired sites under rainforest and mixed forests and found that there was no detectable diﬀerence of mean total soil C between the two forest types, and on average, both −1 contained about 200 Mg·ha of C. Some C in subsoil under rainforests is 3000 years old and retains a chemical signature of pyrogenic C, detectable in NMR spectra, indicating that soil C stocks are buﬀered against the eﬀects of forest succession. ,e mean −1 loss of C in biomass as mixed forests transition to rainforests is estimated to be about 260 Mg·ha over a c. 400-year period, so the −1 −1 mature mixed forest ecosystem emits about 0.65 Mg·ha ·yr of C during its transition to rainforest. For this reason and because of the risk of forest ﬁres, setting aside large areas of wet eucalypt forests as reserves in order to increase landscape C storage is not a sound strategy for long-term climate change mitigation. Maintaining a mosaic of managed native forests, including regenerating eucalypts, mixed forests, rainforests, and reserves, is likely to be the best strategy for maintaining landscape C stocks. −1 soils and biomass is about equal (120 Mg·ha ) [5]. Aus- 1. Introduction tralian forests as a whole contain the lowest forest biomass −1 Natural forest ecosystems contain about 1500 Pg of carbon and soil C for midlatitudes (mean C of 45 Mg·ha for forests −1 (C) [1], and on a worldwide basis, forest soils contain about and 83 Mg·ha for soils to 1 m depth) [2] because of climatic twice the C of the vegetation they support [2]. ,e pro- limitations and the relative infertility of large areas of forest- portions of C in vegetation and soil varies greatly depending supporting soils. However, the natural forests of Australia’s on latitude, altitude, vegetation, climate, and soil [2–4], but wet southeast include the tallest hardwoods in the world (up in boreal and temperate forests, the average C content of to 100 m high) (Figure 1), and according to some authors 2 International Journal of Forestry Research (a) (b) Figure 1: Iconic Tasmanian wet eucalypt forests. ,e tree on the left (a) is 100 m tall. ,e right-hand side image (b) shows mixed forest consisting of mature tall eucalypts (Eucalyptus regnans) and a dense understorey of rainforest species which are about half as tall as the dominant eucalypts. (e.g., Keith et al. [6]), the aboveground C content of these In the northern hemisphere, it has been found that −1 ecosystems is over 2500 Mg·ha , which is over 50 times the temperate old-growth forests continue to accumulate C as −1 mean Australian forest aboveground value (45 Mg·ha ) and they mature [12–14] and protection of old-growth temperate also greatly exceeds mean values for other midlatitude forests has been promoted as a C sequestration policy. A −1 forests (range 32–114 Mg·ha ; [2]), the IPCC ﬁgure similar policy has been argued for wet eucalypt forests of −1 southeast Australia, in order to moderate climate change (96 Mg·ha ; [8]) for biomass C in world temperate forests, and the mean aboveground C for mountain ash (Eucalyptus induced by the eﬀects of increased concentrations of CO in −1 regnans) forests in Victoria (246 Mg·ha ; [7]). However, the atmosphere. For example, Mackey et al. ([15], p. 39), in Sillett et al. [9] found that aboveground biomass C in forests reference to eucalypt forests, proposed that “the remaining similar to those described by Keith et al. [6] was at most intact natural forests constitute a signiﬁcant standing stock −1 −1 706 Mg·ha , but probably lower (438 Mg·ha ) due to the of C that should be protected from carbon-emitting land-use loss of mass in decayed hollow trunks and limbs, and activities.” However, although C stocks will undoubtedly questioned the assertion that the tallest southeast Australian increase as young trees mature, such a policy ignores the fact eucalypt forests are the most C-dense forests in the world, that old-growth eucalypt forests in Tasmania are not the end quoting measurements of North American redwood (Se- point of forest succession [11] and that all eucalypt forests quoia sempervirens) forests which contain more than are highly susceptible to ﬁre. −1 Provided there is no stand-destroying ﬁre and there is a 2000 Mg·ha of biomass C. A typical value for biomass C (including roots) for seed source nearby (for example, in a rainforest-dominated Tasmanian mature eucalypt forests >55 m high, with >40% gully, on a shady slope, or in a nearby unburned patch of crown cover (Class 1 forests in the Tasmanian classiﬁcation), forest), wet eucalypt forests will transition to rainforests, a was obtained by Moroni et al. [10]: these contain process described in the classic paper by Gilbert [16]. After −1 470 Mg·ha of C. If all forests supporting all mature trees ﬁre, a eucalypt forest will normally establish rapidly. By the >55 m are included (i.e., all Class 1 and 2 forests), the ﬁgure time the eucalypt forests are about 100 years old, shade- −1 −1 is lower (387 Mg·ha ) which is close to the 378 Mg·ha tolerant rainforest species may already be established in the ﬁgure obtained by Moroni et al. [11] in a 14-site paired understorey. As the eucalypts reach maturity, the rainforest comparison of aboveground biomass (trees and coarse will form a continuous understorey and a “mixed” tall eu- woody debris, but not litter) under mixed forests and calypt/rainforest results ([16], Figure 1.10). ,e eucalypts cannot regenerate under the rainforest canopy; hence, once rainforest but four times the IPCC ﬁgure for biomass C in −1 world temperate forests (96 Mg·ha [8]). ,e mean ﬁgure the eucalypts reach their maximum age of 300–500 years and for C in all mature Tasmanian “wet” eucalypt forests (those progressively die, they are replaced by the rainforest with a dense understorey of shrubs and/or rainforest trees) is understorey, although eucalypt woody debris may persist for −1 232 Mg·ha [10]. up to two centuries on the forest ﬂoor. Peak aboveground International Journal of Forestry Research 3 wood volumes occur when the eucalypts are tallest and types by Mackey et al. ([15], p. 28), Dean and Wardell- contain an understorey of rainforest species (Figure 1). Johnson [27], and May et al. [28]) did not take into account Rainforests, on average, contain about 45% of the above- published data and exceeded the ﬁgures summarised in ground wood volume and biomass C of mixed forest [11]. To Table 1 by factors of up to 3.5 [29]. halt the progression to rainforests and maintain eucalypt Fedrigo et al. [30] were the ﬁrst to compare measured cover, ﬁre must return before the eucalypt overstorey (the soil C in nearby rainforest, mixed rainforest-wet scle- eucalypt seed source) dies [16, 17]. rophyll stands (“ecotone forest”), and wet sclerophyll forest When considering C trends in Tasmanian forests, an (“eucalypt forest”) in the Yarra Ranges of Victoria, but only important question to ask is whether the lower biomass C in to 30 cm depth. ,ey found no signiﬁcant diﬀerence of soil −1 rainforest than in mixed mature eucalypt forest also indi- C between these forest types: values were 163 Mg·ha −1 cates lower C in the total ecosystem under rainforest. ,e under wet sclerophyll forest and 149 Mg·ha under answer to this question depends on soil C content, does it rainforest. Unfortunately, no information was provided on change with vegetation cover or does it remain the same? whether soils were closely matched across diﬀerent vege- ,is unanswered question prompted Norris et al. [18] to tation plots. Consequently, eﬀects of vegetation or ﬁre on comment that “the uncertainty around soil carbon repre- soil C storage may have been conﬂated with those resulting sents a priority for further work.” One might assume that as from inherent soil diﬀerences such as depth, drainage, and mixed forests contain about twice the biomass of rainforests parent material. [11], they should return more biomass to the forest ﬂoor As the information summarised in Table 1 and in the than rainforests and thus accumulate more soil C than soils literature is insuﬃcient to answer the question of whether under rainforest. Alternatively, one could argue that fre- soil C increases, decreases, or remains the same as wet quent ﬁres in eucalypt forests deplete soil C stocks, negating eucalypt forests transition to rainforest over time, a study any soil C increase caused by greater cycling of C to the was undertaken to measure soil C in paired sites under forest ﬂoor by the maturing eucalypts of greater biomass. A mixed forest and rainforest in Tasmania. ,is paper complicating issue can be the accumulation of pyrogenic C presents the new soil C data obtained in this study. It also in soils under ﬁre-susceptible forests [19, 20]. discusses the ecological processes aﬀecting total ecosys- Previous work on soil C under eucalypts and rainforest tem C and assesses the value of wet eucalypt forests in in Tasmania has been limited to soil proﬁle analysis un- Tasmania for mitigating the eﬀects of CO emissions on dertaken in soil surveys covering a range of soils and parent climate. material and forest types [21–23], supplemented by two additional proﬁles ([24]; P. D. McIntosh, unpublished data). 2. Methods ,e surveys were not speciﬁcally designed to compare amounts of soil C under diﬀerent vegetation types but 2.1. Site Selection. At 14 site pairs, Moroni et al. [11] contain suﬃcient information to enable soil C to 1 m depth compared aboveground C stocks in mixed forests and under diﬀerent forest types to be calculated (Table 1). Wet rainforests representative of mature Tasmanian forest eucalypt forest soils contain about 50% more C to 1 m depth ecosystems. (Mixed forest is deﬁned as a forest of dominant than dry forests, and rainforest soils contain about 50% more mature eucalypts (e.g., Eucalyptus obliqua, E. regnans, E. C than wet forests (Table 1). However, the mean rainforest delegatensis, E. globulus, and E. viminalis) generally −1 soil C ﬁgure (226 Mg·ha ) and the C/N ratio of 19 for soil to 40–100 m high with a rainforest understorey; rainforest 1 m depth under this forest type are based on analysis of only lacks standing eucalypts and the canopy is dominated by three proﬁles in northeast Tasmania and cannot be regarded species such as Nothofagus cunninghamii, Atherosperma as typical. ,e highest mean C/N ratio (24) is found in soils moschatum, and Phyllocladus aspleniifolius.) ,ese forest under dry forests, which experience frequent understorey pairs provided a framework for a soil C comparison. In a ﬁres [25], and in which, charcoal is generally visible on the desktop study, all 14 pairs were examined to establish soil surface and in topsoils. Rainforest sites, which have which were best matched with respect to landform, slope, probably not been burnt for at least 500 years, have topsoil aspect, elevation, and geology. For a short list of pairs, these C/N ratios of 15, probably a result of higher proportion of attributes, together with soil depth and stoniness, were ﬁeld microbial residues in relation to plant remains than in checked. Six pairs showing least diﬀerence between these topsoils under eucalypts. About half the total soil C to 1 m basic soil physical attributes and which were nonstony or depth is held below 30 cm depth. contained only a few stones, and which covered repre- −1 Interestingly, the mean ﬁgure of 123 Mg·ha for soil C sentative areas of mixed eucalypt forest in Tasmania and to 1 m depth under all eucalypt forests in Tasmania over a widespread soil parent rocks, were selected. A seventh range of soil types (Table 1) is indistinguishable from the mixed forest/rainforest pair, not studied by Moroni et al. −1 IPCC default ﬁgure of 122 Mg·ha [8] for all temperate [11], was selected using the same site criteria, to provide forests, higher than the “typical” temperate forest soil ﬁgure information on a soil developed on Precambrian siltstone, a −1 of 100 Mg·ha by Lal and Lorenz [4] but lower than the c. rock type extensive in northwest Tasmania. No site had −1 150 Mg·ha ﬁgure for temperate broadleaf forests studied evidence of human-induced soil disturbance or vegetation by Duarte-Guardia et al. ([26], Figure 5(d)). Previous esti- disturbance in the form of recent ﬁres or clearfelling. Site mates of the soil C pool in forest soils of Tasmania and locations are shown in Figure 2, and brief site details are mainland southeast Australia under diﬀerent vegetation shown in Table 2. 4 International Journal of Forestry Research 1,2 Table 1: Mean carbon and nitrogen to 1 m depth for three Tasmanian forest types . Analyses to 1 m depth Analyses to 0–30 cm depth A1 horizon only Forest type −1 −1 −1 −1 C (Mg·ha ) C (Mg·ha ) Range (Mg·ha ) N (Mg·ha ) C/N ratio C/N ratio Rainforest (n � 3) 102 226 144–321 11.9 19 15 Wet eucalypt forest (n � 19) 71 147 37–273 9.9 15 22 Dry eucalypt forest (n � 14) 53 92 45–168 4.1 22 24 All eucalypt forests (n � 33) 63 123 1 2 From the proﬁle data of Grant et al. [21], Hill et al. [22], Laﬀan et al. [23], McIntosh [24], and P.D. McIntosh (unpublished data). Means are arithmetical and not area-weighted. Generally an overstorey of drought-tolerant species such as Eucalyptus amygdalina, E. globulus, E. viminalis, E. globulus, or E. tenuiramis with a heathy or grassy open understorey. identical sampling approach was used in a national survey of 146°0′0″E 148°0′0″E soil C in rainforests of Papua New Guinea [40], and in addition, at each site, one sample was taken from the subsoil at 30–60 cm depth and another at 60–100 cm depth. Mc- Intosh et al. [41] found that 10 topsoil samples per plot, Smithton Wynyard separately analysed, were suﬃcient to demonstrate signiﬁ- Luncheon Hill cant (P< 0.05) diﬀerences between soil C concentrations Scottsdale under diﬀerent vegetation types and land uses in loessial Launceston soils of southern New Zealand. For reasons of logistics mentioned above, including the limitations imposed by the necessity for manual sampling by a small team in dense forests, sampling followed procedures developed for quantitative (per hectare) soil C measurement in forests of Papua New Guinea [40], except that samples were individually analysed rather than bulked. For topsoil sampling, an Eijkelkamp split-tube sampler taking a 48 mm Maydena 1 diameter core was used. Ten cores to 30 cm depth in mineral Hobart soil (excluding any litter or O horizon) were sampled within a typical and undisturbed area under each forest type (mixed forest or rainforest). Each core was split into 3 depth in- 05 25 0 100 crements (0–10 cm, 10–20 cm, and 20–30 cm) and separately bagged for later analysis. A soil pit was dug to 1 m depth or a kilometers rock contact and photographed, described and classiﬁed. 146°0′0″E 148°0′0″E Subsoils were sampled at 30–60 cm and 60–100 cm depths, Figure 2: Site locations. For site names, see Table 2. or to a rock contact (sites 1 and 7). Subsoil bulk density samples were taken with an aluminium tube (75 mm internal diameter and length 100 mm), at 40–50 cm and 75–85 cm 2.2. Field Sampling. Makip ¨ a¨a¨ et al. [34] and Muukkonen [35] depths, where the rock contact allowed. recognised that in boreal forests, 10–20 samples per plot or Samples were air-dried at about 25 C and weighed. For more are required in order to reasonably estimate amounts of topsoil C on an area basis, and Cunningham et al. [36] each depth increment, subsamples were oven-dried at 105 C recommended at least 30 samples for accurate determination and passed through a 2 mm sieve to remove stones and determine the eﬀective bulk density (EBD) of respective of topsoil C (0–30 cm depth) in plantations on ex-farmland in Victoria, Australia. In practice, ideal sampling strategies <2 mm diameter fractions. such as the mechanised site sampling designed for ﬂat land in New South Wales [37] are usually modiﬁed for logistical 2.3. Organic Matter Analysis. Acid soil pH values and tests reasons (e.g., sampling in steep country, manual transport of with HCl indicated that no free carbonates were present in samples, and time and resources available). For example, ¨ ¨¨ any proﬁle. At sites 1–5, total C and N were analysed by despite their earlier studies, Makipaa et al. [34] and combusting ﬁnely ground 0.1 g subsamples in a Vario Macro Muukkonen et al. [35] recommended only four samples per Cube elemental analyser, and the labile C fraction was plot in their design for soil C assessment in Tanzania. measured using a hot water extraction method as outlined by McKenzie et al. [38] recommended at least four topsoil Ghani et al. [42]. At sites 6 and 7, a similar combustion replicates per site for soil C assessment in Australia but gave method was used: analysis was performed with a Euro EA no guidance for subsoil sampling intensity. In a national Elemental Analyser (HEKAtech GmbH, Wegberg, survey of topsoil C in Australia [39] designed for C ac- Germany). counting purposes, 10 topsoil replicates were taken at each Physical soil organic matter (SOM) fractionation was site, at each of 3 depths (0–10 cm, 10–20 cm, and 20–30 cm), undertaken on sieved air-dried soil samples for two and samples were bulked for each depth increment. An 42°0′0″S 42°0′0″S International Journal of Forestry Research 5 Table 2: Site details. Mean Mean max. FAO soil Moroni Parent annual Forest Altitude Latitude/ Site ID and mean min. classiﬁcation et al. [11] material rainfall type (m) longitude 4 o temp. ( C) (FAO [33]) site (mm) 13.7 42 48.018′ S 1167 Gleysol Rainforest 640 4 (1) Mueller Permian 4.1 146 31.465′ E Road siltstone 13.7 42 47.986′ S 1167 Gleysol Mixed 640 3 4.1 146 31.474′ E 14.2 42 47.768′ S Quaternary 1167 Cambisol Rainforest 560 8 4.4 146 34.930′ E (2) Styx Road colluvium from 14.1 42 47.700′ S siltstone 1167 Cambisol Mixed 587 7 4.3 146 34.928′ E Quaternary 14.2 42 43.076′ S colluvium from 1167 Podzol Rainforest 564 12 4.4 146 31.073′ E (3) Eleven Road sandstone 14.4 42 36.664′ S 1167 Podzol Mixed 533 11 4.5 146 5.362′ E 14.9 42 43.076′ S Mixed 1167 Cambisol Rainforest 464 6 (4) Florentine 4.7 146 1.073′ E quaternary Road 14.9 42 43.068′ S alluvium 1167 Cambisol Mixed 463 5 4.7 146 31.123′ E 15.8 41 7.244′ S 1524 Ferralsol Rainforest 226 24 Precambrian 6.9 145 0.566′ E (5) Sumac Road dolerite 15.8 41 7.303′ S 1524 Ferralsol Mixed 227 23 6.9 145 0.510′ E 13.1 41 23.095′ S 966 Umbrisol Rainforest 793 21 (6) South Esk Devonian 4.9 147 40.227′ E Road granite 13.2 41 23.458′ S 966 Umbrisol Mixed 779 20 5.0 147 40.464′ E 16.1 40 57.806′ S Not 1524 Stagnosol Rainforest 178 Precambrian 7.1 145 16.521′ E applicable (7) Tipunah siltstone 16.1 40 57.761′ S Not 1524 Stagnosol Mixed 184 7.0 145 16.532′ E applicable 1 2 Mean annual rainfall for Maydena weather station 95063 at 281 m altitude [31]. Rainfall at the site is likely to be higher. Mean annual rainfall at Luncheon Hill weather station 91259 at 345 m altitude [31]. Rainfall at the site is likely to be lower. Mean annual rainfall at Scottsdale weather station 91219 at 198 m altitude [31]. Rainfall at the site is likely to be higher. Derived from mean temperatures for Maydena (for sites 1–4), Smithton (sites 5–7), and Scottsdale (site 6) [31], to which the lapse rates of Nunez [32] have been applied. −1 contrasting sites: Sumac Road and Eleven Road (Table 2). A 2200, Bandelin, Berlin, Germany) applying 600 J·ml at 70% composite sample (30 g) derived from the right, middle, and intensity to break up soil aggregates and release occluded left soil proﬁle wall was prepared for the uppermost (B1 or particulate organic matter (oPOM). To avoid strong heating A1) and lowermost (C or Bhs) horizons at these two sites, and consequent mutation of released fragments, the sus- respectively, to identify diﬀerences between forest types as pension was cooled with water during this step. Samples well as between topsoils and subsoils (the Bhs horizon at were centrifuged (3500 rpm, 30 min), and the free-ﬂoating Eleven Road is a B horizon rich in humus and sesquioxides). oPOM was removed by suction. ,e extracted oPOM was ,e fractionation procedure combined density and particle- partitioned into >20 μm (oPOM>20) and <20 μm size fractionation (Kogel-Knabner [43]). ,e bulk sample (oPOM<20) fractions by sieving. Both fractions were −1 was homogenised in a glass beaker and sprayed with water to washed until outﬂows had an EC of 2 μS cm , and then reduce hydrophobicity and remove entrapped air. 150 ml of freeze-dried. ,e mineral residue was repeatedly purged −3 sodium polytungstate (SPT) solution (1.8 g·cm ) was added with deionized water and centrifuged (3500–5000 rpm, gradually to saturate the sample and ﬁnally submerge it. ,e 30 min) until the EC of the supernatant was below 50 μS −1 suspension was left standing overnight to allow separation of cm . ,e residue was wet sieved to obtain coarse sand lighter and heavier fractions. Free-ﬂoating particles, repre- (2000–630 μm), medium sand (630–200 μm), ﬁne sand senting free particulate organic matter (fPOM), were re- (200–63 μm), and coarse silt (63–20 μm) fractions. ,e moved with a vacuum pump and washed using pressure outﬂow was collected to further separate medium silt ﬁltration and 0.22 μm membrane ﬁlters (Berrytec GmbH) (20–6.3 μm) from ﬁne silt and clay (<6.3 μm) by gravity until the outﬂow had an electrical conductivity (EC) of<2 μS sedimentation in Atterberg-type cylinders, making use of −1 cm . Subsequently, the fPOM was freeze-dried. ,e heavier Stokes’ law. ,e two size fractions were freeze-dried, and all fraction was subjected to ultrasonication (Sonoplus HD remaining mineral fractions were dried at 65 C. ,ereupon, 6 International Journal of Forestry Research mineral fractions >20 μm were ground with a swing mill mean soil ﬁgure obtained for wet eucalypt forests in earlier −1 (3 min). ,e fPOM and oPOM>20 fractions were ground by surveys (147 Mg·ha ; Table 1) probably because the earlier hand using a mortar. Processed samples were put in glass surveys were not conﬁned to mixed forest containing mature vials and stored at room temperature until elemental eucalypts and included very stony proﬁles (the stoniest −1 analysis. proﬁle contained c. 70% stones by volume and 37 Mg·ha of ,e chemical composition of fPOM, oPOM>20, C). Such proﬁles were not sampled in this study. oPOM<20, and clay fractions was analysed using 13C- In deep soils sampled to 100 cm depth (Table 3, sites CPMAS NMR spectroscopy. A Bruker Advance III 200 2–6), 39–76% (mean 50%) of the soil C is held below 30 cm Spectrometer was used, coupled with a Bruker MAS II depth (Table 3; Figure 4). ,e clay-rich Styx Road Cambisols control unit, and operated with Bruker BioSpin software. developed in Quaternary colluvium contained most C −1 ,e NMR rotor was spun at a MAS spin rate of 6.8 kHz with (300–326 Mg·ha ). varying recycle delay time and scan counts. (,e recycle ,ere was no signiﬁcant diﬀerence between C/N ratios in delay time refers to the time between individual scans, while uppermost (A1 and B1) horizons sampled under rainforest scan counts refer to the total number of scans required until and mixed forest; mean values were 20–22 (Figure 5). an appropriate spectrum could be identiﬁed.) Fractions with However, in lowermost horizons sampled (generally, B3, C, a relatively high C concentration, i.e., mainly fPOM, BCg, or Bh horizons), C/N ratios were signiﬁcantly higher in oPOM>20, and oPOM<20, for which suﬃcient material was soils under mixed forests (17± 5) than under rainforests available from the fractionation procedure, were scanned (13± 6) (Figure 5). with a recycle delay time of 1.0 s while scan counts ranged During sample preparation, 50% of the samples (n � 102) between 3,400 and 91,400. For fractions showing a low C taken from mixed forest proﬁles were observed to contain concentration, i.e., mainly clay fractions, or fractions con- identiﬁable pieces of pyrogenic C (charcoal) in the >2 mm taining only small amounts of material from the fraction- fraction, but only 16% of the samples (n � 92) taken from ation procedure, the recycle delay time was set to 0.4 s and up rainforest proﬁles contained identiﬁable pyrogenic C. to 370,000 scans were required to obtain quality spectra. ,e Mean total C pools in mixed forests and rainforests were clay fraction of the samples of the Ferralsol at Sumac Road estimated (Table 4) using the soil data of this paper and the required most scan counts: this fraction combined low C biomass data of Moroni et al. [11] and Moroni et al. [10], contents with high contents of pedogenic Fe. Pedogenic Fe who assumed that roots contain 25% of measured above- has the potential to reduce the C signal and thus lead to less ground biomass [51, 52]. distinct peaks [44], which increases the number of scans required. After the samples were scanned, the spectra were processed with line broadening ranging from 0 to 75 Hz, 3.2. Soil Organic Matter Characterisation. ,e distribution of C among SOM fractions varied between sites and with soil depending on the distinctness of their peaks. Furthermore, the spectra were phase adjusted and baseline corrected (in an depth. In the uppermost (B1) horizons at Sumac Road, most C was found in the clay fraction, with slightly more under automatic mode). ,e interpretation of spectra followed principles outlined in detail by Kogel-Knabner ¨ et al. [45]. mixed forest (68%) than rainforest (60%) (in Ferralsol proﬁles under both rainforest and mixed forest, an A1 horizon as deﬁned by the National Committee on Soil and 2.4. Statistical Analysis. Standard deviations were calculated Terrain [53] was not present and the B1 horizon was the for all means (Table 3). ,e eﬀect of forest type on bulk soil uppermost mineral horizon). ,e remaining C was largely in C/N ratios in both top- and subsoil was tested based on the oPOM form with a negligible contribution of fPOM. In estimation graphs as demonstrated by Gardner and Altmann contrast at Sumac Road under rainforest, 75% of total C in [46]. ,e graphs show the 95% conﬁdence interval, which is the C horizon was held in the oPOM<20 fraction compared determined via bootstrap resamples (5000 repetitions) of the to 6% in this fraction below mixed forest. ,is subsoil eﬀect size. Bootstrap was bias-corrected to account for skew. diﬀerence may reﬂect diﬀerent SOM stabilisation mecha- ,is was accomplished using the Python (version 3.7) nisms in subsoils under the two forest types, as observed in module DABEST [47]. forests elsewhere [54] and in agricultural systems [55]. In the uppermost (A1) horizon at Eleven Road under mixed forest, most C was held in the oPOM<20 fraction, but under 3. Results rainforest, most was held in the clay fraction. ,e Bhs horizons under both forest types had highest C contents in 3.1. Carbon Content of Site Pairs and C/N Ratios. ,e soils the clay fraction. under rainforest and mixed forest contain 110 to −1 In the Ferralsols at Sumac Road (site 5), the fPOM 326 Mg·ha (Table 3; Figure 3) of total C. No statistical diﬀerence between soil C values under rainforest and mixed extracted from the <2 mm B1 horizon under mixed forest had a distinct NMR peak in the aromatic aryl-C region forest (t-test, P> 0.05) was detected: mean values were −1 −1 203 Mg·ha in soils under rainforest and 199 Mg·ha in (130 ppm), indicating the presence of pyrogenic C [56, 57] in this horizon under mixed forest; this peak was not detected soils under mixed forest. ,e latter ﬁgure is close to the mean value measured by Cotching ([50], p. 86) to 1 m depth for six in the B1 horizon under rainforest (Figure 6). In the fPOM fraction of the B1 horizon under mixed forest 28% of the soil orders under unspeciﬁed native forest in Tasmania −1 −1 total contribution was derived from aryl-C, whereas under (193 Mg·ha ; range 94–273 Mg·ha ) but higher than the International Journal of Forestry Research 7 Table 3: Mean soil C at seven site pairs in Tasmania, from the data of Hardcastle ([48], sites 1–5) and Kloﬀel ¨ ([49], sites 6 and 7), adjusted for stone content. 2 −1 −3 Soil C by sampling depth (Mg·ha ) EBD by sampling depth (Mg m ) 1 −1 Site ID Forest type Total C (Mg·ha ) 0–10 cm 10–20 cm 20–30 cm 30–60 cm 60–100 cm 0–10 cm 10–20 cm 20–30 cm 30–60 cm 60–100 cm R 37.5± 6.5 31.1± 8.7 27.8± 9.9 35.5 — 0.53± 0.17 1.03± 0.20 0.99± 0.13 1.42 — 132 (1) Mueller Road M 49.9± 8.1 42.7± 11.3 39.8± 15.9 40.0 — 0.53± 0.21 0.83± 0.16 0.96± 0.20 1.12 — 172 R 52.0± 9.0 42.0± 8.3 37.9± 16.4 119.4 74.8 0.43± 0.14 0.64± 0.10 0.74± 0.17 0.91 0.99 326 (2) Styx Road M 45.3± 8.3 39.0± 7.2 34.8± 6.8 101.3 84.0 0.49± 0.19 0.66± 0.18 0.88± 0.15 0.96 0.93 304 R 25.8± 6.4 24.9± 3.7 21.4± 3.6 113.4 114.3 0.59± 0.21 0.81± 0.15 1.05± 0.22 1.10 1.21 300 (3) Eleven Road M 37.5± 13.9 36.5± 10.1 34.5± 13.0 44.6 49.6 0.35± 0.14 0.67± 0.21 1.13± 0.20 1.65 1.30 203 R 23.5± 4.1 16.5± 7.8 11.7± 5.7 27.0 30.8 0.72± 0.13 0.92± 0.21 0.84± 0.23 0.95 1.10 110 (4) Florentine Road M 23.0± 8.5 20.1± 15.5 16.0± 10.3 37.9 27.4 0.65± 0.21 0.87± 0.22 0.94± 0.22 1.11 1.18 124 R 51.9± 4.9 51.4± 4.5 40.5± 10.6 73.5 19.2 0.58± 0.05 0.81± 0.09 0.82± 0.21 1.10 1.09 237 (5) Sumac Road M 42.7± 6.9 47.5± 7.8 44.2± 6.8 94.3 31.3 0.51± 0.08 0.86± 0.08 0.97± 0.15 1.06 1.13 260 R 36.7± 4.6 36.6± 4.4 17.8± 3.5 74.6 35.4 0.62± 0.09 0.83± 0.08 0.55± 0.00 0.72 0.90 201 (6) South Esk Road M 39.9± 12.9 30.6± 11.4 22.6± 8.7 43.1 40.2 0.62± 0.09 0.62± 0.09 0.62± 0.09 0.90 0.79 176 R 44.8± 10.0 25.6± 6.7 5.3± 2.2 18.7 — 0.79± 0.17 1.15± 0.20 0.55± 0 1.36 - 115 (7) Tipunah M 53.5± 8.9 24.7± 6.4 9.0± 2.4 64.6 — 0.61± 0.12 1.04± 0.21 0.55± 0 1.26 - 152 −1 Mean total C values (Mg·ha ) 0–30 cm depth 0–100 cm depth Mean Std. Dev. Range Mean Std. Dev. Range R 95 34 52–144 203 89 110–326 M 105 33 59–134 199 83 124–304 1 2 R � rainforest; M � mixed. Each topsoil ﬁgure (0–10 cm, 10–20 cm, and 20–30 cm depth increments) is the mean of 10 samples. Subsoil ﬁgures are the means of three samples for each depth increment (30–60 cm and 60–100 cm, or to a rock contact) for sites 1–5 and are from analysis of one sample per depth increment for sites 6 and 7. ,e rainforest proﬁle was 42 cm deep on rock and the mixed forest proﬁle was 45 cm deep on rock. C ﬁgures have been calculated to 42 cm depth for both proﬁles. ,e rainforest proﬁle was 95 cm deep on rock and the mixed forest proﬁle was 80 cm deep on rock. C ﬁgures have been calculated to 80 cm depth for both proﬁles. To a rock contact at sites 1 and 7; see also footnotes 3 and 4. 8 International Journal of Forestry Research 400 analysed, and usable spectra for samples under mixed forest were not obtained. ,e NaOH-insoluble fraction of the or- ganic matter in the rainforest Bhs horizon (Figure 7(a)) was radiocarbon dated by accelerator mass spectrometry (AMS) to 2882± 24 yr BP (Wk49496, uncalibrated). In another study (P. McIntosh, unpublished data), organic matter from a podzol pan in northeast Tasmania was dated 3133± 38 BP 150 (Wk17421, uncalibrated), after similar NaOH pretreatment. If it is assumed that C illuviation is a continuous process under the present climate, then these dates are the average age of C illuviation in these soils, and the subsoil horizons analysed may well be around 6000 years old or older. However, some proﬁles are younger: buried charcoal at 1 m depth in the Umbrisol under mixed forest in granitic colluvium under mixed forest at site 6 (South Esk Road) (Table 1 and Figure 2) was dated 1838± 14 yr BP (Wk49497, uncalibrated), and probably dates localised erosion, for example, fall of a large eucalypt tree with its intact root-ball of soil, following a ﬁre. Mixed forest Rainforest 4. Discussion Figure 3: Total C held in soils to 1 m depth or to a rock contact at shallower depth (Mueller Road and Tipunah sites). Green shad- 4.1. Soil Carbon Stocks. ,ere is no evidence that soil C either ing � mixed forest; blue shading � rainforest. ,e bars indicate 1.5x increases or decreases during the transition of mixed forests the interquartile range. Open circles indicate values outside this to rainforests (Table 3). Soil C stocks are within the range of range. previous proﬁle values (Table 1; [50]). It is most unlikely that, even if signiﬁcant soil C diﬀerences were detected by more detailed and statistically robust studies, they would be suﬃciently large to balance the loss of total biomass C (mean −1 value 259 Mg·ha ; Table 4) resulting from the transition of mixed forests to rainforests. In addition, as rainforest sites had little litter cover and litter in mixed forests was mainly in the form of bark accumulations at the base of large eucalypts, taking account of litter (estimated by Mackey et al. ([15], p. 22) to be 2% of total biomass and soil C in mature eucalypt forest) would probably accentuate the biomass C diﬀerence between rainforest and mixed forest, rather than lessen it. ,e mean amount of soil C held below 30 cm depth for all proﬁles is 50%, demonstrating the importance of sam- pling full proﬁles to 1 m depth or a rock contact when calculating ecosystem C storage. ,e mean C contained at 0–30 cm depth under mixed eucalypt forests in this study −1 −1 (Table 3) is 95 Mg·ha (range 52–144 Mg·ha ); previously published estimates of forest soil C under mixed forests and Mixed forest rainforest are too high and not based on information Rainforest available at the time of publication. For example, the −1 Figure 4: Proportion of total C held in topsoils (0–30 cm depth). unreferenced 271 Mg·ha value quoted in 2010 by Dean and Green shading � mixed forest; blue shading � rainforest. ,e bars Wardell-Johnson [27] for soil C at 0–30 cm depth under tall indicate 1.5x the interquartile range. Open circles indicate values old-growth forest in Tasmania is almost three times the outside this range. ﬁgure measured in this study and over twice the highest measured forest C value at 0–30 cm soil depth in Tasmania −1 rainforest, the respective aryl-C contribution was only 15%. available in 2010: 121 Mg·ha for the Stronach soil In Ferralsol subsoils (Figure 7), no clear diﬀerences of aryl-C (Umbrisol) formed in granite colluvium in northeast Tas- −1 under rainforest and mixed forest were apparent. mania [21]. Likewise, the unreferenced 369 Mg·ha of C at Eluviation of pyrogenic C through proﬁles is conﬁrmed 0–30 cm depth quoted by Dean and Wardell-Johnson ([27], by the weak aryl-C peak in NMR analyses of organic/clay Table 1) for rainforest soils in Tasmania is over three times coatings in the Bhs horizon of the rainforest Podzol at Eleven too high: in 2010, the mean measured C value for 0–30 cm Road (Figure 7) which contains 3.4% C. Unfortunately the soils at three rainforests sites in Tasmania (Table 1) was −1 amount of fPOM and oPOM material in Podzol subsoil 102 Mg·ha , almost identical to the mean value of −1 samples was insuﬃcient for all organic fractions to be 105 Mg·ha measured in this study (Table 3). In addition, –1 Topsoil total C share (%) Total soil C stocks (Mg·ha ) Mueller Road Mueller Road Styx Road Styx Road Eleven Road Eleven Road Florentine Road Florentine Road Sumac Road Sumac Road South Esk Road South Esk Road Tipunah Tipunah International Journal of Forestry Research 9 35 35 30 30 25 25 20 20 ∆ ∆ –5 15 15 –10 10 10 –5 –15 5 5 –10 –20 0 0 Rainforest Mixed forest Mixed forest Rainforest Mixed forest Mixed forest n = 21 n = 21 minus n = 22 n = 24 minus rainforest rainforest (a) (b) Figure 5: Estimation graphs [45] comparing C/N ratios of soils under rainforest and mixed forests for (a) uppermost (A1 or B1) horizons and (b) lowermost (B3, C, BCg, or Bhs) horizons. ,e 95% conﬁdence interval, indicated by vertical black bars, is obtained via bootstrap resampling (5000 repetitions). ,e horizontal black lines represent mean C/N ratios for each forest type. ,e vertical curve indicates the resampled distribution. Table 4: Estimated mean C pools in mixed forests and rainforests of Tasmania and C in each pool expressed as a percentage of total C in the ecosystem. Standard deviations in parentheses. −1 Carbon (Mg·ha ) Forest type 1 2 Soil Aboveground biomass Roots Total biomass Total biomass and soil 203 (89) 171 (75) 43 214 417 Rainforest 48% 41% 11% 52% 199 (83) 378 (173) 95 473 672 Mixed forest 30% 56% 14% 70% 1 2 3 ,is study. From Moroni et al. [11], Table 2. ,ere are no studies of root mass in Australian temperate rainforests so the default value based on 25% of aboveground biomass [51] has been used. ,e default ﬁgure, based on 25% of aboveground biomass [51, 52]. ,e actual mean (n � 6) for tall eucalypt forest −1 −1 >30 m high is 90 Mg·ha (SE � 19 Mg·ha ) ([51], p. 62), but as the eucalypts in the mixed forests in this study were mature and close to their maximum height, the larger ﬁgure is more likely to be correct. Dean and Wardell-Johnson [27] provided no evidence to ([28], p. 17) refers to “<30 cm” soils and another refers to support their assertion that C levels in soils at 0–30 cm depth “A and B horizons” (p. 274). Whether their average soil C −1 soils under rainforest exceed those under old-growth (euca- ﬁgure of 280 Mg·ha “assumed to represent all eucalypt −1 lypt) forest by 98 Mg·ha ; no signiﬁcant diﬀerence of 0–30 cm forests in southeast Australia” ([28], p. 274) refers to soil C between these two forest types was found in this study 0–30 cm or 0–1 m depth, it is at least two times too high for (Table 3) or in the study conducted by Fedrigo et al. [30]. Tasmanian eucalypt forests, in which the mean soil C in −1 ,e unreferenced c. 700 Mg·ha “baseline” soil C ﬁgure information available to May and co-authors in 2012 for all −1 for the “full soil proﬁle” of undeﬁned depth under Tas- eucalypt forests in Tasmania was 123 Mg·ha for 0–1 m −1 manian E. regnans forest used by Dean and Wardell-Johnson depth and 63 Mg·ha for 0–30 cm depth (Table 1). Con- ([27], Figure 8) is more than twice the maximum measured sequently modelled potential C sequestration ﬁgures soil C value under E. regnans available to these authors in published by May et al. ([28], p. 54, Figure 4 and Table 5), −1 2010 which was 273 Mg·ha in the previously mentioned unfortunately promoted with the aim to “develop a clear −1 Stronach proﬁle [21]. ,e 700·Mg·ha value also exceeds the accounting framework for carbon stocks and ﬂows in highest published forest soil C ﬁgure known to the authors Tasmania’s forests” ([28], p. 10), are overestimated and for a forested nonswamp site in the Australasian region incorrect. Similarly, the modelled losses in soil C from “old- −1 (600 Mg·ha ) measured in a Papua New Guinea soil formed growth” (mixed) eucalypt forests after land-use change −1 in gabbroic alluvium [58] and is more than three times the (e.g., the 300 Mg·ha · C loss modelled by Dean and mean mixed forest soil C ﬁgure in this study (Table 3). Wardell-Johnson [27]) based on the unreferenced −1 ,ere are numerous references to total soil C in Tas- 700 Mg·ha baseline is, in turn, overestimated; the esti- manian forests by May et al. [28] but the soil depth to which mated loss exceeds the total C held in most forest soils as their ﬁgures refer is not speciﬁed, although one estimate determined in this and previous studies (Table 1). C/N ratio C/N ratio 10 International Journal of Forestry Research fPOM fPOM oPOM>20 oPOM>20 oPOM<20 oPOM<20 Clay Clay 300 200 100 0 300 200 100 0 Chemical shi (ppm) Chemical shi (ppm) (a) (b) Figure 6: Site 5 Ferralsols: NMR spectra of fPOM, oPOM, and clay fractions from the B1 horizons (uppermost mineral soil horizons) under rainforest (a) and mixed forest (b). ,e prominent aryl-C peak in the fPOM fraction under mixed forest is arrowed. In their baseline map of soil C at 0–30 cm depth, Is the similarity of mean soil C values under mixed forest and designed to support national carbon accounting in Australia, rainforest a result of soils under rainforest not yet having Viscarra Rossell et al. [39] estimated that the largely forested attained equilibrium with their “new” lower-biomass forest −1 lands of western Tasmania contained 161–220 Mg·ha of C cover? (2) Has soil C under mixed forest been enhanced in soils at 0–30 cm depth. When compared to the range of C (above levels expected in forests subject to frequent ﬁre) by values found in this study for soils with few or no stones an intermittent supply of relatively inert charcoal? (3) Does −1 (52–144 Mg·ha ; Table 3), the modelled range appears to be the soil C measured in these studies represent the balance of an overestimate, especially as large areas of forested western C input and breakdown over millennia rather than centuries Tasmania are steeplands with stony soils. Consequently, the and is what we measure now the result of inherited soil C mean C ﬁgure for all Tasmanian soils at 0–30 cm depth accumulation independent of present vegetation cover? −1 Although NMR results for a greater range of sites would (134 Mg·ha , [39]) may also have been overestimated. ,e similarity of mean measured soil C stocks under be advantageous for determining organic matter processes mixed forest and rainforest, when contrasted with the large in these soils, the NMR results obtained, together with the diﬀerence in the biomass C of the two forest types (Table 4) observation that charcoal is present in both eucalypt and and likely greater biomass and C inputs to the soil under rainforest soils, and the radiocarbon ages on buried charcoal, mixed forest than under rainforest, raises three questions: (1) allow tentative answers to be provided for the 3 questions International Journal of Forestry Research 11 300 200 100 0 –100 Chemical shi (ppm) (a) (b) Figure 7: (a) Organic matter/clay coatings on blocky peds in the subsoil (the Bhs horizon) of the Podzol in sandy colluvium under rainforest at Eleven Road. Vertical section, 10 cm from top to bottom. (b) NMR spectrum for the organic matter/clay coatings; from right to left, the four peaks represent alkyl-C (0–50 ppm), O-alkyl-C (50–110 ppm), aryl-C (110–145 ppm; arrowed), and O-aryl-C (145–160 ppm). −1 posed above. All three propositions appear to be correct: (1) average 3.5% increase of topsoil C (from 145 to 150 Mg·ha at rainforest soils have not yet attained equilibrium with their 0–10 cm depth) after harvest and burning in three wet eucalypt “new” lower-biomass forest cover; they still contain a coupes (harvest areas). ,e results were not statistically ana- chemical signature of past ﬁres; (2) NMR results show that lysed, but Slijepcevic ([61], p. 285) considered that they “did not soil C levels under mixed forest (and to a lesser extent under provide any evidence of carbon loss or gain from the upper rainforest) have been enhanced by intermittent supply of layers after burning.” Ellis and Graley [62] found that imme- relatively inert (pyrogenic) carbon, as well as other stable diately after “hot” (intense) regeneration burns following eu- carbon fractions; and (3) measured soil C under rainforest calypt harvest at two locations in Tasmania, C in topsoil −1 retains characteristics of mixed forest cover, and both the (0–10 cm depth) decreased by 6.4–7.4 Mg·ha , but long-term amount and character of the C in the soil under these trends were not investigated. In Victoria, Australia, Polglase vegetation types is probably largely inherited and has been et al. [63] found that in E. regnans forests regenerating after ﬁre, determined by processes acting over thousands of years. soil C reached 86% of equilibrium (steady state) values when the However, soil organic matter characteristics are also forest was aged 30 years, with “true equilibrium” not being inﬂuenced by present-day vegetation. ,e greater relative reached until about 150 years. ,e eﬀect of regular ﬁres on soil amounts of pyrogenic C (and aryl-C) in Ferralsol surface B1 properties in wet eucalypt forests was addressed by Guinto et al. horizons under mixed forest and the higher mean C/N ratio [64] who measured a signiﬁcant 1.9% decline in C in soils at of subsoils under mixed forest than under rainforest is likely 0–10 cm depth in wet sclerophyll forest soils burnt every two to be a consequence of greater ﬁre frequency in the eucalypt- years for 20 years in southeast Queensland. Assuming a topsoil −1 dominated mixed forests than in the adjacent rainforests. EBD of 0.5 Mg·ha for these 0–10 cm soils, this 20-year decline −1 ,ese eﬀects are discussed below. equates to a C loss of 9 Mg·ha . In contrast, Krishnaraj [19] found that topsoil C under eucalypt forests of the Otway Ranges of southern Victoria increased by 2% after a forest ﬁre due to 4.2. Changes of Soil C after Fire. Measures of short-term accumulation of charcoal. change of soil C stocks after ﬁres have yielded ambiguous ,e diﬀerent results obtained by these authors probably results. In a global meta-analysis, Johnson and Curtis [59] noted reﬂect the problem of adequately covering postdisturbance an increase of soil C after wildﬁre (attributed to the seques- soil variation by the sampling strategies used. Until more tration of pyrogenic C and contributions from postﬁre nitro- comprehensive work is done, results of these case studies gen-ﬁxing plants) but a decrease after prescribed fuel-reduction should not be used to generalise about the short-term eﬀect burns. Pennington et al. [60] noted signiﬁcant declines of topsoil of harvest and burning on soil C. ,e eucalypt forests in- C concentration at 0–5 cm and 5–10 cm depth (from 8.04 to vestigated in this study must have experienced stand- 5.40% and from 4.12 to 3.41%, respectively) after harvest and a destroying ﬁres more recently than the rainforests, but the regeneration burn in tall wet eucalypt forests at a southern eﬀects of ﬁres is not evident in mean total C values. Tasmania site. However, the diﬀerence between unburnt and −1 burnt topsoils at 0–30 cm depth (5 Mg·ha ) was not statistically signiﬁcant once bulk density had been taken into account. No 4.3. Sequestration of Carbon by Wet Eucalypt Forest. correction was made for stones, but stone content “was shown Many authors (e.g., Keith et al. [6], Mackey et al. [15], Dean to be very low” [60]. In contrast, Slijepcevic [61] noted an and Wardell-Johnson [27], May et al. [28], and Dean et al. 12 International Journal of Forestry Research [65]) have promoted the protection of eucalypt forests to harvesting, or storm damage) nor that present after dis- mitigate the eﬀects of climate change. ,e weaknesses in the turbance represents the potential biomass in the landscape as a whole, and the true biomass potential at a landscape scale arguments presented by these authors have been highlighted by Moroni et al. [67] and McIntosh and Moroni [29] who will always lie between the disturbed and undisturbed ex- pointed out that (1) mature eucalypt forests are not the tremes. ,e biomass C in the landscape is always in a state of steady-state end point of forest succession and (2) that a ﬂux and cannot be deﬁned. concept originally designed to assess C sequestration po- Even the concepts of “undisturbed” (Mackey et al. [15]) tential in ecosystems (Gupta and Rao [68]) cannot be applied and “natural” (Keith et al. [66]) must be used cautiously, to landscapes as a management tool. ,e relevance of this since Aboriginal management has profoundly aﬀected study to these two issues is discussed below. forest structure in Tasmania for many thousands of years. Early European settlers remarked on ﬁre use by the Ab- original population [69–72]. Frequent ﬁres probably lit by 4.3.1. Forest Succession. ,e soil studies summarised in the Aboriginal population maintained eucalypts and Table 3 and Figures 3 and 4 indicate that during the suc- grasslands in upland northeast Tasmania and prevented cession from mixed forest to rainforest, no change in mean expansion of rainforests [73]. ,e pollen and charcoal soil C stocks is detectable. Given the long-term stability of record in deep peats on Surrey Hills in northwest Tas- the soils as indicated by NMR analyses and the lower bio- mania and on the Nicholas Range in northeast Tasmania mass (and probably lower biomass inputs into the soil) provides evidence of ﬁres and ﬁre-induced open eucalypt under rainforest, and previous measurements of total C in woodlands being present for at least the last 12 000 years Tasmanian proﬁles (Table 1), it appears unlikely that more [74–76]. detailed and statistically robust studies would demonstrate We can conclude that many eucalypt forests had a more −1 that the loss of biomass C (259 Mg·ha ) during the mixed open character under Aboriginal management than they do forest/rainforest transition is compensated by an increase in at present, and that regular low-intensity ﬁres (mostly soil C. (,e mean value of soil C measured under rainforests burning understorey only) periodically released only in this study would need to be 2.5 times higher to com- modest amounts of C to the atmosphere. We can be rea- pensate for the biomass C loss.) Consequently, it can be sonably certain that at least during the Holocene, the concluded that the C loss resulting from the mixed forest/ Tasmanian lowland and midaltitude vegetation pattern −1 rainforest transition is around 260 Mg·ha . If we assume before European arrival was a mosaic of ﬁre-induced forest, that wet eucalypt forests are tallest (and contain most C) rainforest, open woodland, grassland, and sedge-domi- when they are 400–500 years old, that all eucalypt trees have nated moorlands, a pattern noted by early surveyors such fallen after another 150 years, and that fallen trees have as Lawrence [77] in the Florentine Valley near Maydena rotted away after another 250 years, then we can calculate and Hellyer [71] in northwest Tasmania, which in part that during the mixed forest/rainforest transition, was anthropogenically induced and which, incidentally, −1 260 Mg·ha of C will be released to the atmosphere over an makes description of the Tasmanian wet forests as a approximately 400-year period, i.e., C is lost at a rate of “wilderness,” e.g., “Tasmanian Wilderness World Heritage −1 −1 about 0.65 Mg·ha year . ,us, during natural succession, Area,” a misnomer. For a brief period from approximately mature wet eucalypt forests are signiﬁcant C emitters, not C 1825 (by which time, the Aboriginal population was se- accumulators. verely depleted) to 1920 (when mechanised harvest began in earnest), forest cover was neither controlled by frequent Aboriginal land management nor disturbed by clearfelling 4.3.2. Forest Management. Despite acknowledging that “the role of ﬁre in natural forests is complex and must be con- operations, although timber was extracted from more ac- sidered on a landscape-wide basis” (Mackey et al. [15], p. 20), cessible sites [78]. In this period, for the ﬁrst time in 12000 these authors underestimated the eﬀect of ﬁres on C ac- years, many forests previously managed by Aboriginal cumulation at the landscape scale, even though eucalypt burning grew more densely and regenerated without large- scale human intervention, greatly increasing aboveground ecosystems are ﬁre-induced. For example, Mackey et al. [15] (p. 23 and 26) excluded bushﬁre-aﬀected areas when cal- biomass and fuel for ﬁres. Possibly partly because of this post-1825 fuel increase culating gross primary production, and living biomass C was calculated only for “undisturbed” forests. ,e limitation of and cessation of low-intensity burning, there have been a number of large ﬁres since European settlement in Tas- ﬁres on C accumulation is evident in areas of Tasmania and the southeast Australian mainland aﬀected by the 2019 and mania. ,e largest was in 1898 when over 1 million ha and 2020 bushﬁres (discussed below): while not all burnt eu- possibly up to 2 million ha of forests in the southwest burned calypts are killed by ﬁre, the overall eﬀect of crown ﬁres is to [28]. Other very large ﬁres were in 1934 when 800 000–900 halt landscape-scale C accumulation or reverse it. Because 000 ha burned [79] and in 1967 when approximately 250 eucalypt forests in their natural state will either burn and 000 ha burned [80]. Recently, over 2400 recorded lightning regenerate or transition to rainforest in the absence of ﬁre, it strikes in Tasmania in December 2018 and January 2019 caused 72 vegetation ﬁres extending over 205 000 ha in- is not correct to assume that mature eucalypt forests are a “baseline” against which to judge land-use change, as done, cluding c. 100 000 ha of wet eucalypt forests and 14% (296 ha) of Tasmania’s tall (>70 m high) eucalypt forests and for example, by May et al. ([28], p. 274). Neither the biomass present before disturbance (whether the result of burning, 7000 ha of rainforest [81]. International Journal of Forestry Research 13 of C stocks from the soil data of Grant et al. [21], Hill et al. 5. Conclusions [22], and Laﬀan et al. [23] are available as an excel Previous studies showed that soil C to 1 m depth (or a rock spreadsheet from the corresponding author. contact) under wet eucalypt cover ranged from 35 to −1 325 Mg·ha , depending on stoniness, soil depth, and parent Disclosure material, but provided little data on soil C in rainforest soils, ,is paper is partly based on the unpublished research theses so the soil C trend (if any) during the ecological transition of of J. Hardcastle and T. Kloﬀel. ¨ mixed eucalypt forests (mature tall wet eucalypts with a rainforest understorey) to rainforests could not be established. Conflicts of Interest ,is study of 7 paired sites found no consistent diﬀer- ence between soil C stocks under mixed forests or rainforest None of the authors have conﬂicts of interest regarding the in Tasmania, and the mean soil C value under both forest −1 conduct of this study or the interpretation of its results. types is about 200 Mg·ha . Consequently, at a landscape −1 scale, the loss of biomass C (259 Mg·ha ) during the Acknowledgments transition of mixed eucalypt forests (mean biomass C, −1 −1 473 Mg·ha ) to rainforest (mean biomass C, 214 Mg·ha ) ,e authors are grateful to Elizabeth Brewer, Adrian Slee, (Table 4) approximates to the total ecosystem loss and Carsten Muller ¨ who assisted with ﬁeldwork, Peter Schad −1 (269 Mg·ha ) during this ecological transition (Table 4). for checking FAO soil classiﬁcations, and Carsten Muller, ¨ If it is assumed that the transition to rainforest takes Mark Poynter, and two anonymous reviewers for their 400 years, then the mixed forests are emitting about valuable comments on drafts of the text. ,e Board of the −1 −1 0.65 Mg·ha yr until they reach pure rainforest. During Forest Practices Authority provided funds to support the this ecological transition, the soils remain (on average) research and travel costs of James Hardcastle, Tobias Kloﬀel, ¨ relatively stable: deep soil horizons under rainforest retain and Carsten Muller. ¨ ,e authors thank Sustainable Timber an imprint of pyrogenic C from past ﬁres, and the soils Tasmania and the Department of Primary Industries, Parks, appear to be buﬀered against the eﬀects of vegetation change Water, and the Environment for allowing access to sites. and variable C inputs. ,e observation that Tasmanian tall wet eucalypt forests References become net C emitters as they mature and transition to rainforests is contrary to trends monitored in northern [1] R. A. Houghton, “Balancing the global carbon budget,” An- nual Review of Earth and Planetary Sciences, vol. 35, no. 1, hemisphere temperate forests in which C continues to ac- pp. 313–347, 2007. cumulate as forests age [12–14]. 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Publisher: Hindawi Publishing Corporation
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Abstract

Hindawi International Journal of Forestry Research Volume 2020, Article ID 6509659, 16 pages https://doi.org/10.1155/2020/6509659 Research Article Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account 1 2 3 4 Peter D. McIntosh , James L. Hardcastle, Tobias Klo ¨ﬀel, Martin Moroni, and Talitha C. Santini Forest Practices Authority, 30 Patrick Street, Hobart, TAS 7000, Australia School of Earth and Environmental Sciences, University of Queensland, Brisbane, QLD 4072, Australia Research Department of Ecology and Ecosystem Management, Technical University of Munich, Freising, Germany Private Forests Tasmania, 30 Patrick Street, Hobart, TAS 7000, Australia UWA School of Agriculture and Environment, University of Western Australia, Crawley, WA 6009, Australia Correspondence should be addressed to Peter D. McIntosh; peter.mcintosh@fpa.tas.gov.au Received 11 November 2019; Revised 25 February 2020; Accepted 28 March 2020; Published 30 July 2020 Academic Editor: Kurt Johnsen Copyright © 2020 Peter D. McIntosh et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Small areas of the wetter parts of southeast Australia including Tasmania support high-biomass “wet” eucalypt forests, including “mixed” forests consisting of mature eucalypts up to 100 m high with a rainforest understorey. In Tasmania, mixed forests transition to lower biomass rainforests over time. In the scientiﬁc and public debate on ways to mitigate climate change, these forests have received attention for their ability to store large amounts of carbon (C), but the contribution of soil C stocks to the total C in these two ecosystems has not been systematically researched, and consequently, the potential of wet eucalypt forests to serve as long-term C sinks is uncertain. ,is study compared soil C stocks to 1 m depth at paired sites under rainforest and mixed forests and found that there was no detectable diﬀerence of mean total soil C between the two forest types, and on average, both −1 contained about 200 Mg·ha of C. Some C in subsoil under rainforests is 3000 years old and retains a chemical signature of pyrogenic C, detectable in NMR spectra, indicating that soil C stocks are buﬀered against the eﬀects of forest succession. ,e mean −1 loss of C in biomass as mixed forests transition to rainforests is estimated to be about 260 Mg·ha over a c. 400-year period, so the −1 −1 mature mixed forest ecosystem emits about 0.65 Mg·ha ·yr of C during its transition to rainforest. For this reason and because of the risk of forest ﬁres, setting aside large areas of wet eucalypt forests as reserves in order to increase landscape C storage is not a sound strategy for long-term climate change mitigation. Maintaining a mosaic of managed native forests, including regenerating eucalypts, mixed forests, rainforests, and reserves, is likely to be the best strategy for maintaining landscape C stocks. −1 soils and biomass is about equal (120 Mg·ha ) [5]. Aus- 1. Introduction tralian forests as a whole contain the lowest forest biomass −1 Natural forest ecosystems contain about 1500 Pg of carbon and soil C for midlatitudes (mean C of 45 Mg·ha for forests −1 (C) [1], and on a worldwide basis, forest soils contain about and 83 Mg·ha for soils to 1 m depth) [2] because of climatic twice the C of the vegetation they support [2]. ,e pro- limitations and the relative infertility of large areas of forest- portions of C in vegetation and soil varies greatly depending supporting soils. However, the natural forests of Australia’s on latitude, altitude, vegetation, climate, and soil [2–4], but wet southeast include the tallest hardwoods in the world (up in boreal and temperate forests, the average C content of to 100 m high) (Figure 1), and according to some authors 2 International Journal of Forestry Research (a) (b) Figure 1: Iconic Tasmanian wet eucalypt forests. ,e tree on the left (a) is 100 m tall. ,e right-hand side image (b) shows mixed forest consisting of mature tall eucalypts (Eucalyptus regnans) and a dense understorey of rainforest species which are about half as tall as the dominant eucalypts. (e.g., Keith et al. [6]), the aboveground C content of these In the northern hemisphere, it has been found that −1 ecosystems is over 2500 Mg·ha , which is over 50 times the temperate old-growth forests continue to accumulate C as −1 mean Australian forest aboveground value (45 Mg·ha ) and they mature [12–14] and protection of old-growth temperate also greatly exceeds mean values for other midlatitude forests has been promoted as a C sequestration policy. A −1 forests (range 32–114 Mg·ha ; [2]), the IPCC ﬁgure similar policy has been argued for wet eucalypt forests of −1 southeast Australia, in order to moderate climate change (96 Mg·ha ; [8]) for biomass C in world temperate forests, and the mean aboveground C for mountain ash (Eucalyptus induced by the eﬀects of increased concentrations of CO in −1 regnans) forests in Victoria (246 Mg·ha ; [7]). However, the atmosphere. For example, Mackey et al. ([15], p. 39), in Sillett et al. [9] found that aboveground biomass C in forests reference to eucalypt forests, proposed that “the remaining similar to those described by Keith et al. [6] was at most intact natural forests constitute a signiﬁcant standing stock −1 −1 706 Mg·ha , but probably lower (438 Mg·ha ) due to the of C that should be protected from carbon-emitting land-use loss of mass in decayed hollow trunks and limbs, and activities.” However, although C stocks will undoubtedly questioned the assertion that the tallest southeast Australian increase as young trees mature, such a policy ignores the fact eucalypt forests are the most C-dense forests in the world, that old-growth eucalypt forests in Tasmania are not the end quoting measurements of North American redwood (Se- point of forest succession [11] and that all eucalypt forests quoia sempervirens) forests which contain more than are highly susceptible to ﬁre. −1 Provided there is no stand-destroying ﬁre and there is a 2000 Mg·ha of biomass C. A typical value for biomass C (including roots) for seed source nearby (for example, in a rainforest-dominated Tasmanian mature eucalypt forests >55 m high, with >40% gully, on a shady slope, or in a nearby unburned patch of crown cover (Class 1 forests in the Tasmanian classiﬁcation), forest), wet eucalypt forests will transition to rainforests, a was obtained by Moroni et al. [10]: these contain process described in the classic paper by Gilbert [16]. After −1 470 Mg·ha of C. If all forests supporting all mature trees ﬁre, a eucalypt forest will normally establish rapidly. By the >55 m are included (i.e., all Class 1 and 2 forests), the ﬁgure time the eucalypt forests are about 100 years old, shade- −1 −1 is lower (387 Mg·ha ) which is close to the 378 Mg·ha tolerant rainforest species may already be established in the ﬁgure obtained by Moroni et al. [11] in a 14-site paired understorey. As the eucalypts reach maturity, the rainforest comparison of aboveground biomass (trees and coarse will form a continuous understorey and a “mixed” tall eu- woody debris, but not litter) under mixed forests and calypt/rainforest results ([16], Figure 1.10). ,e eucalypts cannot regenerate under the rainforest canopy; hence, once rainforest but four times the IPCC ﬁgure for biomass C in −1 world temperate forests (96 Mg·ha [8]). ,e mean ﬁgure the eucalypts reach their maximum age of 300–500 years and for C in all mature Tasmanian “wet” eucalypt forests (those progressively die, they are replaced by the rainforest with a dense understorey of shrubs and/or rainforest trees) is understorey, although eucalypt woody debris may persist for −1 232 Mg·ha [10]. up to two centuries on the forest ﬂoor. Peak aboveground International Journal of Forestry Research 3 wood volumes occur when the eucalypts are tallest and types by Mackey et al. ([15], p. 28), Dean and Wardell- contain an understorey of rainforest species (Figure 1). Johnson [27], and May et al. [28]) did not take into account Rainforests, on average, contain about 45% of the above- published data and exceeded the ﬁgures summarised in ground wood volume and biomass C of mixed forest [11]. To Table 1 by factors of up to 3.5 [29]. halt the progression to rainforests and maintain eucalypt Fedrigo et al. [30] were the ﬁrst to compare measured cover, ﬁre must return before the eucalypt overstorey (the soil C in nearby rainforest, mixed rainforest-wet scle- eucalypt seed source) dies [16, 17]. rophyll stands (“ecotone forest”), and wet sclerophyll forest When considering C trends in Tasmanian forests, an (“eucalypt forest”) in the Yarra Ranges of Victoria, but only important question to ask is whether the lower biomass C in to 30 cm depth. ,ey found no signiﬁcant diﬀerence of soil −1 rainforest than in mixed mature eucalypt forest also indi- C between these forest types: values were 163 Mg·ha −1 cates lower C in the total ecosystem under rainforest. ,e under wet sclerophyll forest and 149 Mg·ha under answer to this question depends on soil C content, does it rainforest. Unfortunately, no information was provided on change with vegetation cover or does it remain the same? whether soils were closely matched across diﬀerent vege- ,is unanswered question prompted Norris et al. [18] to tation plots. Consequently, eﬀects of vegetation or ﬁre on comment that “the uncertainty around soil carbon repre- soil C storage may have been conﬂated with those resulting sents a priority for further work.” One might assume that as from inherent soil diﬀerences such as depth, drainage, and mixed forests contain about twice the biomass of rainforests parent material. [11], they should return more biomass to the forest ﬂoor As the information summarised in Table 1 and in the than rainforests and thus accumulate more soil C than soils literature is insuﬃcient to answer the question of whether under rainforest. Alternatively, one could argue that fre- soil C increases, decreases, or remains the same as wet quent ﬁres in eucalypt forests deplete soil C stocks, negating eucalypt forests transition to rainforest over time, a study any soil C increase caused by greater cycling of C to the was undertaken to measure soil C in paired sites under forest ﬂoor by the maturing eucalypts of greater biomass. A mixed forest and rainforest in Tasmania. ,is paper complicating issue can be the accumulation of pyrogenic C presents the new soil C data obtained in this study. It also in soils under ﬁre-susceptible forests [19, 20]. discusses the ecological processes aﬀecting total ecosys- Previous work on soil C under eucalypts and rainforest tem C and assesses the value of wet eucalypt forests in in Tasmania has been limited to soil proﬁle analysis un- Tasmania for mitigating the eﬀects of CO emissions on dertaken in soil surveys covering a range of soils and parent climate. material and forest types [21–23], supplemented by two additional proﬁles ([24]; P. D. McIntosh, unpublished data). 2. Methods ,e surveys were not speciﬁcally designed to compare amounts of soil C under diﬀerent vegetation types but 2.1. Site Selection. At 14 site pairs, Moroni et al. [11] contain suﬃcient information to enable soil C to 1 m depth compared aboveground C stocks in mixed forests and under diﬀerent forest types to be calculated (Table 1). Wet rainforests representative of mature Tasmanian forest eucalypt forest soils contain about 50% more C to 1 m depth ecosystems. (Mixed forest is deﬁned as a forest of dominant than dry forests, and rainforest soils contain about 50% more mature eucalypts (e.g., Eucalyptus obliqua, E. regnans, E. C than wet forests (Table 1). However, the mean rainforest delegatensis, E. globulus, and E. viminalis) generally −1 soil C ﬁgure (226 Mg·ha ) and the C/N ratio of 19 for soil to 40–100 m high with a rainforest understorey; rainforest 1 m depth under this forest type are based on analysis of only lacks standing eucalypts and the canopy is dominated by three proﬁles in northeast Tasmania and cannot be regarded species such as Nothofagus cunninghamii, Atherosperma as typical. ,e highest mean C/N ratio (24) is found in soils moschatum, and Phyllocladus aspleniifolius.) ,ese forest under dry forests, which experience frequent understorey pairs provided a framework for a soil C comparison. In a ﬁres [25], and in which, charcoal is generally visible on the desktop study, all 14 pairs were examined to establish soil surface and in topsoils. Rainforest sites, which have which were best matched with respect to landform, slope, probably not been burnt for at least 500 years, have topsoil aspect, elevation, and geology. For a short list of pairs, these C/N ratios of 15, probably a result of higher proportion of attributes, together with soil depth and stoniness, were ﬁeld microbial residues in relation to plant remains than in checked. Six pairs showing least diﬀerence between these topsoils under eucalypts. About half the total soil C to 1 m basic soil physical attributes and which were nonstony or depth is held below 30 cm depth. contained only a few stones, and which covered repre- −1 Interestingly, the mean ﬁgure of 123 Mg·ha for soil C sentative areas of mixed eucalypt forest in Tasmania and to 1 m depth under all eucalypt forests in Tasmania over a widespread soil parent rocks, were selected. A seventh range of soil types (Table 1) is indistinguishable from the mixed forest/rainforest pair, not studied by Moroni et al. −1 IPCC default ﬁgure of 122 Mg·ha [8] for all temperate [11], was selected using the same site criteria, to provide forests, higher than the “typical” temperate forest soil ﬁgure information on a soil developed on Precambrian siltstone, a −1 of 100 Mg·ha by Lal and Lorenz [4] but lower than the c. rock type extensive in northwest Tasmania. No site had −1 150 Mg·ha ﬁgure for temperate broadleaf forests studied evidence of human-induced soil disturbance or vegetation by Duarte-Guardia et al. ([26], Figure 5(d)). Previous esti- disturbance in the form of recent ﬁres or clearfelling. Site mates of the soil C pool in forest soils of Tasmania and locations are shown in Figure 2, and brief site details are mainland southeast Australia under diﬀerent vegetation shown in Table 2. 4 International Journal of Forestry Research 1,2 Table 1: Mean carbon and nitrogen to 1 m depth for three Tasmanian forest types . Analyses to 1 m depth Analyses to 0–30 cm depth A1 horizon only Forest type −1 −1 −1 −1 C (Mg·ha ) C (Mg·ha ) Range (Mg·ha ) N (Mg·ha ) C/N ratio C/N ratio Rainforest (n � 3) 102 226 144–321 11.9 19 15 Wet eucalypt forest (n � 19) 71 147 37–273 9.9 15 22 Dry eucalypt forest (n � 14) 53 92 45–168 4.1 22 24 All eucalypt forests (n � 33) 63 123 1 2 From the proﬁle data of Grant et al. [21], Hill et al. [22], Laﬀan et al. [23], McIntosh [24], and P.D. McIntosh (unpublished data). Means are arithmetical and not area-weighted. Generally an overstorey of drought-tolerant species such as Eucalyptus amygdalina, E. globulus, E. viminalis, E. globulus, or E. tenuiramis with a heathy or grassy open understorey. identical sampling approach was used in a national survey of 146°0′0″E 148°0′0″E soil C in rainforests of Papua New Guinea [40], and in addition, at each site, one sample was taken from the subsoil at 30–60 cm depth and another at 60–100 cm depth. Mc- Intosh et al. [41] found that 10 topsoil samples per plot, Smithton Wynyard separately analysed, were suﬃcient to demonstrate signiﬁ- Luncheon Hill cant (P< 0.05) diﬀerences between soil C concentrations Scottsdale under diﬀerent vegetation types and land uses in loessial Launceston soils of southern New Zealand. For reasons of logistics mentioned above, including the limitations imposed by the necessity for manual sampling by a small team in dense forests, sampling followed procedures developed for quantitative (per hectare) soil C measurement in forests of Papua New Guinea [40], except that samples were individually analysed rather than bulked. For topsoil sampling, an Eijkelkamp split-tube sampler taking a 48 mm Maydena 1 diameter core was used. Ten cores to 30 cm depth in mineral Hobart soil (excluding any litter or O horizon) were sampled within a typical and undisturbed area under each forest type (mixed forest or rainforest). Each core was split into 3 depth in- 05 25 0 100 crements (0–10 cm, 10–20 cm, and 20–30 cm) and separately bagged for later analysis. A soil pit was dug to 1 m depth or a kilometers rock contact and photographed, described and classiﬁed. 146°0′0″E 148°0′0″E Subsoils were sampled at 30–60 cm and 60–100 cm depths, Figure 2: Site locations. For site names, see Table 2. or to a rock contact (sites 1 and 7). Subsoil bulk density samples were taken with an aluminium tube (75 mm internal diameter and length 100 mm), at 40–50 cm and 75–85 cm 2.2. Field Sampling. Makip ¨ a¨a¨ et al. [34] and Muukkonen [35] depths, where the rock contact allowed. recognised that in boreal forests, 10–20 samples per plot or Samples were air-dried at about 25 C and weighed. For more are required in order to reasonably estimate amounts of topsoil C on an area basis, and Cunningham et al. [36] each depth increment, subsamples were oven-dried at 105 C recommended at least 30 samples for accurate determination and passed through a 2 mm sieve to remove stones and determine the eﬀective bulk density (EBD) of respective of topsoil C (0–30 cm depth) in plantations on ex-farmland in Victoria, Australia. In practice, ideal sampling strategies <2 mm diameter fractions. such as the mechanised site sampling designed for ﬂat land in New South Wales [37] are usually modiﬁed for logistical 2.3. Organic Matter Analysis. Acid soil pH values and tests reasons (e.g., sampling in steep country, manual transport of with HCl indicated that no free carbonates were present in samples, and time and resources available). For example, ¨ ¨¨ any proﬁle. At sites 1–5, total C and N were analysed by despite their earlier studies, Makipaa et al. [34] and combusting ﬁnely ground 0.1 g subsamples in a Vario Macro Muukkonen et al. [35] recommended only four samples per Cube elemental analyser, and the labile C fraction was plot in their design for soil C assessment in Tanzania. measured using a hot water extraction method as outlined by McKenzie et al. [38] recommended at least four topsoil Ghani et al. [42]. At sites 6 and 7, a similar combustion replicates per site for soil C assessment in Australia but gave method was used: analysis was performed with a Euro EA no guidance for subsoil sampling intensity. In a national Elemental Analyser (HEKAtech GmbH, Wegberg, survey of topsoil C in Australia [39] designed for C ac- Germany). counting purposes, 10 topsoil replicates were taken at each Physical soil organic matter (SOM) fractionation was site, at each of 3 depths (0–10 cm, 10–20 cm, and 20–30 cm), undertaken on sieved air-dried soil samples for two and samples were bulked for each depth increment. An 42°0′0″S 42°0′0″S International Journal of Forestry Research 5 Table 2: Site details. Mean Mean max. FAO soil Moroni Parent annual Forest Altitude Latitude/ Site ID and mean min. classiﬁcation et al. [11] material rainfall type (m) longitude 4 o temp. ( C) (FAO [33]) site (mm) 13.7 42 48.018′ S 1167 Gleysol Rainforest 640 4 (1) Mueller Permian 4.1 146 31.465′ E Road siltstone 13.7 42 47.986′ S 1167 Gleysol Mixed 640 3 4.1 146 31.474′ E 14.2 42 47.768′ S Quaternary 1167 Cambisol Rainforest 560 8 4.4 146 34.930′ E (2) Styx Road colluvium from 14.1 42 47.700′ S siltstone 1167 Cambisol Mixed 587 7 4.3 146 34.928′ E Quaternary 14.2 42 43.076′ S colluvium from 1167 Podzol Rainforest 564 12 4.4 146 31.073′ E (3) Eleven Road sandstone 14.4 42 36.664′ S 1167 Podzol Mixed 533 11 4.5 146 5.362′ E 14.9 42 43.076′ S Mixed 1167 Cambisol Rainforest 464 6 (4) Florentine 4.7 146 1.073′ E quaternary Road 14.9 42 43.068′ S alluvium 1167 Cambisol Mixed 463 5 4.7 146 31.123′ E 15.8 41 7.244′ S 1524 Ferralsol Rainforest 226 24 Precambrian 6.9 145 0.566′ E (5) Sumac Road dolerite 15.8 41 7.303′ S 1524 Ferralsol Mixed 227 23 6.9 145 0.510′ E 13.1 41 23.095′ S 966 Umbrisol Rainforest 793 21 (6) South Esk Devonian 4.9 147 40.227′ E Road granite 13.2 41 23.458′ S 966 Umbrisol Mixed 779 20 5.0 147 40.464′ E 16.1 40 57.806′ S Not 1524 Stagnosol Rainforest 178 Precambrian 7.1 145 16.521′ E applicable (7) Tipunah siltstone 16.1 40 57.761′ S Not 1524 Stagnosol Mixed 184 7.0 145 16.532′ E applicable 1 2 Mean annual rainfall for Maydena weather station 95063 at 281 m altitude [31]. Rainfall at the site is likely to be higher. Mean annual rainfall at Luncheon Hill weather station 91259 at 345 m altitude [31]. Rainfall at the site is likely to be lower. Mean annual rainfall at Scottsdale weather station 91219 at 198 m altitude [31]. Rainfall at the site is likely to be higher. Derived from mean temperatures for Maydena (for sites 1–4), Smithton (sites 5–7), and Scottsdale (site 6) [31], to which the lapse rates of Nunez [32] have been applied. −1 contrasting sites: Sumac Road and Eleven Road (Table 2). A 2200, Bandelin, Berlin, Germany) applying 600 J·ml at 70% composite sample (30 g) derived from the right, middle, and intensity to break up soil aggregates and release occluded left soil proﬁle wall was prepared for the uppermost (B1 or particulate organic matter (oPOM). To avoid strong heating A1) and lowermost (C or Bhs) horizons at these two sites, and consequent mutation of released fragments, the sus- respectively, to identify diﬀerences between forest types as pension was cooled with water during this step. Samples well as between topsoils and subsoils (the Bhs horizon at were centrifuged (3500 rpm, 30 min), and the free-ﬂoating Eleven Road is a B horizon rich in humus and sesquioxides). oPOM was removed by suction. ,e extracted oPOM was ,e fractionation procedure combined density and particle- partitioned into >20 μm (oPOM>20) and <20 μm size fractionation (Kogel-Knabner [43]). ,e bulk sample (oPOM<20) fractions by sieving. Both fractions were −1 was homogenised in a glass beaker and sprayed with water to washed until outﬂows had an EC of 2 μS cm , and then reduce hydrophobicity and remove entrapped air. 150 ml of freeze-dried. ,e mineral residue was repeatedly purged −3 sodium polytungstate (SPT) solution (1.8 g·cm ) was added with deionized water and centrifuged (3500–5000 rpm, gradually to saturate the sample and ﬁnally submerge it. ,e 30 min) until the EC of the supernatant was below 50 μS −1 suspension was left standing overnight to allow separation of cm . ,e residue was wet sieved to obtain coarse sand lighter and heavier fractions. Free-ﬂoating particles, repre- (2000–630 μm), medium sand (630–200 μm), ﬁne sand senting free particulate organic matter (fPOM), were re- (200–63 μm), and coarse silt (63–20 μm) fractions. ,e moved with a vacuum pump and washed using pressure outﬂow was collected to further separate medium silt ﬁltration and 0.22 μm membrane ﬁlters (Berrytec GmbH) (20–6.3 μm) from ﬁne silt and clay (<6.3 μm) by gravity until the outﬂow had an electrical conductivity (EC) of<2 μS sedimentation in Atterberg-type cylinders, making use of −1 cm . Subsequently, the fPOM was freeze-dried. ,e heavier Stokes’ law. ,e two size fractions were freeze-dried, and all fraction was subjected to ultrasonication (Sonoplus HD remaining mineral fractions were dried at 65 C. ,ereupon, 6 International Journal of Forestry Research mineral fractions >20 μm were ground with a swing mill mean soil ﬁgure obtained for wet eucalypt forests in earlier −1 (3 min). ,e fPOM and oPOM>20 fractions were ground by surveys (147 Mg·ha ; Table 1) probably because the earlier hand using a mortar. Processed samples were put in glass surveys were not conﬁned to mixed forest containing mature vials and stored at room temperature until elemental eucalypts and included very stony proﬁles (the stoniest −1 analysis. proﬁle contained c. 70% stones by volume and 37 Mg·ha of ,e chemical composition of fPOM, oPOM>20, C). Such proﬁles were not sampled in this study. oPOM<20, and clay fractions was analysed using 13C- In deep soils sampled to 100 cm depth (Table 3, sites CPMAS NMR spectroscopy. A Bruker Advance III 200 2–6), 39–76% (mean 50%) of the soil C is held below 30 cm Spectrometer was used, coupled with a Bruker MAS II depth (Table 3; Figure 4). ,e clay-rich Styx Road Cambisols control unit, and operated with Bruker BioSpin software. developed in Quaternary colluvium contained most C −1 ,e NMR rotor was spun at a MAS spin rate of 6.8 kHz with (300–326 Mg·ha ). varying recycle delay time and scan counts. (,e recycle ,ere was no signiﬁcant diﬀerence between C/N ratios in delay time refers to the time between individual scans, while uppermost (A1 and B1) horizons sampled under rainforest scan counts refer to the total number of scans required until and mixed forest; mean values were 20–22 (Figure 5). an appropriate spectrum could be identiﬁed.) Fractions with However, in lowermost horizons sampled (generally, B3, C, a relatively high C concentration, i.e., mainly fPOM, BCg, or Bh horizons), C/N ratios were signiﬁcantly higher in oPOM>20, and oPOM<20, for which suﬃcient material was soils under mixed forests (17± 5) than under rainforests available from the fractionation procedure, were scanned (13± 6) (Figure 5). with a recycle delay time of 1.0 s while scan counts ranged During sample preparation, 50% of the samples (n � 102) between 3,400 and 91,400. For fractions showing a low C taken from mixed forest proﬁles were observed to contain concentration, i.e., mainly clay fractions, or fractions con- identiﬁable pieces of pyrogenic C (charcoal) in the >2 mm taining only small amounts of material from the fraction- fraction, but only 16% of the samples (n � 92) taken from ation procedure, the recycle delay time was set to 0.4 s and up rainforest proﬁles contained identiﬁable pyrogenic C. to 370,000 scans were required to obtain quality spectra. ,e Mean total C pools in mixed forests and rainforests were clay fraction of the samples of the Ferralsol at Sumac Road estimated (Table 4) using the soil data of this paper and the required most scan counts: this fraction combined low C biomass data of Moroni et al. [11] and Moroni et al. [10], contents with high contents of pedogenic Fe. Pedogenic Fe who assumed that roots contain 25% of measured above- has the potential to reduce the C signal and thus lead to less ground biomass [51, 52]. distinct peaks [44], which increases the number of scans required. After the samples were scanned, the spectra were processed with line broadening ranging from 0 to 75 Hz, 3.2. Soil Organic Matter Characterisation. ,e distribution of C among SOM fractions varied between sites and with soil depending on the distinctness of their peaks. Furthermore, the spectra were phase adjusted and baseline corrected (in an depth. In the uppermost (B1) horizons at Sumac Road, most C was found in the clay fraction, with slightly more under automatic mode). ,e interpretation of spectra followed principles outlined in detail by Kogel-Knabner ¨ et al. [45]. mixed forest (68%) than rainforest (60%) (in Ferralsol proﬁles under both rainforest and mixed forest, an A1 horizon as deﬁned by the National Committee on Soil and 2.4. Statistical Analysis. Standard deviations were calculated Terrain [53] was not present and the B1 horizon was the for all means (Table 3). ,e eﬀect of forest type on bulk soil uppermost mineral horizon). ,e remaining C was largely in C/N ratios in both top- and subsoil was tested based on the oPOM form with a negligible contribution of fPOM. In estimation graphs as demonstrated by Gardner and Altmann contrast at Sumac Road under rainforest, 75% of total C in [46]. ,e graphs show the 95% conﬁdence interval, which is the C horizon was held in the oPOM<20 fraction compared determined via bootstrap resamples (5000 repetitions) of the to 6% in this fraction below mixed forest. ,is subsoil eﬀect size. Bootstrap was bias-corrected to account for skew. diﬀerence may reﬂect diﬀerent SOM stabilisation mecha- ,is was accomplished using the Python (version 3.7) nisms in subsoils under the two forest types, as observed in module DABEST [47]. forests elsewhere [54] and in agricultural systems [55]. In the uppermost (A1) horizon at Eleven Road under mixed forest, most C was held in the oPOM<20 fraction, but under 3. Results rainforest, most was held in the clay fraction. ,e Bhs horizons under both forest types had highest C contents in 3.1. Carbon Content of Site Pairs and C/N Ratios. ,e soils the clay fraction. under rainforest and mixed forest contain 110 to −1 In the Ferralsols at Sumac Road (site 5), the fPOM 326 Mg·ha (Table 3; Figure 3) of total C. No statistical diﬀerence between soil C values under rainforest and mixed extracted from the <2 mm B1 horizon under mixed forest had a distinct NMR peak in the aromatic aryl-C region forest (t-test, P> 0.05) was detected: mean values were −1 −1 203 Mg·ha in soils under rainforest and 199 Mg·ha in (130 ppm), indicating the presence of pyrogenic C [56, 57] in this horizon under mixed forest; this peak was not detected soils under mixed forest. ,e latter ﬁgure is close to the mean value measured by Cotching ([50], p. 86) to 1 m depth for six in the B1 horizon under rainforest (Figure 6). In the fPOM fraction of the B1 horizon under mixed forest 28% of the soil orders under unspeciﬁed native forest in Tasmania −1 −1 total contribution was derived from aryl-C, whereas under (193 Mg·ha ; range 94–273 Mg·ha ) but higher than the International Journal of Forestry Research 7 Table 3: Mean soil C at seven site pairs in Tasmania, from the data of Hardcastle ([48], sites 1–5) and Kloﬀel ¨ ([49], sites 6 and 7), adjusted for stone content. 2 −1 −3 Soil C by sampling depth (Mg·ha ) EBD by sampling depth (Mg m ) 1 −1 Site ID Forest type Total C (Mg·ha ) 0–10 cm 10–20 cm 20–30 cm 30–60 cm 60–100 cm 0–10 cm 10–20 cm 20–30 cm 30–60 cm 60–100 cm R 37.5± 6.5 31.1± 8.7 27.8± 9.9 35.5 — 0.53± 0.17 1.03± 0.20 0.99± 0.13 1.42 — 132 (1) Mueller Road M 49.9± 8.1 42.7± 11.3 39.8± 15.9 40.0 — 0.53± 0.21 0.83± 0.16 0.96± 0.20 1.12 — 172 R 52.0± 9.0 42.0± 8.3 37.9± 16.4 119.4 74.8 0.43± 0.14 0.64± 0.10 0.74± 0.17 0.91 0.99 326 (2) Styx Road M 45.3± 8.3 39.0± 7.2 34.8± 6.8 101.3 84.0 0.49± 0.19 0.66± 0.18 0.88± 0.15 0.96 0.93 304 R 25.8± 6.4 24.9± 3.7 21.4± 3.6 113.4 114.3 0.59± 0.21 0.81± 0.15 1.05± 0.22 1.10 1.21 300 (3) Eleven Road M 37.5± 13.9 36.5± 10.1 34.5± 13.0 44.6 49.6 0.35± 0.14 0.67± 0.21 1.13± 0.20 1.65 1.30 203 R 23.5± 4.1 16.5± 7.8 11.7± 5.7 27.0 30.8 0.72± 0.13 0.92± 0.21 0.84± 0.23 0.95 1.10 110 (4) Florentine Road M 23.0± 8.5 20.1± 15.5 16.0± 10.3 37.9 27.4 0.65± 0.21 0.87± 0.22 0.94± 0.22 1.11 1.18 124 R 51.9± 4.9 51.4± 4.5 40.5± 10.6 73.5 19.2 0.58± 0.05 0.81± 0.09 0.82± 0.21 1.10 1.09 237 (5) Sumac Road M 42.7± 6.9 47.5± 7.8 44.2± 6.8 94.3 31.3 0.51± 0.08 0.86± 0.08 0.97± 0.15 1.06 1.13 260 R 36.7± 4.6 36.6± 4.4 17.8± 3.5 74.6 35.4 0.62± 0.09 0.83± 0.08 0.55± 0.00 0.72 0.90 201 (6) South Esk Road M 39.9± 12.9 30.6± 11.4 22.6± 8.7 43.1 40.2 0.62± 0.09 0.62± 0.09 0.62± 0.09 0.90 0.79 176 R 44.8± 10.0 25.6± 6.7 5.3± 2.2 18.7 — 0.79± 0.17 1.15± 0.20 0.55± 0 1.36 - 115 (7) Tipunah M 53.5± 8.9 24.7± 6.4 9.0± 2.4 64.6 — 0.61± 0.12 1.04± 0.21 0.55± 0 1.26 - 152 −1 Mean total C values (Mg·ha ) 0–30 cm depth 0–100 cm depth Mean Std. Dev. Range Mean Std. Dev. Range R 95 34 52–144 203 89 110–326 M 105 33 59–134 199 83 124–304 1 2 R � rainforest; M � mixed. Each topsoil ﬁgure (0–10 cm, 10–20 cm, and 20–30 cm depth increments) is the mean of 10 samples. Subsoil ﬁgures are the means of three samples for each depth increment (30–60 cm and 60–100 cm, or to a rock contact) for sites 1–5 and are from analysis of one sample per depth increment for sites 6 and 7. ,e rainforest proﬁle was 42 cm deep on rock and the mixed forest proﬁle was 45 cm deep on rock. C ﬁgures have been calculated to 42 cm depth for both proﬁles. ,e rainforest proﬁle was 95 cm deep on rock and the mixed forest proﬁle was 80 cm deep on rock. C ﬁgures have been calculated to 80 cm depth for both proﬁles. To a rock contact at sites 1 and 7; see also footnotes 3 and 4. 8 International Journal of Forestry Research 400 analysed, and usable spectra for samples under mixed forest were not obtained. ,e NaOH-insoluble fraction of the or- ganic matter in the rainforest Bhs horizon (Figure 7(a)) was radiocarbon dated by accelerator mass spectrometry (AMS) to 2882± 24 yr BP (Wk49496, uncalibrated). In another study (P. McIntosh, unpublished data), organic matter from a podzol pan in northeast Tasmania was dated 3133± 38 BP 150 (Wk17421, uncalibrated), after similar NaOH pretreatment. If it is assumed that C illuviation is a continuous process under the present climate, then these dates are the average age of C illuviation in these soils, and the subsoil horizons analysed may well be around 6000 years old or older. However, some proﬁles are younger: buried charcoal at 1 m depth in the Umbrisol under mixed forest in granitic colluvium under mixed forest at site 6 (South Esk Road) (Table 1 and Figure 2) was dated 1838± 14 yr BP (Wk49497, uncalibrated), and probably dates localised erosion, for example, fall of a large eucalypt tree with its intact root-ball of soil, following a ﬁre. Mixed forest Rainforest 4. Discussion Figure 3: Total C held in soils to 1 m depth or to a rock contact at shallower depth (Mueller Road and Tipunah sites). Green shad- 4.1. Soil Carbon Stocks. ,ere is no evidence that soil C either ing � mixed forest; blue shading � rainforest. ,e bars indicate 1.5x increases or decreases during the transition of mixed forests the interquartile range. Open circles indicate values outside this to rainforests (Table 3). Soil C stocks are within the range of range. previous proﬁle values (Table 1; [50]). It is most unlikely that, even if signiﬁcant soil C diﬀerences were detected by more detailed and statistically robust studies, they would be suﬃciently large to balance the loss of total biomass C (mean −1 value 259 Mg·ha ; Table 4) resulting from the transition of mixed forests to rainforests. In addition, as rainforest sites had little litter cover and litter in mixed forests was mainly in the form of bark accumulations at the base of large eucalypts, taking account of litter (estimated by Mackey et al. ([15], p. 22) to be 2% of total biomass and soil C in mature eucalypt forest) would probably accentuate the biomass C diﬀerence between rainforest and mixed forest, rather than lessen it. ,e mean amount of soil C held below 30 cm depth for all proﬁles is 50%, demonstrating the importance of sam- pling full proﬁles to 1 m depth or a rock contact when calculating ecosystem C storage. ,e mean C contained at 0–30 cm depth under mixed eucalypt forests in this study −1 −1 (Table 3) is 95 Mg·ha (range 52–144 Mg·ha ); previously published estimates of forest soil C under mixed forests and Mixed forest rainforest are too high and not based on information Rainforest available at the time of publication. For example, the −1 Figure 4: Proportion of total C held in topsoils (0–30 cm depth). unreferenced 271 Mg·ha value quoted in 2010 by Dean and Green shading � mixed forest; blue shading � rainforest. ,e bars Wardell-Johnson [27] for soil C at 0–30 cm depth under tall indicate 1.5x the interquartile range. Open circles indicate values old-growth forest in Tasmania is almost three times the outside this range. ﬁgure measured in this study and over twice the highest measured forest C value at 0–30 cm soil depth in Tasmania −1 rainforest, the respective aryl-C contribution was only 15%. available in 2010: 121 Mg·ha for the Stronach soil In Ferralsol subsoils (Figure 7), no clear diﬀerences of aryl-C (Umbrisol) formed in granite colluvium in northeast Tas- −1 under rainforest and mixed forest were apparent. mania [21]. Likewise, the unreferenced 369 Mg·ha of C at Eluviation of pyrogenic C through proﬁles is conﬁrmed 0–30 cm depth quoted by Dean and Wardell-Johnson ([27], by the weak aryl-C peak in NMR analyses of organic/clay Table 1) for rainforest soils in Tasmania is over three times coatings in the Bhs horizon of the rainforest Podzol at Eleven too high: in 2010, the mean measured C value for 0–30 cm Road (Figure 7) which contains 3.4% C. Unfortunately the soils at three rainforests sites in Tasmania (Table 1) was −1 amount of fPOM and oPOM material in Podzol subsoil 102 Mg·ha , almost identical to the mean value of −1 samples was insuﬃcient for all organic fractions to be 105 Mg·ha measured in this study (Table 3). In addition, –1 Topsoil total C share (%) Total soil C stocks (Mg·ha ) Mueller Road Mueller Road Styx Road Styx Road Eleven Road Eleven Road Florentine Road Florentine Road Sumac Road Sumac Road South Esk Road South Esk Road Tipunah Tipunah International Journal of Forestry Research 9 35 35 30 30 25 25 20 20 ∆ ∆ –5 15 15 –10 10 10 –5 –15 5 5 –10 –20 0 0 Rainforest Mixed forest Mixed forest Rainforest Mixed forest Mixed forest n = 21 n = 21 minus n = 22 n = 24 minus rainforest rainforest (a) (b) Figure 5: Estimation graphs [45] comparing C/N ratios of soils under rainforest and mixed forests for (a) uppermost (A1 or B1) horizons and (b) lowermost (B3, C, BCg, or Bhs) horizons. ,e 95% conﬁdence interval, indicated by vertical black bars, is obtained via bootstrap resampling (5000 repetitions). ,e horizontal black lines represent mean C/N ratios for each forest type. ,e vertical curve indicates the resampled distribution. Table 4: Estimated mean C pools in mixed forests and rainforests of Tasmania and C in each pool expressed as a percentage of total C in the ecosystem. Standard deviations in parentheses. −1 Carbon (Mg·ha ) Forest type 1 2 Soil Aboveground biomass Roots Total biomass Total biomass and soil 203 (89) 171 (75) 43 214 417 Rainforest 48% 41% 11% 52% 199 (83) 378 (173) 95 473 672 Mixed forest 30% 56% 14% 70% 1 2 3 ,is study. From Moroni et al. [11], Table 2. ,ere are no studies of root mass in Australian temperate rainforests so the default value based on 25% of aboveground biomass [51] has been used. ,e default ﬁgure, based on 25% of aboveground biomass [51, 52]. ,e actual mean (n � 6) for tall eucalypt forest −1 −1 >30 m high is 90 Mg·ha (SE � 19 Mg·ha ) ([51], p. 62), but as the eucalypts in the mixed forests in this study were mature and close to their maximum height, the larger ﬁgure is more likely to be correct. Dean and Wardell-Johnson [27] provided no evidence to ([28], p. 17) refers to “<30 cm” soils and another refers to support their assertion that C levels in soils at 0–30 cm depth “A and B horizons” (p. 274). Whether their average soil C −1 soils under rainforest exceed those under old-growth (euca- ﬁgure of 280 Mg·ha “assumed to represent all eucalypt −1 lypt) forest by 98 Mg·ha ; no signiﬁcant diﬀerence of 0–30 cm forests in southeast Australia” ([28], p. 274) refers to soil C between these two forest types was found in this study 0–30 cm or 0–1 m depth, it is at least two times too high for (Table 3) or in the study conducted by Fedrigo et al. [30]. Tasmanian eucalypt forests, in which the mean soil C in −1 ,e unreferenced c. 700 Mg·ha “baseline” soil C ﬁgure information available to May and co-authors in 2012 for all −1 for the “full soil proﬁle” of undeﬁned depth under Tas- eucalypt forests in Tasmania was 123 Mg·ha for 0–1 m −1 manian E. regnans forest used by Dean and Wardell-Johnson depth and 63 Mg·ha for 0–30 cm depth (Table 1). Con- ([27], Figure 8) is more than twice the maximum measured sequently modelled potential C sequestration ﬁgures soil C value under E. regnans available to these authors in published by May et al. ([28], p. 54, Figure 4 and Table 5), −1 2010 which was 273 Mg·ha in the previously mentioned unfortunately promoted with the aim to “develop a clear −1 Stronach proﬁle [21]. ,e 700·Mg·ha value also exceeds the accounting framework for carbon stocks and ﬂows in highest published forest soil C ﬁgure known to the authors Tasmania’s forests” ([28], p. 10), are overestimated and for a forested nonswamp site in the Australasian region incorrect. Similarly, the modelled losses in soil C from “old- −1 (600 Mg·ha ) measured in a Papua New Guinea soil formed growth” (mixed) eucalypt forests after land-use change −1 in gabbroic alluvium [58] and is more than three times the (e.g., the 300 Mg·ha · C loss modelled by Dean and mean mixed forest soil C ﬁgure in this study (Table 3). Wardell-Johnson [27]) based on the unreferenced −1 ,ere are numerous references to total soil C in Tas- 700 Mg·ha baseline is, in turn, overestimated; the esti- manian forests by May et al. [28] but the soil depth to which mated loss exceeds the total C held in most forest soils as their ﬁgures refer is not speciﬁed, although one estimate determined in this and previous studies (Table 1). C/N ratio C/N ratio 10 International Journal of Forestry Research fPOM fPOM oPOM>20 oPOM>20 oPOM<20 oPOM<20 Clay Clay 300 200 100 0 300 200 100 0 Chemical shi (ppm) Chemical shi (ppm) (a) (b) Figure 6: Site 5 Ferralsols: NMR spectra of fPOM, oPOM, and clay fractions from the B1 horizons (uppermost mineral soil horizons) under rainforest (a) and mixed forest (b). ,e prominent aryl-C peak in the fPOM fraction under mixed forest is arrowed. In their baseline map of soil C at 0–30 cm depth, Is the similarity of mean soil C values under mixed forest and designed to support national carbon accounting in Australia, rainforest a result of soils under rainforest not yet having Viscarra Rossell et al. [39] estimated that the largely forested attained equilibrium with their “new” lower-biomass forest −1 lands of western Tasmania contained 161–220 Mg·ha of C cover? (2) Has soil C under mixed forest been enhanced in soils at 0–30 cm depth. When compared to the range of C (above levels expected in forests subject to frequent ﬁre) by values found in this study for soils with few or no stones an intermittent supply of relatively inert charcoal? (3) Does −1 (52–144 Mg·ha ; Table 3), the modelled range appears to be the soil C measured in these studies represent the balance of an overestimate, especially as large areas of forested western C input and breakdown over millennia rather than centuries Tasmania are steeplands with stony soils. Consequently, the and is what we measure now the result of inherited soil C mean C ﬁgure for all Tasmanian soils at 0–30 cm depth accumulation independent of present vegetation cover? −1 Although NMR results for a greater range of sites would (134 Mg·ha , [39]) may also have been overestimated. ,e similarity of mean measured soil C stocks under be advantageous for determining organic matter processes mixed forest and rainforest, when contrasted with the large in these soils, the NMR results obtained, together with the diﬀerence in the biomass C of the two forest types (Table 4) observation that charcoal is present in both eucalypt and and likely greater biomass and C inputs to the soil under rainforest soils, and the radiocarbon ages on buried charcoal, mixed forest than under rainforest, raises three questions: (1) allow tentative answers to be provided for the 3 questions International Journal of Forestry Research 11 300 200 100 0 –100 Chemical shi (ppm) (a) (b) Figure 7: (a) Organic matter/clay coatings on blocky peds in the subsoil (the Bhs horizon) of the Podzol in sandy colluvium under rainforest at Eleven Road. Vertical section, 10 cm from top to bottom. (b) NMR spectrum for the organic matter/clay coatings; from right to left, the four peaks represent alkyl-C (0–50 ppm), O-alkyl-C (50–110 ppm), aryl-C (110–145 ppm; arrowed), and O-aryl-C (145–160 ppm). −1 posed above. All three propositions appear to be correct: (1) average 3.5% increase of topsoil C (from 145 to 150 Mg·ha at rainforest soils have not yet attained equilibrium with their 0–10 cm depth) after harvest and burning in three wet eucalypt “new” lower-biomass forest cover; they still contain a coupes (harvest areas). ,e results were not statistically ana- chemical signature of past ﬁres; (2) NMR results show that lysed, but Slijepcevic ([61], p. 285) considered that they “did not soil C levels under mixed forest (and to a lesser extent under provide any evidence of carbon loss or gain from the upper rainforest) have been enhanced by intermittent supply of layers after burning.” Ellis and Graley [62] found that imme- relatively inert (pyrogenic) carbon, as well as other stable diately after “hot” (intense) regeneration burns following eu- carbon fractions; and (3) measured soil C under rainforest calypt harvest at two locations in Tasmania, C in topsoil −1 retains characteristics of mixed forest cover, and both the (0–10 cm depth) decreased by 6.4–7.4 Mg·ha , but long-term amount and character of the C in the soil under these trends were not investigated. In Victoria, Australia, Polglase vegetation types is probably largely inherited and has been et al. [63] found that in E. regnans forests regenerating after ﬁre, determined by processes acting over thousands of years. soil C reached 86% of equilibrium (steady state) values when the However, soil organic matter characteristics are also forest was aged 30 years, with “true equilibrium” not being inﬂuenced by present-day vegetation. ,e greater relative reached until about 150 years. ,e eﬀect of regular ﬁres on soil amounts of pyrogenic C (and aryl-C) in Ferralsol surface B1 properties in wet eucalypt forests was addressed by Guinto et al. horizons under mixed forest and the higher mean C/N ratio [64] who measured a signiﬁcant 1.9% decline in C in soils at of subsoils under mixed forest than under rainforest is likely 0–10 cm depth in wet sclerophyll forest soils burnt every two to be a consequence of greater ﬁre frequency in the eucalypt- years for 20 years in southeast Queensland. Assuming a topsoil −1 dominated mixed forests than in the adjacent rainforests. EBD of 0.5 Mg·ha for these 0–10 cm soils, this 20-year decline −1 ,ese eﬀects are discussed below. equates to a C loss of 9 Mg·ha . In contrast, Krishnaraj [19] found that topsoil C under eucalypt forests of the Otway Ranges of southern Victoria increased by 2% after a forest ﬁre due to 4.2. Changes of Soil C after Fire. Measures of short-term accumulation of charcoal. change of soil C stocks after ﬁres have yielded ambiguous ,e diﬀerent results obtained by these authors probably results. In a global meta-analysis, Johnson and Curtis [59] noted reﬂect the problem of adequately covering postdisturbance an increase of soil C after wildﬁre (attributed to the seques- soil variation by the sampling strategies used. Until more tration of pyrogenic C and contributions from postﬁre nitro- comprehensive work is done, results of these case studies gen-ﬁxing plants) but a decrease after prescribed fuel-reduction should not be used to generalise about the short-term eﬀect burns. Pennington et al. [60] noted signiﬁcant declines of topsoil of harvest and burning on soil C. ,e eucalypt forests in- C concentration at 0–5 cm and 5–10 cm depth (from 8.04 to vestigated in this study must have experienced stand- 5.40% and from 4.12 to 3.41%, respectively) after harvest and a destroying ﬁres more recently than the rainforests, but the regeneration burn in tall wet eucalypt forests at a southern eﬀects of ﬁres is not evident in mean total C values. Tasmania site. However, the diﬀerence between unburnt and −1 burnt topsoils at 0–30 cm depth (5 Mg·ha ) was not statistically signiﬁcant once bulk density had been taken into account. No 4.3. Sequestration of Carbon by Wet Eucalypt Forest. correction was made for stones, but stone content “was shown Many authors (e.g., Keith et al. [6], Mackey et al. [15], Dean to be very low” [60]. In contrast, Slijepcevic [61] noted an and Wardell-Johnson [27], May et al. [28], and Dean et al. 12 International Journal of Forestry Research [65]) have promoted the protection of eucalypt forests to harvesting, or storm damage) nor that present after dis- mitigate the eﬀects of climate change. ,e weaknesses in the turbance represents the potential biomass in the landscape as a whole, and the true biomass potential at a landscape scale arguments presented by these authors have been highlighted by Moroni et al. [67] and McIntosh and Moroni [29] who will always lie between the disturbed and undisturbed ex- pointed out that (1) mature eucalypt forests are not the tremes. ,e biomass C in the landscape is always in a state of steady-state end point of forest succession and (2) that a ﬂux and cannot be deﬁned. concept originally designed to assess C sequestration po- Even the concepts of “undisturbed” (Mackey et al. [15]) tential in ecosystems (Gupta and Rao [68]) cannot be applied and “natural” (Keith et al. [66]) must be used cautiously, to landscapes as a management tool. ,e relevance of this since Aboriginal management has profoundly aﬀected study to these two issues is discussed below. forest structure in Tasmania for many thousands of years. Early European settlers remarked on ﬁre use by the Ab- original population [69–72]. Frequent ﬁres probably lit by 4.3.1. Forest Succession. ,e soil studies summarised in the Aboriginal population maintained eucalypts and Table 3 and Figures 3 and 4 indicate that during the suc- grasslands in upland northeast Tasmania and prevented cession from mixed forest to rainforest, no change in mean expansion of rainforests [73]. ,e pollen and charcoal soil C stocks is detectable. Given the long-term stability of record in deep peats on Surrey Hills in northwest Tas- the soils as indicated by NMR analyses and the lower bio- mania and on the Nicholas Range in northeast Tasmania mass (and probably lower biomass inputs into the soil) provides evidence of ﬁres and ﬁre-induced open eucalypt under rainforest, and previous measurements of total C in woodlands being present for at least the last 12 000 years Tasmanian proﬁles (Table 1), it appears unlikely that more [74–76]. detailed and statistically robust studies would demonstrate We can conclude that many eucalypt forests had a more −1 that the loss of biomass C (259 Mg·ha ) during the mixed open character under Aboriginal management than they do forest/rainforest transition is compensated by an increase in at present, and that regular low-intensity ﬁres (mostly soil C. (,e mean value of soil C measured under rainforests burning understorey only) periodically released only in this study would need to be 2.5 times higher to com- modest amounts of C to the atmosphere. We can be rea- pensate for the biomass C loss.) Consequently, it can be sonably certain that at least during the Holocene, the concluded that the C loss resulting from the mixed forest/ Tasmanian lowland and midaltitude vegetation pattern −1 rainforest transition is around 260 Mg·ha . If we assume before European arrival was a mosaic of ﬁre-induced forest, that wet eucalypt forests are tallest (and contain most C) rainforest, open woodland, grassland, and sedge-domi- when they are 400–500 years old, that all eucalypt trees have nated moorlands, a pattern noted by early surveyors such fallen after another 150 years, and that fallen trees have as Lawrence [77] in the Florentine Valley near Maydena rotted away after another 250 years, then we can calculate and Hellyer [71] in northwest Tasmania, which in part that during the mixed forest/rainforest transition, was anthropogenically induced and which, incidentally, −1 260 Mg·ha of C will be released to the atmosphere over an makes description of the Tasmanian wet forests as a approximately 400-year period, i.e., C is lost at a rate of “wilderness,” e.g., “Tasmanian Wilderness World Heritage −1 −1 about 0.65 Mg·ha year . ,us, during natural succession, Area,” a misnomer. For a brief period from approximately mature wet eucalypt forests are signiﬁcant C emitters, not C 1825 (by which time, the Aboriginal population was se- accumulators. verely depleted) to 1920 (when mechanised harvest began in earnest), forest cover was neither controlled by frequent Aboriginal land management nor disturbed by clearfelling 4.3.2. Forest Management. Despite acknowledging that “the role of ﬁre in natural forests is complex and must be con- operations, although timber was extracted from more ac- sidered on a landscape-wide basis” (Mackey et al. [15], p. 20), cessible sites [78]. In this period, for the ﬁrst time in 12000 these authors underestimated the eﬀect of ﬁres on C ac- years, many forests previously managed by Aboriginal cumulation at the landscape scale, even though eucalypt burning grew more densely and regenerated without large- scale human intervention, greatly increasing aboveground ecosystems are ﬁre-induced. For example, Mackey et al. [15] (p. 23 and 26) excluded bushﬁre-aﬀected areas when cal- biomass and fuel for ﬁres. Possibly partly because of this post-1825 fuel increase culating gross primary production, and living biomass C was calculated only for “undisturbed” forests. ,e limitation of and cessation of low-intensity burning, there have been a number of large ﬁres since European settlement in Tas- ﬁres on C accumulation is evident in areas of Tasmania and the southeast Australian mainland aﬀected by the 2019 and mania. ,e largest was in 1898 when over 1 million ha and 2020 bushﬁres (discussed below): while not all burnt eu- possibly up to 2 million ha of forests in the southwest burned calypts are killed by ﬁre, the overall eﬀect of crown ﬁres is to [28]. Other very large ﬁres were in 1934 when 800 000–900 halt landscape-scale C accumulation or reverse it. Because 000 ha burned [79] and in 1967 when approximately 250 eucalypt forests in their natural state will either burn and 000 ha burned [80]. Recently, over 2400 recorded lightning regenerate or transition to rainforest in the absence of ﬁre, it strikes in Tasmania in December 2018 and January 2019 caused 72 vegetation ﬁres extending over 205 000 ha in- is not correct to assume that mature eucalypt forests are a “baseline” against which to judge land-use change, as done, cluding c. 100 000 ha of wet eucalypt forests and 14% (296 ha) of Tasmania’s tall (>70 m high) eucalypt forests and for example, by May et al. ([28], p. 274). Neither the biomass present before disturbance (whether the result of burning, 7000 ha of rainforest [81]. International Journal of Forestry Research 13 of C stocks from the soil data of Grant et al. [21], Hill et al. 5. Conclusions [22], and Laﬀan et al. [23] are available as an excel Previous studies showed that soil C to 1 m depth (or a rock spreadsheet from the corresponding author. contact) under wet eucalypt cover ranged from 35 to −1 325 Mg·ha , depending on stoniness, soil depth, and parent Disclosure material, but provided little data on soil C in rainforest soils, ,is paper is partly based on the unpublished research theses so the soil C trend (if any) during the ecological transition of of J. Hardcastle and T. Kloﬀel. ¨ mixed eucalypt forests (mature tall wet eucalypts with a rainforest understorey) to rainforests could not be established. Conflicts of Interest ,is study of 7 paired sites found no consistent diﬀer- ence between soil C stocks under mixed forests or rainforest None of the authors have conﬂicts of interest regarding the in Tasmania, and the mean soil C value under both forest −1 conduct of this study or the interpretation of its results. types is about 200 Mg·ha . Consequently, at a landscape −1 scale, the loss of biomass C (259 Mg·ha ) during the Acknowledgments transition of mixed eucalypt forests (mean biomass C, −1 −1 473 Mg·ha ) to rainforest (mean biomass C, 214 Mg·ha ) ,e authors are grateful to Elizabeth Brewer, Adrian Slee, (Table 4) approximates to the total ecosystem loss and Carsten Muller ¨ who assisted with ﬁeldwork, Peter Schad −1 (269 Mg·ha ) during this ecological transition (Table 4). for checking FAO soil classiﬁcations, and Carsten Muller, ¨ If it is assumed that the transition to rainforest takes Mark Poynter, and two anonymous reviewers for their 400 years, then the mixed forests are emitting about valuable comments on drafts of the text. ,e Board of the −1 −1 0.65 Mg·ha yr until they reach pure rainforest. During Forest Practices Authority provided funds to support the this ecological transition, the soils remain (on average) research and travel costs of James Hardcastle, Tobias Kloﬀel, ¨ relatively stable: deep soil horizons under rainforest retain and Carsten Muller. ¨ ,e authors thank Sustainable Timber an imprint of pyrogenic C from past ﬁres, and the soils Tasmania and the Department of Primary Industries, Parks, appear to be buﬀered against the eﬀects of vegetation change Water, and the Environment for allowing access to sites. and variable C inputs. ,e observation that Tasmanian tall wet eucalypt forests References become net C emitters as they mature and transition to rainforests is contrary to trends monitored in northern [1] R. A. Houghton, “Balancing the global carbon budget,” An- nual Review of Earth and Planetary Sciences, vol. 35, no. 1, hemisphere temperate forests in which C continues to ac- pp. 313–347, 2007. cumulate as forests age [12–14]. 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Journal

International Journal of Forestry Research – Hindawi Publishing Corporation

Published: Jul 30, 2020

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M. J. Gardner and D. G. Altman
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Comparison of soil organic matter quality under wet eucalypt and old-growth rainforests in Tasmania,

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P. Pennington, M. Laffan, R. Lewis, and P. Otahal
Loss of carbon during controlled regeneration burns in Eucalyptus forest,

A. Slijepcevic
Gains and losses in soil nutrients associated with harvesting and burning eucalypt rainforest,

R. C. Ellis and A. M. Graley
Soil chemical properties and forest floor nutrients under repeated prescribed burning in eucalypt forests of south-east Queensland, Australia,

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Forecasting landscape-level carbon sequestration using gridded, spatially adjusted tree growth,

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H. Keith, B. Mackey, S. Berry, and D. Lindenmayer
The wood, the trees, or the forest? Carbon in trees in Tasmanian state forest: a response to comments,

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R. K. Gupta and D. L. N. Rao
Fire-stick farming,

R. Jones
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O. Moss, A. Farrell, J. Vink, P. McIntosh, and A. Slee
Historic timber-getting in the southern forests. Industry overview and assessment of its technology,

P. Kostoglou
Changes in southwestern Tasmanian fire regimes since the early 1800s,

J. Marsden-Smedley
Section 4: Tasmania,

B. Walker and K. Felton
A review of the management of the Tasmanian Fires of December 2018–March 2019,

M. Cronstedt, G. Thomas, and P. Considine
Carbon sequestration in forests, addressing the scale question,

M. E. Harmon
Leadbeater’s possum and Victoria’s Central Highlands’ forests: flawed science and environmental activism as drivers of forest management change,

M. Poynter and M. Ryan

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Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account

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Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account

Can Carbon Sequestration in Tasmanian “Wet” Eucalypt Forests Be Used to Mitigate Climate Change? Forest Succession, the Buffering Effects of Soils, and Landscape Processes Must Be Taken into Account

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