Mycorrhizal responses to nitrogen fertilization in boreal ecosystems: potential consequences for soil carbon storage

Mycorrhizal fungi can contribute to soil carbon sequestration by immobilizing carbon in living fungal tissues and by producing recalcitrant compounds that remain in the soil following fungal senescence. We hypothesized that nitrogen (N) fertilization would decrease these carbon stocks, because plants should reduce investment of carbon in mycorrhizal fungi when N availability is high. We measured the abundance of two major groups of mycorrhizal fungi, arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi, in the top 10 cm of soil in control and N-fertilized plots within three Alaskan boreal ecosystems that represented different recovery stages following severe fire. Pools of mycorrhizal carbon included root-associated AM and ECM structures; soil-associated AM hyphae; and glomalin, a glycoprotein produced by AM fungi. Total mycorrhizal carbon pools decreased by approximately 50 g C m⁻² in the youngest site under N fertilization, and this reduction was driven mostly by glomalin. Total mycorrhizal carbon did not change significantly in the other sites. Root-associated AM structures were more abundant under N fertilization across all sites, and root-associated ECM structures increased marginally significantly. We found no significant N effects on AM hyphae. Carbon sequestered within living mycorrhizal structures (0.051–0.21 g m⁻²) was modest compared with that of glomalin (33–203 g m⁻²). We conclude that our hypothesis was only supported in relation to glomalin stocks within one of the three study sites. As N effects on glomalin were inconsistent among sites, an understanding of the mechanisms underlying this variation would improve our ability to predict ecosystem feedbacks to global change.     

Long-term impacts of anthropogenic perturbations on dynamics and speciation of organic carbon in tropical forest and subtropical grassland ecosystems

Anthropogenic perturbations have profoundly modified the Earth's biogeochemical cycles, the most prominent of these changes being manifested by global carbon (C) cycling. We investigated long‐term effects of human‐induced land‐use and land‐cover changes from native tropical forest (Kenya) and subtropical grassland (South Africa) ecosystems to agriculture on the dynamics and structural composition of soil organic C (SOC) using elemental analysis and integrated ¹³C nuclear magnetic resonance (NMR), near‐edge X‐ray absorption fine structure (NEXAFS) and synchrotron‐based Fourier transform infrared‐attenuated total reflectance (Sr‐FTIR‐ATR) spectroscopy. Anthropogenic interventions led to the depletion of 76%, 86% and 67% of the total SOC; and 77%, 85% and 66% of the N concentrations from the surface soils of Nandi, Kakamega and the South African sites, respectively, over a period of up to 100 years. Significant proportions of the total SOC (46–73%) and N (37–73%) losses occurred during the first 4 years of conversion indicating that these forest‐ and grassland‐derived soils contain large amounts of labile soil organic matter (SOM), potentially vulnerable to degradation upon human‐induced land‐use and land‐cover changes. Anthropogenic perturbations altered not only the C sink capacity of these soils, but also the functional group composition and dynamics of SOC with time, rendering structural composition of the resultant organic matter in the agricultural soils to be considerably different from the SOM under natural forest and grassland ecosystems. These molecular level compositional changes were manifested: (i) by the continued degradation of O‐alkyl and acetal‐C structures found in carbohydrate and holocellulose biomolecules, some labile aliphatic‐C functionalities, (ii) by side‐chain oxidation of phenylpropane units of lignin and (iii) by the continued aromatization and aliphatization of the humic fractions possibly through selective accumulation of recalcitrant H and C substituted aryl‐C and aliphatic‐C components such as (poly)‐methylene units, respectively. These changes appeared as early as the fourth year after transition, and their intensity increased with duration of cultivation until a new quasi‐equilibrium of SOC was approached at about 20 years after conversion. However, subtle but persistent changes in molecular structures of the resultant SOM continued long after (up to 100 years) a steady state for SOC was approached. These molecular level changes in the inherent structural composition of SOC may exert considerable influence on biogeochemical cycling of C and bioavailability of essential nutrients present in association with SOM, and may significantly affect the sustainability of agriculture as well as potentials of the soils to sequester C in these tropical and subtropical highland agroecosystems.     

土壤溶解性有机碳在陆地生态系统碳循环中的作用

土壤溶解性有机碳(DOC)是有机碳库的活跃组分,在陆地生态系统碳循环中发挥重要作用.本文从碳循环重要性着手,综述了土壤DOC在土壤碳固持与温室气体排放中的作用;结合我国的现实情况(如土壤酸化、气候变暖等),探讨了土壤DOC的相关影响因素如土壤性质、环境因素、人为活动对土壤DOC的影响及作用机制,对进一步理解土壤DOC在陆地生态系统碳循环与温室气体减排中的作用具有重要意义.     

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

To evaluate the carbon budget of a boreal deciduous forest, we measured CO₂ fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m^–2 y^–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m^–2 y^–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 10⁵ km²), it is likely they can sequester 40–60 Tg C y^–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m^–2 y^–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m^–2 y^–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence.     

Net ecosystem carbon exchange in two experimental grassland ecosystems

Increases in net primary production (NPP) may not necessarily result in increased C sequestration since an increase in uptake can be negated by concurrent increases in ecosystem C losses via respiratory processes. Continuous measurements of net ecosystem C exchange between the atmosphere and two experimental cheatgrass (Bromus tectorum L.) ecosystems in large dynamic flux chambers (EcoCELLs) showed net ecosystem C losses to the atmosphere in excess of 300 g C m^?2 over two growing cycles. Even a doubling of net ecosystem production (NEP) after N fertilization in the second growing season did not compensate for soil C losses incurred during the fallow period. Fertilization not only increased C uptake in biomass but also enhanced C losses through soil respiration from 287 to 469 g C m^?2, mainly through an increase in rhizosphere respiration. Fertilization decreased dissolved inorganic C losses through leaching of from 45 to 10 g C m^?2. Unfertilized cheatgrass added 215 g C m^?2 as root‐derived organic matter but the contribution of these inputs to long‐term C sequestration was limited as these deposits rapidly decomposed. Fertilization increased NEP but did not increase belowground C inputs most likely due to a concurrent increase in the production and decomposition of rhizodeposits. Decomposition of soil organic matter (SOM) was reduced by fertilizer additions. The results from our study show that, although annual grassland ecosystems can add considerable amounts of C to soils during the growing season, it is unlikely that they sequester large amounts of C because of high respiratory losses during dormancy periods. Although fertilization could increase NEP, fertilization might reduce soil C inputs as heterotrophic organisms favor root‐derived organic matter over native SOM.     

基于森林清查资料的江西和浙江森林植被固碳潜力

以我国江西、浙江两省的森林植被为研究对象,基于1999-2003年间第六次全国森林清查数据及收集的1030个亚热带森林样地文献资料,依据林分生长的经验方程,估算了两个地区森林2004-2013年的固碳潜力,并基于455个样点的调查数据研究了不同森林管理措施(纯林间种、间伐、施肥)对森林未来固碳潜力的影响.结果表明:第六次森林清查以来的10年(2004-2013)间,江西森林植被年均自然固碳潜力约11.37 Tg C·a-1(1Tg=1012g),而浙江省森林植被年均自然固碳潜力约4.34 Tg C·a-1.纯林间种对江西、浙江两省森林植被固碳潜力影响最大,其次为间伐抚育,施肥的影响最小,纯林间种、间伐和施肥3种森林管理措施使江西省森林植被固碳潜力分别提高(6.54±3.9)、(3.81±2.02)和(2.35±0.6) Tg C·a-1,浙江省森林植被固碳潜力分别提高(2.64±1.28)、(1.42±0.69)和(1.15±0.29) Tg C·a-1.     

Quantifying uncertainty from large-scale model predictions of forest carbon dynamics

Linking environmental computer simulation models and geographic information systems (GIS) is now a common practice to scale up simulations of complex ecosystem processes for decision support. Unfortunately, several important issues of upscaling using GIS are rarely considered; in particular scale dependency of models, availability of input data, support of input and validation data, and uncertainty in prediction including error propagation from the GIS. We linked the biogeochemical Forest‐DNDC model to a GIS database to predict growth of Eucalyptus globulus plantations at two different scales (～0.045 ha plot^?1 scale and ～100 ha grid^?1 scale) across Victoria, in south‐eastern Australia. Results showed that Forest‐DNDC was not scale dependent across the range of scales investigated. Reduced availability of input data at the larger scale may introduce severe prediction errors, but did not require adjustment of the model in this study. Differences in the support of input and validation data led to an underestimation of predictive precision but an overestimation of prediction accuracy. Increasing data support, produced a high level of prediction accuracy (^?e%), but a medium level of predictive precision (r²=0.474, ME=0.318) after statistical validation. GIS error contribution could be detected but was not readily or reliably quantified. In a regional case study for 2653 ha of E. globulus plantations, the linked model GIS system estimated a total standing biomass of 95 260 t C for mid‐2003 and a net CO₂ balance of ?45 671 t CO₂‐C yr^?1 for the entire year of 2002. This study showed that regional predictions of forest growth and carbon sequestration can be produced with greater confidence after a comprehensive assessment of upscaling issues.     

The drivers of woody species richness and density in a Neotropical savannah

Environmental filtering prevents species without certain attributes from occurring in local communities. Traits respond differently to different abiotic factors, assembling communities with varying composition along environmental gradients. Here, we measured proxies of soil fertility, disturbance by fire, response and physiological traits to assess how these variables interact to determine woody species richness and density in a Neotropical savannah. We explicitly incorporated our assumptions about how different abiotic filters influence different subsets of traits into a statistical model using structural equation modelling, yielding a more accurate representation of the assembly process. Fire had an effect on resistance traits, whereas soil fertility influenced physiological traits. Resistance traits explained both the richness and density of plots, whereas physiological traits explained only the density. Fewer fire events led to richer and denser plots. Similarly, areas with lower cation exchange capacity assembled less dense communities. Furthermore, we showed that structural equation modelling yielded a realistic representation of the bivariate interactions of distinct environmental filters with different subsets of traits.     

Contemporary carbon stocks of mineral forest soils in the Swiss Alps

Soil organic carbon (SOC) has been identified as the main globalterrestrial carbon reservoir, but considerable uncertainty remains as toregional SOC variability and the distribution of C between vegetationand soil. We used gridded forest soil data (8–km × 8–km)representative of Swiss forests in terms of climate and forest typedistribution to analyse spatial patterns of mineral SOC stocks alonggradients in the European Alps for the year 1993. At stand level, meanSOC stocks of 98 t C ha^–1 (N = 168,coefficient of variation: 70%) were obtained for the entiremineral soil profile, 76 t C ha^–1 (N =137, CV: 50%) in 0–30 cm topsoil, and 62 t Cha^–1 (N = 156, CV: 46%) in0–20 cm topsoil. Extrapolating to national scale, we calculatedcontemporary SOC stocks of 110 Tg C (entire mineral soil, standarderror: 6 Tg C), 87 Tg C (0–30 cm topsoil, standarderror: 3.5 Tg C) and 70 Tg C (0–20 cm topsoil, standarderror: 2.5 Tg C) for mineral soils of accessible Swiss forests(1.1399 Mha). According to our estimate, the 0–20 cm layers ofmineral forest soils in Switzerland store about half of the Csequestered by forest trees (136 Tg C) and more than five times morethan organic horizons (13.2 Tg C).At stand level, regression analyses on the entire data set yielded nostrong climatic or topographic signature for forest SOC stocks in top(0–20 cm) and entire mineral soils across the Alps, despite thewide range of values of site parameters. Similarly, geostatisticalanalyses revealed no clear spatial trends for SOC in Switzerland at thescale of sampling. Using subsets, biotic, abiotic controls andcategorial variables (forest type, region) explained nearly 60%of the SOC variability in topsoil mineral layers (0–20 cm) forbroadleaf stands (N = 56), but only little of thevariability in needleleaf stands (N = 91,R ² = 0.23 for topsoil layers).Considerable uncertainties remain in assessments of SOC stocks, due tounquantified errors in soil density and rock fraction, lack of data onwithin-site SOC variability and missing or poorly quantifiedenvironmental control parameters. Considering further spatial SOCvariability, replicate pointwise soil sampling at 8–km × 8–kmresolution without organic horizons will thus hardly allow to detectchanges in SOC stocks in strongly heterogeneous mountain landscapes.     

Estimating soil carbon sequestration under elevated CO2 by combining carbon isotope labelling with soil carbon cycle modelling

Elevated CO₂ concentrations generally stimulate grassland productivity, but herbaceous plants have only a limited capacity to sequester extra carbon (C) in biomass. However, increased primary productivity under elevated CO₂ could result in increased transfer of C into soils where it could be stored for prolonged periods and exercise a negative feedback on the rise in atmospheric CO₂. Measuring soil C sequestration directly is notoriously difficult for a number of methodological reasons. Here, we present a method that combines C isotope labelling with soil C cycle modelling to partition net soil sequestration into changes in new C fixed over the experimental duration (C_new) and pre‐experimental C (C_old). This partitioning is advantageous because the C_new accumulates whereas C_old is lost in the course of time (ΔC_new>0 whereas ΔC_old<0). We applied this method to calcareous grassland exposed to 600 μL CO₂ L^?1 for 6 years. The CO₂ used for atmospheric enrichment was depleted in ¹³C relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil C_new fluxes under elevated CO₂. Using ¹³C/¹²C mass balance and inverse modelling, the Rothamsted model ‘RothC’ predicted gross soil C_new inputs under elevated CO₂ and the decomposition of C_old. The modelled soil C pools and fluxes were in good agreement with experimental data. C isotope data indicated a net sequestration of ≈90 g C_new m^?2 yr^?1 in elevated CO₂. Accounting for C_old‐losses, this figure was reduced to ≈30 g C m^?2 yr^?1 at elevated CO₂; the elevated CO₂‐effect on net C sequestration was in the range of≈10 g C m^?2 yr^?1. A sensitivity and error analysis suggests that the modelled data are relatively robust. However, elevated CO₂‐specific mechanisms may necessitate a separate parameterization at ambient and elevated CO₂; these include increased soil moisture due to reduced leaf conductance, soil disaggregation as a consequence of increased soil moisture, and priming effects. These effects could accelerate decomposition of C_old in elevated CO₂ so that the CO₂ enrichment effect may be zero or even negative. Overall, our findings suggest that the C sequestration potential of this grassland under elevated CO₂ is rather limited.     

Climate change effects on net carbon exchange of a boreal aspen–hazelnut forest: estimates from the ecosystem model ecosys

Although boreal forests are currently sinks for atmospheric C, there is some concern that they may not remain so under hypothesized warming of the boreal climate. The ecosystem model ecosys was used to evaluate possible changes in ecosystem C exchange and accumulation under changes in atmospheric CO₂ concentration (C_a) proposed in emissions scenario IS92a, and accompanying changes in air temperature and precipitation proposed by general circulation models running under IS92a. Ecosys was first tested under current climate by comparing modelled rates of C exchange and accumulation with those measured in a mixed aspen–hazelnut stand in central Saskatchewan. The model was then run with daily increments of C_a, temperature and precipitation, and differences in C exchange and accumulation between current and changing climates were evaluated. Model results indicated that over a 120‐y period, a mixed aspen–hazelnut stand currently accumulates about 14 kg C m^?2. Under the hypothesized changes in climate this stand would accumulate an additional 8.5 kg C m^?2, largely through higher rates of CO₂ fixation and longer growing seasons under higher C_a and temperature. This additional accumulation would be entirely as aspen wood, while soil organic matter would change little. This accumulation would therefore be vulnerable to losses from fire and insects.     

13C natural abundance variations in carbonates and organic carbon from boreal forest wetlands

¹³C natural abundance variations were measured in peat soil and vegetation from two contrasting boreal forest wetlands: an upland watershed basin and a permanently saturated lowland mire. Evidence of methane oxidation was shown in the permanently saturated wetland with δ¹³C values as low as -97 ‰ in carbonate minerals found in floating peat mats. It is postulated that¹³C depleted CH₄ is oxidized in the mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Methane flux measurements during the summer of 1992 showed much lower fluxes in areas with floating peat mats relative to open water. Secondary carbonates in the basin peat have isotope compositions close to the δ¹³C values of the peat organic carbon (-25 ‰), indicating their origin from fermentation and possibly from sulfate-reduction. In the upland basin peat deposits, the δ¹³C_PDB values of organic C were constant with depth, while the permanently saturated mire had localities of¹³C enrichment in deeper layers of the peat. The¹³C enrichment may reflect areas of intense CH₄ production in which¹³C enriched residual substrate is left behind during the production of highly¹³C depleted CH₄.     

Contribution of trees to carbon storage in soils of silvopastoral systems in Florida, USA

Silvopastoral systems that integrate trees in pasture production systems are likely to enhance soil carbon (C) storage in lower soil layers due to the presence of deep tree roots. To quantify the relative soil C contribution from trees (C3 plants) and warm season grasses (C4 plants) in silvopastoral systems, soil samples were collected and analyzed from silvopastures of slash pine ( Pinus elliottii )+bahiagrass ( Paspalum notatum ), and adjacent open pasture (OP), at six depths down to 125 cm, at four sites representing two major soil orders (Spodosols and Ultisols) of Florida. The plant sources of C in whole (nonfractionated) and three soil fraction sizes (250–2000, 53–250, and <53 μm) were traced using stable C isotope signatures. The silvopasture sites contained higher amounts of C3-derived soil organic carbon (SOC) compared with OP sites, at all soil depths. Slash pine trees (C3 plants) seemed to have contributed more C in the silt+clay-sized (<53 μm) fractions than bahiagrass (C4 plants), particularly deeper in the soil profile. Spodosols sites contained more C in the <53 μm fraction at and below the spodic horizon (occurring between 15 and 50 cm) in silvopasture compared with OP. The results indicate that most of SOC in deeper soil profiles and the relatively stable <53 μm C fraction were derived from tree components (C3 plants) in all the sites, suggesting that the tree-based pasture system has greater potential to store more stable C in the soil compared with the treeless system.     

吉林省森林生态系统的碳储量、碳密度及其分布

利用森林资源二类调查汇总数据和标准地实测数据,研究吉林省森林生态系统的碳密度、碳储量及其组分和分布特征.结果表明:吉林省森林生态系统碳储量为1827.293TgC,其中乔木层、灌草层、枯落物层和土壤层的碳储量分别为439.152、5.195、45.600和1330.466TgC,分别占总碳量的24.1%、0.3%、2.5%和73.1%.吉林省森林生态系统碳密度为225.304MgC.hm-2,各层碳密度的大小顺序为土壤层(164.666MgC.hm-2)>乔木层(54.352MgC.hm-2)>枯落物层(5.644MgC.hm-2)>灌草层(0.643MgC.hm-2).不同类型森林生态系统碳储量在9.357～959.716TgC,碳密度在180.648～254.627MgC.hm-2之间,各林型分配特征表现为土壤层最大、灌草层最小.全省森林生态系统碳储量和碳密度的空间分布总体上为东部山区高、中西部平原地区低.吉林省森林中中龄林分比重大,若对现有森林加以更好的管理,可以增加其碳吸存潜力.     

Effects of rest grazing on organic carbon storage in Stipa Baicalensis steppe in Inner Mongolia

In order to evaluate the potential effects of rest grazing on organic carbon storage on the Stipa baicalensis steppe in Inner Mongolia, compared the S. baicalensis steppes after rest grazing for 3 years, 6 years, and 9 years, using potassium dichromate heating method, this study analyzed the organic carbon storage of plant and soil in the steppes among different periods of rest grazing. The results indicated that as the rest grazing years prolonged, the biomass included above-ground parts, litters and underground plant parts(roots) of the plant communities all increased, meanwhile the carbon content of the biomass increased with the rest grazing years prolonged. For the zero rest grazing (RG0) steppe and the steppes after a rest grazing of 3 years (RG3a), 6 years (RG6a), 9 years (RG9a), the carbon storage in above-ground parts of plant communities were 42.60 g C/m², 66.33 g C/m², 83.46 g C/m², 100.29 g C/m² respectively; the carbon storage of litters were 7.85 g C/m², 9.12 g C/m², 9.18 g C/m², 11.54 g C/m² separately; the carbon storage of underground plant parts (0–100 cm) were 281.40 g C/m², 576.38 g C/m², 745.33 g C/m², 1279.61 g C/m² respectively; and the carbon storage in 0–100 cm soil were 22991.14 g C/m², 24687.75 g C/m², 26564.86 g C/m²,33041.55 g C/m². The results suggested that as the rest grazing years prolonged, the organic carbon storage in plant communities and soil increased. The carbon storage of underground plant parts and soil organic carbon mainly concentrated in 0–40 cm soil. After rest grazing for 3 years, 6 years, and 9 years, the increased soil organic carbon were as the 81.14%, 85.84%, and 89.46% of the total increased carbon; From the perspective of carbon sequestration cost, the total cost of RG3a, RG6a and RG9a were 2903.40 RMB/hm², 5806.80 RMB/hm², and 8710.20 RMB/hm². The cost reduced with the extension of rest grazing years, 0.17 RMB/kg C, 0.16 RMB/kg C, 0.09 RMB/kg C for RG3a, RG6a and RG9a respectively. From the growth characteristics of grassland plants, the spring was one of the two avoid grazing periods, timely rest grazing could effectively restore and update grassland vegetation, and was beneficial to the sustainable use of grassland. From the available data, the organic carbon storage of RG9a was the highest, while the cost of carbon sequestration was the lowest. Therefore, spring rest grazing should be encouraged to continue for it was proved to be a very efficient grassland use measures.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

Estimating carbon carrying capacity in natural forest ecosystems across heterogeneous landscapes: addressing sources of error

Evaluating contributions of forest ecosystems to climate change mitigation requires well‐calibrated carbon cycle models with quantified baseline carbon stocks. An appropriate baseline for carbon accounting of natural forests at landscape scales is carbon carrying capacity (CCC); defined as the mass of carbon stored in an ecosystem under prevailing environmental conditions and natural disturbance regimes but excluding anthropogenic disturbance. Carbon models require empirical measurements for input and calibration, such as net primary production (NPP) and total ecosystem carbon stock (equivalent to CCC at equilibrium). We sought to improve model calibration by addressing three sources of errors that cause uncertainty in carbon accounting across heterogeneous landscapes: (1) data‐model representation, (2) data‐object representation, (3) up‐scaling. We derived spatially explicit empirical models based on environmental variables across landscape scales to estimate NPP (based on a synthesis of global site data of NPP and gross primary productivity, n=27), and CCC (based on site data of carbon stocks in natural eucalypt forests of southeast Australia, n=284). The models significantly improved predictions, each accounting for 51% of the variance. Our methods to reduce uncertainty in baseline carbon stocks, such as using appropriate calibration data from sites with minimal human disturbance, measurements of large trees and incorporating environmental variability across the landscape, have generic application to other regions and ecosystem types. These analyses resulted in forest CCC in southeast Australia (mean total biomass of 360 t C ha^?1, with cool moist temperate forests up to 1000 t C ha^?1) that are larger than estimates from other national and international (average biome 202 t C ha^?1) carbon accounting systems. Reducing uncertainty in estimates of carbon stocks in natural forests is important to allow accurate accounting for losses of carbon due to human activities and sequestration of carbon by forest growth.