Assessing the influence of historic net and gross land changes on the carbon fluxes of Europe

Legacy effects of land cover/use on carbon fluxes require considering both present and past land cover/use change dynamics. To assess past land use dynamics, model‐based reconstructions of historic land cover/use are needed. Most historic reconstructions consider only the net area difference between two time steps (net changes) instead of accounting for all area gains and losses (gross changes). Studies about the impact of gross and net land change accounting methods on the carbon balance are still lacking. In this study, we assessed historic changes in carbon in soils for five land cover/use types and of carbon in above‐ground biomass of forests. The assessment focused on Europe for the period 1950 to 2010 with decadal time steps at 1‐km spatial resolution using a bookkeeping approach. To assess the implications of gross land change data, we also used net land changes for comparison. Main contributors to carbon sequestration between 1950 and 2010 were afforestation and cropland abandonment leading to 14.6 PgC sequestered carbon (of which 7.6 PgC was in forest biomass). Sequestration was highest for old‐growth forest areas. A sequestration dip was reached during the 1970s due to changes in forest management practices. Main contributors to carbon emissions were deforestation (1.7 PgC) and stable cropland areas on peaty soils (0.8 PgC). In total, net fluxes summed up to 203 TgC yr^?1 (98 TgC yr^?1 in forest biomass and 105 TgC yr^?1 in soils). For areas that were subject to land changes in both reconstructions (35% of total area), the differences in carbon fluxes were about 68%. Overall for Europe the difference between accounting for either gross or net land changes led to 7% difference (up to 11% per decade) in carbon fluxes with systematically higher fluxes for gross land change data.     

Emissions of carbon from forestry and land-use change in tropical Asia

The net emissions of carbon from forestry and changes in land use in south and southeast Asia were calculated here with a book-keeping model that used rates of land-use change and associated per hectare changes in vegetation and soil to calculate changes in the amount of carbon held in terrestrial ecosystems and wood products. The total release of carbon to the atmosphere over the period 1850–1995 was 43.5 PgC. The clearing of forests for permanent croplands released 33.5 PgC, about 75% of the total. The reduction of biomass in the remaining forests, as a result of shifting cultivation, logging, fuelwood extraction, and associated regrowth, was responsible for a net loss of 11.5 PgC, and the establishment of plantations withdrew from the atmosphere 1.5 PgC, most of it since 1980. Based on comparisons with other estimates, the uncertainty of this long-term flux is estimated to be within ±30%. Reducing this uncertainty will be difficult because of the difficulty of documenting the biomass of forests in existence >40 years ago. For the 15-y period 1981–1995, annual emissions averaged 1.07 PgC y^–1, about 50% higher than reported for the 1980s in an earlier study. The uncertainty of recent emissions is probably within ± 50% but could be reduced significantly with systematic use of satellite data on changes in forest area. In tropical Asia, the emissions of carbon from land-use change in the 1980s accounted for approximately 75% of the region’s total carbon emissions. Since 1990 rates of deforestation and their associated emissions have declined, while emissions of carbon from combustion of fossil fuels have increased. The net effect has been a reduction in emissions of CO₂ from this region since 1990.     

Changes in terrestrial carbon storage in the United States. 1: The roles of agriculture and forestry

• 1 Changes in the areas of croplands and pastures, and rates of wood harvest in seven regions of the United States, including Alaska, were derived from historical statistics for the period 1700–1990. These rates of land‐use change were used in a cohort model, together with equations defining the changes in live vegetation, slash, wood products and soil that follow a change in land use, to calculate the annual flux of carbon to the atmosphere from changes in land use.
• 2 The calculated flux increased from less than 10 TgC/yr in 1700 to a maximum of about 400 TgC/yr around 1880 and then decreased to approximately zero by 1950. The total flux for the 290‐year period was a release of 32.6 PgC. The area of forests and woodlands declined by 42% (160 × 10⁶ ha), releasing 29 PgC, or 90% of the total flux. Cultivation of soils accounted for about 25% of the carbon loss. Between 1950 and 1990 the annual flux of carbon was approximately zero, although eastern forests were accumulating carbon.
• 3 When the effects of fire and fire exclusion (reported in a companion paper) were added to this analysis of land‐use change, the uptake of carbon calculated for forests was similar in magnitude to the uptake measured in forest inventories, suggesting that past harvests account for a significant fraction of the observed carbon sink in forests.
• 4 Changes in the management of croplands between 1965 and 1990 may have led to an additional accumulation of carbon, not included in the 32.6 PgC release, but even with this additional non‐forest sink, the calculated accumulation of carbon in the United States was an order of magnitude smaller than the North American carbon sink inferred recently from atmospheric data and models.

     

Land-use change and the carbon cycle

Changes in land use between 1850 and 1980 are estimated to have increased the global areas in croplands, pastures, and shifting cultivation by 891, 1308, and 30 × 10⁶ ha, respectively, reducing the area of forests by about 600 × 10⁶ ha, releasing about 100 PgC to the atmosphere, and transferring about 23 PgC from live vegetation to dead plant material and wood products. Another 1069 × 10⁶ ha are estimated to have been logged during this period, and the net release of carbon from the combined processes of logging and regrowth contributed 23 PgC to the 100-PgC release. Annual rates of land-use change and associated emissions of carbon have decreased over the last several decades in temperate and boreal zones and have increased in the tropics. The average release of carbon from global changes in land use over the decade of the 1980s Is estimated to have been 1.6 ± 0.7 PgC y^?1 almost entirely from the tropics. This estimate of carbon flux is higher than estimates reported in recent summaries because it is limited here to studies concerned only with changes in land use. Other recent analyses, based on data from forest inventories, have reported net accumulations of carbon as high as 1.1 PgC y^?1 in temperate and boreal zones. Because the accumulation of carbon in forests may result from natural processes unrelated to land-use change, estimates based on these inventories should be distinguished from estimates based on changes in land use. Both approaches identify terrestrial sinks of carbon. The argument is made here, however, that differences between the two approaches may help identify the location and magnitude of heretofore ‘missing’ sinks. Before different estimates can be used in this way, analyses must consider similar geographical regions and dates, and they must account for the accumulation and loss of carbon in forest products in a consistent fashion.     

Disturbance and climate effects on carbon stocks and fluxes across Western Oregon USA

We used a spatially nested hierarchy of field and remote‐sensing observations and a process model, Biome‐BGC, to produce a carbon budget for the forested region of Oregon, and to determine the relative influence of differences in climate and disturbance among the ecoregions on carbon stocks and fluxes. The simulations suggest that annual net uptake (net ecosystem production (NEP)) for the whole forested region (8.2 million hectares) was 13.8 Tg C (168 g C m^?2 yr^?1), with the highest mean uptake in the Coast Range ecoregion (226 g C m^?2 yr^?1), and the lowest mean NEP in the East Cascades (EC) ecoregion (88 g C m^?2 yr^?1). Carbon stocks totaled 2765 Tg C (33 700 g C m^?2), with wide variability among ecoregions in the mean stock and in the partitioning above‐ and belowground. The flux of carbon from the land to the atmosphere that is driven by wildfire was relatively low during the late 1990s (～0.1 Tg C yr^?1), however, wildfires in 2002 generated a much larger C source (～4.1 Tg C). Annual harvest removals from the study area over the period 1995–2000 were ～5.5 Tg C yr^?1. The removals were disproportionately from the Coast Range, which is heavily managed for timber production (approximately 50% of all of Oregon's forest land has been managed for timber in the past 5 years). The estimate for the annual increase in C stored in long‐lived forest products and land fills was 1.4 Tg C yr^?1. Net biome production (NBP) on the land, the net effect of NEP, harvest removals, and wildfire emissions indicates that the study area was a sink (8.2 Tg C yr^?1). NBP of the study area, which is the more heavily forested half of the state, compensated for ～52% of Oregon's fossil carbon dioxide emissions of 15.6 Tg C yr^?1 in 2000. The Biscuit Fire in 2002 reduced NBP dramatically, exacerbating net emissions that year. The regional total reflects the strong east–west gradient in potential productivity associated with the climatic gradient, and a disturbance regime that has been dominated in recent decades by commercial forestry.     

Negative emissions from stopping deforestation and forest degradation,globally

Forest growth provides negative emissions of carbon that could help keep the earth's surface temperature from exceeding 2°C, but the global potential is uncertain. Here we use land‐use information from the FAO and a bookkeeping model to calculate the potential negative emissions that would result from allowing secondary forests to recover. We find the current gross carbon sink in forests recovering from harvests and abandoned agriculture to be ?4.4 PgC/year, globally. The sink represents the potential for negative emissions if positive emissions from deforestation and wood harvest were eliminated. However, the sink is largely offset by emissions from wood products built up over the last century. Accounting for these committed emissions, we estimate that stopping deforestation and allowing secondary forests to grow would yield cumulative negative emissions between 2016 and 2100 of about 120 PgC, globally. Extending the lifetimes of wood products could potentially remove another 10 PgC from the atmosphere, for a total of approximately 130 PgC, or about 13 years of fossil fuel use at today's rate. As an upper limit, the estimate is conservative. It is based largely on past and current practices. But if greater negative emissions are to be realized, they will require an expansion of forest area, greater efficiencies in converting harvested wood to long‐lasting products and sources of energy, and novel approaches for sequestering carbon in soils. That is, they will require current management practices to change.     

Carbon storage and fluxes in ponderosa pine forests at different developmental stages

We compared carbon storage and fluxes in young and old ponderosa pine stands in Oregon, including plant and soil storage, net primary productivity, respiration fluxes, eddy flux estimates of net ecosystem exchange (NEE), and Biome‐BGC simulations of fluxes. The young forest (Y site) was previously an old‐growth ponderosa pine forest that had been clearcut in 1978, and the old forest (O site), which has never been logged, consists of two primary age classes (50 and 250 years old). Total ecosystem carbon content (vegetation, detritus and soil) of the O forest was about twice that of the Y site (21 vs. 10 kg C m^?2 ground), and significantly more of the total is stored in living vegetation at the O site (61% vs. 15%). Ecosystem respiration (R_e) was higher at the O site (1014 vs. 835 g C m^?2 year^?1), and it was largely from soils at both sites (77% of R_e). The biological data show that above‐ground net primary productivity (ANPP), NPP and net ecosystem production (NEP) were greater at the O site than the Y site. Monte Carlo estimates of NEP show that the young site is a source of CO₂ to the atmosphere, and is significantly lower than NEP(O) by c. 100 g C m^?2 year^?1. Eddy covariance measurements also show that the O site was a stronger sink for CO₂ than the Y site. Across a 15‐km swath in the region, ANPP ranged from 76 g C m^?2 year^?1 at the Y site to 236 g C m^?2 year^?1 (overall mean 158 ± 14 g C m^?2 year^?1). The lowest ANPP values were for the youngest and oldest stands, but there was a large range of ANPP for mature stands. Carbon, water and nitrogen cycle simulations with the Biome‐BGC model suggest that disturbance type and frequency, time since disturbance, age‐dependent changes in below‐ground allocation, and increasing atmospheric concentration of CO₂ all exert significant control on the net ecosystem exchange of carbon at the two sites. Model estimates of major carbon flux components agree with budget‐based observations to within ± 20%, with larger differences for NEP and for several storage terms. Simulations showed the period of regrowth required to replace carbon lost during and after a stand‐replacing fire (O) or a clearcut (Y) to be between 50 and 100 years. In both cases, simulations showed a shift from net carbon source to net sink (on an annual basis) 10–20 years after disturbance. These results suggest that the net ecosystem production of young stands may be low because heterotrophic respiration, particularly from soils, is higher than the NPP of the regrowth. The amount of carbon stored in long‐term pools (biomass and soils) in addition to short‐term fluxes has important implications for management of forests in the Pacific North‐west for carbon sequestration.     

Spatial and temporal variability in forest–atmosphere CO2 exchange

Seven years of carbon dioxide flux measurements indicate that a ～90‐year‐old spruce dominated forest in Maine, USA, has been sequestering 174±46 g C m^?2 yr^?1 (mean±1 standard deviation, nocturnal friction velocity (u_*) threshold >0.25 m s^?1). An analysis of monthly flux anomalies showed that above‐average spring and fall temperatures were significantly correlated with greater monthly C uptake while above‐average summer temperatures were correlated with decreased net C uptake. Summer months with significantly drier or wetter soils than normal were also characterized by lower rates of C uptake. Years with above‐average C storage were thus typically characterized by warmer than average spring and fall temperatures and adequate summer soil moisture. Environmental and forest–atmosphere flux data recorded from a second tower surrounded by similar forest, but sufficiently distant that flux source regions (‘footprints’), did not overlap significantly showed almost identical temperature and solar radiation conditions, but some differences in energy partitioning could be seen. Half‐hourly as well as integrated (annual) C exchange values recorded at the separate towers were very similar, with average annual net C uptake differing between the two towers by <6%. Interannual variability in net C exchange was found to be much greater than between tower variability. Simultaneous measurements from two towers were used to estimate flux data uncertainty from a single tower. Carbon‐flux model parameters derived independently from each flux tower data set were not significantly different, demonstrating that flux towers can provide a robust method for establishing C exchange model parameters.     

Carbon emissions from fires in tropical and subtropical ecosystems

Global carbon emissions from fires are difficult to quantify and have the potential to influence interannual variability and long‐term trends in atmospheric CO₂ concentrations. We used 4 years of Tropical Rainfall Measuring Mission (TRMM) Visible and Infrared Scanner (VIRS) satellite data and a biogeochemical model to assess spatial and temporal variability of carbon emissions from tropical fires. The TRMM satellite data extended between 38°N and 38°S and covered the period from 1998 to 2001. A relationship between TRMM fire counts and burned area was derived using estimates of burned area from other satellite fire products in Africa and Australia and reported burned areas from the United States. We modified the Carnegie‐Ames‐Stanford‐Approach (CASA) biogeochemical model to account for both direct combustion losses and the decomposition from fire‐induced mortality, using both TRMM and Sea‐viewing Wide Field of view Sensor (SeaWiFS) satellite data as model drivers. Over the 1998–2001 period, we estimated that the sum of carbon emissions from tropical fires and fuel wood use was 2.6 Pg C yr^?1. An additional flux of 1.2 Pg C yr^?1 was released indirectly, as a result of decomposition of vegetation killed by fire but not combusted. The sum of direct and indirect carbon losses from fires represented 9% of tropical and subtropical net primary production (NPP). We found that including fire processes in the tropics substantially alters the seasonal cycle of net biome production by shifting carbon losses to months with low soil moisture and low rates of soil microbial respiration. Consequently, accounting for fires increases growing season net flux by ～12% between 38°N and 38°S, with the greatest effect occurring in highly productive savanna regions.     

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

The exchange of carbon dioxide (CO₂) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO₂ uptake flux was ?3 µmol m^?2s^?1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m^?2s^?1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms^?1). On average, 164 mmol CO₂ m^?2d^?1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO₂ m^?2 (180 g CO₂ m^?2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ?0.04 yr^?1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.     

Dynamics of carbon stocks in soils and detritus across chronosequences of different forest types in the Pacific Northwest, USA

We investigated variation in carbon stock in soils and detritus (forest floor and woody debris) in chronosequences that represent the range of forest types in the US Pacific Northwest. Stands range in age from <13 to >600 years. Soil carbon, to a depth of 100 cm, was highest in coastal Sitka spruce/western hemlock forests (36±10 kg C m^?2) and lowest in semiarid ponderosa pine forests (7±10 kg C m^?2). Forests distributed across the Cascade Mountains had intermediate values between 10 and 25 kg C m^?2. Soil carbon stocks were best described as a linear function of net primary productivity (r²=0.52), annual precipitation (r²=0.51), and a power function of forest floor mean residence time (r²=0.67). The highest rates of soil and detritus carbon turnover were recorded on mesic sites of Douglas‐fir/western hemlock forests in the Cascade Mountains with lower rates in wetter and drier habitats, similar to the pattern of site productivity. The relative contribution of soil and detritus carbon to total ecosystem carbon decreased as a negative exponential function of stand age to a value of ～35% between 150 and 200 years across the forest types. These age‐dependent trends in the portioning of carbon between biomass and necromass were not different among forest types. Model estimates of soil carbon storage based on decomposition of legacy carbon and carbon accumulation following stand‐replacing disturbance showed that soil carbon storage reached an asymptote between 150 and 200 years, which has significant implications to modeling carbon dynamics of the temperate coniferous forests following a stand‐replacing disturbance.     

Environmental change and the carbon balance of Amazonian forests

Extreme climatic events and land‐use change are known to influence strongly the current carbon cycle of Amazonia, and have the potential to cause significant global climate impacts. This review intends to evaluate the effects of both climate and anthropogenic perturbations on the carbon balance of the Brazilian Amazon and to understand how they interact with each other. By analysing the outputs of the Intergovernmental Panel for Climate Change (IPCC) Assessment Report 4 (AR4) model ensemble, we demonstrate that Amazonian temperatures and water stress are both likely to increase over the 21st Century. Curbing deforestation in the Brazilian Amazon by 62% in 2010 relative to the 1990s mean decreased the Brazilian Amazon's deforestation contribution to global land use carbon emissions from 17% in the 1990s and early 2000s to 9% by 2010. Carbon sources in Amazonia are likely to be dominated by climatic impacts allied with forest fires (48.3% relative contribution) during extreme droughts. The current net carbon sink (net biome productivity, NBP) of +0.16 (ranging from +0.11 to +0.21) Pg C year^?1 in the Brazilian Amazon, equivalent to 13.3% of global carbon emissions from land‐use change for 2008, can be negated or reversed during drought years [NBP = ?0.06 (?0.31 to +0.01) Pg C year^?1]. Therefore, reducing forest fires, in addition to reducing deforestation, would be an important measure for minimizing future emissions. Conversely, doubling the current area of secondary forests and avoiding additional removal of primary forests would help the Amazonian gross forest sink to offset approximately 42% of global land‐use change emissions. We conclude that a few strategic environmental policy measures are likely to strengthen the Amazonian net carbon sink with global implications. Moreover, these actions could increase the resilience of the net carbon sink to future increases in drought frequency.     

The spatial distribution of forest biomass in the Brazilian Amazon: a comparison of estimates

The amount of carbon released to the atmosphere as a result of deforestation is determined, in part, by the amount of carbon held in the biomass of the forests converted to other uses. Uncertainty in forest biomass is responsible for much of the uncertainty in current estimates of the flux of carbon from land‐use change. In the present contribution several estimates of forest biomass are compared for the Brazilian Amazon, based on spatial interpolations of direct measurements, relationships to climatic variables, and remote sensing data. Three questions were posed: First, do the methods yield similar estimates? Second, do they yield similar spatial patterns of distribution of biomass? And, third, what factors need most attention if we are to predict more accurately the distribution of forest biomass over large areas? The answer to the first two questions is that estimates of biomass for Brazil's Amazonian forests (including dead and belowground biomass) vary by more than a factor of two, from a low of 39 PgC to a high of 93 PgC. Furthermore, the estimates disagree as to the regions of high and low biomass. The lack of agreement among estimates confirms the need for reliable determination of aboveground biomass over large areas. Potential methods include direct measurement of biomass through forest inventories with improved allometric regression equations, dynamic modelling of forest recovery following observed stand‐replacing disturbances, and estimation of aboveground biomass from airborne or satellite‐based instruments sensitive to the vertical structure plant canopies.     

Modeling the spatial and temporal heterogeneity of deforestation‐driven carbon emissions: the INPE‐EM framework applied to the Brazilian Amazon

We present a generic spatially explicit modeling framework to estimate carbon emissions from deforestation (INPE‐EM). The framework incorporates the temporal dynamics related to the deforestation process and accounts for the biophysical and socioeconomic heterogeneity of the region under study. We build an emission model for the Brazilian Amazon combining annual maps of new clearings, four maps of biomass, and a set of alternative parameters based on the recent literature. The most important results are as follows: (a) Using different biomass maps leads to large differences in estimates of emission; for the entire region of the Brazilian Amazon in the last decade, emission estimates of primary forest deforestation range from 0.21 to 0.26 Pg C yr^?1. (b) Secondary vegetation growth presents a small impact on emission balance because of the short duration of secondary vegetation. In average, the balance is only 5% smaller than the primary forest deforestation emissions. (c) Deforestation rates decreased significantly in the Brazilian Amazon in recent years, from 27 Mkm² in 2004 to 7 Mkm² in 2010. INPE‐EM process‐based estimates reflect this decrease even though the agricultural frontier is moving to areas of higher biomass. The decrease is slower than a non‐process instantaneous model would estimate as it considers residual emissions (slash, wood products, and secondary vegetation). The average balance, considering all biomass, decreases from 0.28 in 2004 to 0.15 Pg C yr^?1 in 2009; the non‐process model estimates a decrease from 0.33 to 0.10 Pg C yr^?1. We conclude that the INPE‐EM is a powerful tool for representing deforestation‐driven carbon emissions. Biomass estimates are still the largest source of uncertainty in the effective use of this type of model for informing mechanisms such as REDD+. The results also indicate that efforts to reduce emissions should focus not only on controlling primary forest deforestation but also on creating incentives for the restoration of secondary forests.     

Aboveground Forest Biomass and the Global Carbon Balance

The long‐term net flux of carbon between terrestrial ecosystems and the atmosphere has been dominated by two factors: changes in the area of forests and per hectare changes in forest biomass resulting from management and regrowth. While these factors are reasonably well documented in countries of the northern mid‐latitudes as a result of systematic forest inventories, they are uncertain in the tropics. Recent estimates of carbon emissions from tropical deforestation have focused on the uncertainty in rates of deforestation. By using the same data for biomass, however, these studies have underestimated the total uncertainty of tropical emissions and may have biased the estimates. In particular, regional and country‐specific estimates of forest biomass reported by three successive assessments of tropical forest resources by the FAO indicate systematic changes in biomass that have not been taken into account in recent estimates of tropical carbon emissions. The ‘changes’ more likely represent improved information than real on‐the‐ground changes in carbon storage. In either case, however, the data have a significant effect on current estimates of carbon emissions from the tropics and, hence, on understanding the global carbon balance.     

Post‐Soviet farmland abandonment,forest recovery,and carbon sequestration in western Ukraine

Land use is a critical factor in the global carbon cycle, but land‐use effects on carbon fluxes are poorly understood in many regions. One such region is Eastern Europe and the former Soviet Union, where land‐use intensity decreased substantially after the collapse of socialism, and farmland abandonment and forest expansion have been widespread. Our goal was to examine how land‐use trends affected net carbon fluxes in western Ukraine (57 000 km²) and to assess the region's future carbon sequestration potential. Using satellite‐based forest disturbance and farmland abandonment rates from 1988 to 2007, historic forest resource statistics, and a carbon bookkeeping model, we reconstructed carbon fluxes from land use in the 20th century and assessed potential future carbon fluxes until 2100 for a range of forest expansion and logging scenarios. Our results suggested that the low‐point in forest cover occurred in the 1920s. Forest expansion between 1930 and 1970 turned the region from a carbon source to a sink, despite intensive logging during socialism. The collapse of the Soviet Union created a vast, but currently largely untapped carbon sequestration potential (up to～150 Tg C in our study region). Future forest expansion will likely maintain or even increase the region's current sink strength of 1.48 Tg C yr^?1. This may offer substantial opportunities for offsetting industrial carbon emissions and for rural development in regions with otherwise diminishing income opportunities. Throughout Eastern Europe and the former Soviet Union, millions of hectares of farmland were abandoned after the collapse of socialism; thus similar reforestation opportunities may exist in other parts of this region.     

Conversion from forests to pastures in the Colombian Amazon leads to contrasting soil carbon dynamics depending on land management practices

Strategies to mitigate climate change by reducing deforestation and forest degradation (e.g. REDD+) require country‐ or region‐specific information on temporal changes in forest carbon (C) pools to develop accurate emission factors. The soil C pool is one of the most important C reservoirs, but is rarely included in national forest reference emission levels due to a lack of data. Here, we present the soil organic C (SOC) dynamics along 20 years of forest‐to‐pasture conversion in two subregions with different management practices during pasture establishment in the Colombian Amazon: high‐grazing intensity (HG) and low‐grazing intensity (LG) subregions. We determined the pattern of SOC change resulting from the conversion from forest (C3 plants) to pasture (C4 plants) by analysing total SOC stocks and the natural abundance of the stable isotopes ¹³C along two 20‐year chronosequences identified in each subregion. We also analysed soil N stocks and the natural abundance of ¹⁵N during pasture establishment. In general, total SOC stocks at 30 cm depth in the forest were similar for both subregions, with an average of 47.1 ± 1.8 Mg C ha^?1 in HG and 48.7 ± 3.1 Mg C ha^?1 in LG. However, 20 years after forest‐to‐pasture conversion SOC in HG decreased by 20%, whereas in LG SOC increased by 41%. This net SOC decrease in HG was due to a larger reduction in C3‐derived input and to a comparatively smaller increase in C4‐derived C input. In LG both C3‐ and C4‐derived C input increased along the chronosequence. N stocks were generally similar in both subregions and soil N stock changes during pasture establishment were correlated with SOC changes. These results emphasize the importance of management practices involving low‐grazing intensity in cattle activities to preserve SOC stocks and to reduce C emissions after land‐cover change from forest to pasture in the Colombian Amazon.     

Nitrogen deposition in tropical forests from savanna and deforestation fires

We used satellite‐derived estimates of global fire emissions and a chemical transport model to estimate atmospheric nitrogen (N) fluxes from savanna and deforestation fires in tropical ecosystems. N emissions and reactive N deposition led to a net transport of N equatorward, from savannas and areas undergoing deforestation to tropical forests. Deposition of fire‐emitted N in savannas was only 26% of emissions – indicating a net export from this biome. On average, net N loss from fires (the sum of emissions and deposition) was equivalent to approximately 22% of biological N fixation (BNF) in savannas (4.0 kg N ha^?1 yr^?1) and 38% of BNF in ecosystems at the deforestation frontier (9.3 kg N ha^?1 yr^?1). Net N gains from fires occurred in interior tropical forests at a rate equivalent to 3% of their BNF (0.8 kg N ha^?1 yr^?1). This percentage was highest for African tropical forests in the Congo Basin (15%; 3.4 kg N ha^?1 yr^?1) owing to equatorward transport from frequently burning savannas north and south of the basin. These results provide evidence for cross‐biome atmospheric fluxes of N that may help to sustain productivity in some tropical forest ecosystems on millennial timescales. Anthropogenic fires associated with slash and burn agriculture and deforestation in the southern part of the Amazon Basin and across Southeast Asia have substantially increased N deposition in these regions in recent decades and may contribute to increased rates of carbon accumulation in secondary forests and other N‐limited ecosystems.     

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.     

Net biome production of the Amazon Basin in the 21st century

Global change includes multiple stressors to natural ecosystems ranging from direct climate and land‐use impacts to indirect degradation processes resulting from fire. Humid tropical forests are vulnerable to projected climate change and possible synergistic interactions with deforestation and fire, which may initiate a positive feedback to rising atmospheric CO₂. Here, we present results from a multifactorial impact analysis that combined an ensemble of climate change models with feedbacks from deforestation and accidental fires to quantify changes in Amazon Basin carbon cycling. Using the LPJmL Dynamic Global Vegetation Model, we modelled spatio‐temporal changes in net biome production (NBP); the difference between carbon fluxes from fire, deforestation, soil respiration and net primary production. By 2050, deforestation and fire (with no CO₂ increase or climate change) resulted in carbon losses of 7.4–20.3 Pg C with the range of uncertainty depending on socio‐economic storyline. During the same time period, interactions between climate and land use either compensated for carbon losses due to wetter climate and CO₂ fertilization or exacerbated carbon losses from drought‐induced forest mortality (?20.1 to +4.3 Pg C). By the end of the 21st century, depending on climate projection and the rate of deforestation (including its interaction with fire), carbon stocks either increased (+12.6 Pg C) or decreased (?40.6 Pg C). The synergistic effect of deforestation and fire with climate change contributed up to 26–36 Pg C of the overall decrease in carbon stocks. Agreement between climate projections (n=9), not accounting for deforestation and fire, in 2050 and 2098 was relatively low for the directional change in basin‐wide NBP (19–37%) and aboveground live biomass (13–24%). The largest uncertainty resulted from climate projections, followed by implementation of ecosystem dynamics and deforestation. Our analysis partitions the drivers of tropical ecosystem change and is relevant for guiding mitigation and adaptation policy related to global change.