Integrated Terrestrial Ecosystem Carbon Research Articles

Evaluation of five models for constructing forest NPP–age relationships in China based on 3121 field survey samples

Abstract. Forest net primary productivity (NPP), representing the net carbon gain from the atmosphere, varies significantly with forest age. Reliable forest NPP–age relationships are essential for forest carbon cycle modeling and prediction. These relationships can be derived from forest inventory or field survey data, but it is unclear which model is the most effective in simulating forest NPP variation with age. Here, we aim to establish NPP–age relationships for China's forests based on 3121 field survey samples. Five models, including the semi-empirical mathematical (SEM) function, the second-degree polynomial (SDP) function, the logarithmic (L) function, the Michaelis–Menten (M) function, and the Γ function, were compared against field data. Results of the comparison showed that the SEM and Γ functions performed much better than the other three models, but due to the limited field survey samples at old ages, the Γ function showed a sharp decrease in NPP (decreased to almost zero) at old ages when building some forest NPP–age curves, while SEM could capture the variations in forest NPP at old ages reasonably well. Considering the overall performance with currently available forest field survey samples, SEM was regarded as the optimal NPP–age model. The finalized forest NPP–age curves for five forest types in six regions of China can facilitate forest carbon cycle modeling and future projection by using the process-based Integrated Terrestrial Ecosystem Carbon (InTEC) model in China and may also be useful for other regions.

Relationship between Net Primary Productivity and Forest Stand Age under Different Site Conditions and Its Implications for Regional Carbon Cycle Study

Net primary productivity (NPP) is a key component in the terrestrial ecosystem carbon cycle, and it varies according to stand age and site class index (SCI) for different forest types. Here we report an improved method for describing the relationships between NPP and stand age at various SCI values for the main forest types and groups in Heilongjiang Province, China, using existing yield tables, biomass equations, and forest inventory data. We calculated NPP as the sum of four components: Annual accumulation of live biomass, annual mortality of biomass, foliage turnover, and fine root turnover in soil. We also consider the NPP of understory vegetation or moss. These NPP-age relationships under different site conditions indicate that the NPP values of broadleaved and coniferous, as well as broadleaved mixed forests increase rapidly and reach a maximum when in young forests. However, for coniferous forest types, the maximum NPP generally occurs in mature forests. In addition, a higher SCI leads to a higher NPP value. Finally, we input these NPP-age relationships at various SCI values into the Integrated Terrestrial Ecosystem Carbon (InTEC) model to modify NPP modeling to estimate NPP in Heilongjiang Province in China from 2001 to 2010. All of the results showed that the methods reported in this study provide a reliable approach for estimating regional forest carbon budgets.

Forest sector carbon analyses support land management planning and projects: assessing the influence of anthropogenic and natural factors

Management of forest carbon stocks on public lands is critical to maintaining or enhancing carbon dioxide removal from the atmosphere. Acknowledging this, an array of federal regulations and policies have emerged that requires US National Forests to report baseline carbon stocks and changes due to disturbance and management and assess how management activities and forest plans affect carbon stocks. To address these requirements with the best-available science, we compiled empirical and remotely sensed data covering the National Forests (one fifth of the area of US forest land) and analyzed this information using a carbon modeling framework. We demonstrate how integration of various data and models provides a comprehensive evaluation of key drivers of observed carbon trends, for individual National Forests. The models in this framework complement each other with different strengths: the Carbon Calculation Tool uses inventory data to report baseline carbon stocks; the Forest Carbon Management Framework integrates inventory data, disturbance histories, and growth and yield trajectories to report relative effects of disturbances on carbon stocks; and the Integrated Terrestrial Ecosystem Carbon Model incorporates disturbance, climate, and atmospheric data to determine their relative impacts on forest carbon accumulation and loss. We report results for several National Forests across the USA and compare their carbon dynamics. Results show that recent disturbances are causing some forests to transition from carbon sinks to sources, particularly in the West. Meanwhile, elevated atmospheric carbon dioxide and nitrogen deposition are consistently increasing carbon stocks, partially offsetting declines due to disturbances and aging. Climate variability introduces concomitant interannual variability in net carbon uptake or release. Targeting forest disturbance and post-disturbance regrowth is critical to management objectives that involve maintaining or enhancing future carbon sequestration.

Global change induced biomass growth offsets carbon released via increased forest fire and respiration of the central Canadian boreal forest

AbstractNorthern boreal forests are sensitive to many effects of global change. This is of particular concern due to the proportionally greater climate change projected for the area in which these forests occur. One of the sensitive areas is the Far North of Ontario (FNO), consisting of one of the world's largest remaining tracts of unmanaged boreal forest, the world's third largest area of wetland, and the most southerly area of tundra. We studied past, present, and potential future carbon (C) balance of FNO forests using the Integrated Terrestrial Ecosystem Carbon Model and the Canadian Regional Climate Model with stand‐replacing fire disturbance. The forced simulations of past (1901–2004) C balances indicated that vegetation C stock remained stable, while soil C stock gradually declined (−0.07 t C ha−1 yr−1, p < 0.001), resulting in an overall significant decrease in total ecosystem C balance (−0.07 t C ha−1 yr−1, p < 0.001). Two Representative Concentration Pathways (RCPs), RCP8.5 and RCP4.5, simulations of future (2007–2100) C balances indicated that the carbon dioxide fertilization and climate growth‐enhancing effects of global change will outweigh C loss through increased ecosystem respiration, disturbance, and changes in forest age class structure resulting in an increase in total FNO ecosystem C stock by mid‐21st century. However, the projected simulations also indicated that the relative sizes of forest C stocks will change, with relatively less in the soil and more in vegetation, increasing fuel loads and making the entire ecosystem susceptible to forest fire and insect disturbances.

Quantitative simulation of C budgets in a forest in Heilongjiang province, China

Recently, forest carbon (C) budgets have been significantly affected by climate variability, nitrogen (N) deposition, an increasing global atmospheric CO2 concentration, and disturbances (i.e., harvests, fires, and insect infestations). In this study, we quantitatively simulated the annual carbon balance of forests in Heilongjiang, China, from 1901 to 2013 using the Integrated Terrestrial Ecosystem Carbon (InTEC) model, which integrated the effects of nondisturbance (i.e., atmospheric CO2 concentration, N deposition, and climate variability) and disturbance factors. The average net primary production (NPP) of Heilongjiang was 284 g C m-2 a-1 in 1901 and increased in 1950 to 339 g C m-2 a-1; a rapid increase occurred after 1980, with an increase of 48% in 2013 compared with the NPP in 1901. The average NPP of the entire Heilongjiang region increased significantly and became more stable in 2013. However, the NPP in the northern region of the Xiaoxing’an Mountains was lower than that in the other regions. The fluctuation in average net ecosystem production (NEP) was relatively large because Heilongjiang was a carbon source for many years before the 1930s and again in the early 21st century, due to serious disturbances and intensified human activities. In recent years, NEP began to increase again, and in 2013 the forests became a large carbon sink (188 g C m-2 a-1). The spatial distribution of the average NEP was similar to that of NPP, though the largest increment in the average NEP from 1901 to 2013 was in the Changbai Mountains.

Simulation of terrestrial carbon equilibrium state by using a detachable carbon cycle scheme

Determining the equilibrium state of terrestrial carbon is a prerequisite for scientific analysis on the carbon cycle. However, the mechanism through which the carbon cycle reaches the equilibrium state remains unclear. Moreover, the carbon cycle in most of the short–term field experiments rarely reaches the equilibrium state. In this study, a detachable carbon cycle (DCC) model was proposed to simulate the equilibrium state of each carbon pool. The model was established based on a pool–and–flux scheme and contained 14 carbon pools, or carbon flow processes, each process could be detached from the main model and evaluated as an independent component. The environmental scalar algorithms of the Integrated Terrestrial Ecosystem Carbon budget model (InTEC) and Community Atmosphere Biosphere Land Exchange (CABLE) were incorporated in the DCC model. Four situations were compared using the two environmental scalar algorithms and model structure (9 vs. 14 carbon pools). Furthermore, the size and turnover time of each carbon pool were analyzed at the equilibrium state. A sensitivity analysis was then conducted to investigate the responses of carbon density and equilibrium time to 12 key parameters of the model. Results indicated that the combination of the CABLE environmental scalar algorithm and 14 pools exhibited improved performance on carbon storage simulation than that of the other combinations, and the effect of the environmental scalar algorithm was considerably larger than that of the carbon pool number. Sensitivity analysis indicated that the carbon density of grassland and cropland was more vulnerable and sensitive to key parameters of the model than that of the other biomes. This study elucidates influencing factors and underlying control mechanisms in the carbon accumulation, and provides a framework for quantitative analysis of each component of the carbon cycle.

Carbon dynamics of China’s forests during 1901–2010: The importance of forest age

The forest ecosystem plays a significant role in long-term carbon sequestration. The magnitude of a forest carbon sink is determined by the combination of intrinsic biotic factors (i.e. forest age) and extrinsic environmental driving factors (i.e. temperature, precipitation, nitrogen deposition). The mechanism of how forest age affects the carbon balance is still unclear. Furthermore, forest age is a useful surrogate variable for analyses of the impact of disturbances on forest carbon and an important parameter for assessing carbon mitigation potential of forests at regional scale. Without considering forest age, large uncertainties exist in the simulating forest carbon cycle at regional scale. In this study, we analyzed the effect of forest age information on carbon uptake of China’s forests based on a process-based model (Integrated Terrestrial Ecosystem Carbon budget, InTEC model) in the last century, with integration of two forest age maps at 1 km spatial resolution which were derived from inventory data and remote sensing AVHRR NDVI. Inter-annual variability of net primary productivity in China’s forests during 1901–2010 based on InTEC model coupled with inventory data and remote sensing data was similar with each other. Meanwhile, the comparison approach indicated that the errors of 5 years in forest age might cause uncertainties of 25% in simulated net ecosystem productivity of China’s forests. We also discussed the significance of forest age in ecosystem process-based model and the uncertainties of carbon balance of China’s forest ecosystem during 1901–2010. Besides, the hypothesis assumed that no large-scale deforestation occurred since1950s and mature forests (and older forests) were covered the whole China. We adjusted forest age structure for different forest types and regions to simulate the effect of changes in forest age on distribution of carbon balance in China. The important finding is that the loss of carbon sink in China’s forests induced by disturbances (i.e. deforestation, afforestation or natural disturbances) is about 3.80 Pg C during last century. Based this hypothesis, China’s forests in the last two decade manifested a large carbon sink (0.23 Pg C a - 1). Currently, many process-based models are not capable of reflecting the impact of land disturbances because the explicit forest age information are lacking. Our study incorporated the forest age in models successfully and explored the effect of forest age on the simulating carbon balance of forests. The application of forest age in large-scale carbon modeling can improve the accuracy of simulated carbon dynamics and explore the uncertainties in carbon cycle models.

Carbon dynamics in woody biomass of forest ecosystem in China with forest management practices under future climate change and rising CO2 concentration

It is critical to study how different forest management practices affect forest carbon sequestration under global climate change regime. Previous researches focused on the stand-level forest carbon sequestration with rare investigation of forest carbon stocks influenced by forest management practices and climate change at regional scale. In this study, a general integrative approach was used to simulate spatial and temporal variations of woody biomass and harvested biomass of forest in China during the 21st century under different scenarios of climate and CO2 concentration changes and management tasks by coupling Integrated Terrestrial Ecosystem Carbon budget (InTEC) model with Global Forest Model (G4M). The results showed that forest management practices have more predominant effects on forest stem stocking biomass than climate and CO2 concentration change. Meanwhile, the concurrent future changes in climate and CO2 concentration will enhance the amounts of stem stocking biomass in forests of China by 12%–23% during 2001–2100 relative to that with climate change only. The task for maximizing stem stocking biomass will dramatically enhance the stem stocking biomass from 2001–2100, while the task for maximum average increment will result in an increment of stem stocking biomass before 2050 then decline. The difference of woody biomass responding to forest management tasks was owing to the current age structure of forests in China. Meanwhile, the sensitivity of long-term woody biomass to management practices for different forest types (coniferous forest, mixed forest and deciduous forest) under changing climate and CO2 concentration was also analyzed. In addition, longer rotation length under future climate change and rising CO2 concentration scenario will dramatically increase the woody biomass of China during 2001–2100. Therefore, our estimation indicated that taking the role of forest management in the carbon cycle into the consideration at regional or national level is very important to project the forest carbon sequestration under future climate change and rising atmospheric CO2 concentration.

Attributing carbon changes in conterminous U.S. forests to disturbance and non‐disturbance factors from 1901 to 2010

Recent climate variability (increasing temperature, droughts) and atmospheric composition changes (nitrogen deposition, rising CO2concentration) along with harvesting, wildfires, and insect infestations have had significant effects on U.S. forest carbon (C) uptake. In this study, we attribute C changes in the conterminous U.S. forests to disturbance and non‐disturbance factors with the help of forest inventory data, a continental stand age map, and an updated Integrated Terrestrial Ecosystem Carbon Cycle model (InTEC). We grouped factors into disturbances (harvesting, fire, insect infestation) and non‐disturbances (CO2 concentration, N deposition, and climate variability) and estimated their subsequent impacts on forest regrowth patterns. Results showed that on average, the C sink in the conterminous U.S. forests from 1950 to 2010 was 206 Tg C yr−1 with 87% (180 Tg C yr−1) of the sink in living biomass. Compared with the simulation of all factors combined, the estimated C sink would be reduced by 95 Tg C yr−1 if disturbance factors were omitted, and reduced by 50 Tg C yr−1if non‐disturbance factors were omitted. Our study also showed diverse regional patterns of C sinks related to the importance of driving factors. During 1980–2010, disturbance effects dominated the C changes in the South and Rocky Mountain regions, were almost equal to non‐disturbance effects in the North region, and had minor effects compared with non‐disturbance effects in the West Coast region.

Relationships between net primary productivity and stand age for several forest types and their influence on China’s carbon balance

Affected by natural and anthropogenic disturbances such as forest fires, insect-induced mortality and harvesting, forest stand age plays an important role in determining the distribution of carbon pools and fluxes in a variety of forest ecosystems. An improved understanding of the relationship between net primary productivity (NPP) and stand age (i.e., age-related increase and decline in forest productivity) is essential for the simulation and prediction of the global carbon cycle at annual, decadal, centurial, or even longer temporal scales. In this paper, we developed functions describing the relationship between national mean NPP and stand age using stand age information derived from forest inventory data and NPP simulated by the BEPS (Boreal Ecosystem Productivity Simulator) model in 2001. Due to differences in ecobiophysical characteristics of different forest types, NPP-age equations were developed for five typical forest ecosystems in China (deciduous needleleaf forest (DNF), evergreen needleleaf forest in tropic and subtropical zones (ENF-S), deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), and mixed broadleaf forest (MBF)). For DNF, ENF-S, EBF, and MBF, changes in NPP with age were well fitted with a common non-linear function, with R2 values equal to 0.90, 0.75, 0.66, and 0.67, respectively. In contrast, a second order polynomial was best suitable for simulating the change of NPP for DBF, with an R2 value of 0.79. The timing and magnitude of the maximum NPP varied with forest types. DNF, EBF, and MBF reached the peak NPP at the age of 54, 40, and 32 years, respectively, while the NPP of ENF-S maximizes at the age of 13 years. The highest NPP of DBF appeared at 122 years. NPP was generally lower in older stands with the exception of DBF, and this particular finding runs counter to the paradigm of age-related decline in forest growth.Evaluation based on measurements of NPP and stand age at the plot-level demonstrates the reliability and applicability of the fitted NPP-age relationships. These relationships were used to replace the normalized NPP-age relationship used in the original InTEC (Integrated Terrestrial Ecosystem Carbon) model, to improve the accuracy of estimated carbon balance for China’s forest ecosystems. With the revised NPP-age relationship, the InTEC model simulated a larger carbon source from 1950–1980 and a larger carbon sink from 1985–2001 for China’s forests than the original InTEC model did because of the modification to the age-related carbon dynamics in forests. This finding confirms the importance of considering the dynamics of NPP related to forest age in estimating regional and global terrestrial carbon budgets.

Spatially simulating changes of soil water content and their effects on carbon sequestration in Canada's forests and wetlands

The variations of soil water content (SWC) and its influences on the carbon (C) cycle in Canada’s forests and wetlands were studied through model simulations using the Integrated Terrestrial Ecosystem Carbon (InTEC) model. It shows that Canada’s forests and wetlands experienced spatially and temporally heterogeneous changes in SWC from 1901 to 2000. SWC changes caused average NPP to decrease 40.8 Tg C yr−1 from 1901 to 2000, whereas the integrated effect of non-disturbance factors (climate change, CO2 fertilization and N deposition) enhanced NPP by 9.9%. During 1981–2000, the reduction of NPP caused by changes in SWC was 58.1 Tg C yr−1 whereas non-disturbance factors together caused NPP to increase by 16.6%. SWC changes resulted in an average increase of 4.1 Tg C yr−1 in the net C uptake during 1901–2000, relatively small compared with the enhancement in C uptake of 50.2 Tg C yr−1 by the integrated effect of non-disturbance factors. During 1981–2000, changes in SWC caused a reduction of 3.8 Tg C yr−1 in net C sequestration whereas the integrated factors increased net C sequestration by 54.1 Tg C yr−1. Increase in SWC enhanced C sequestration in all ecozones.

Simulating the effects of past changes in climate, atmospheric composition, and fire disturbance on soil carbon in Canada's forests and wetlands

Using the Integrated Terrestrial Ecosystem Carbon Cycle model (InTEC), six simulations with different input scenarios of climate, CO2, and nitrogen (N) deposition are conducted to study the changes of soil carbon (C) content in Canada's forests and wetlands during 1901–2000. Simulated total C stored in Canada's forest and wetland soils is 164.5 Pg C and accounts for about 7% of the global total of 2400 Pg C to the depth of 2 m, implying the significance of Canada's forest and wetland soils in the global terrestrial C cycle. Soils of Canadian forests and wetlands sequestrated 3.9 Pg C (2.6 Pg C in forests and 1.3 Pg C in wetlands) during 1901–2000 because of the integrated effects of climate, CO2fertilization, N deposition, and forest age factors. The changes of soil C content during 1901–2000 ranged spatially from −2 kg C m−2to 4 kg C m−2, depending on fire disturbance history, climate change pattern, and N deposition rates. Soil C increased by 2 to 4 kg C m−2in Eastern Hudson Plains, Eastern Middle Boreal Shield, Southern Boreal Shield, and Atlantic Maritime and decreased by more than 1 kg C m−2in Southern Boreal Plains. Simulations shows that climate influences growing conditions, growing season length, net N mineralization, and N fixation and therefore was the biggest driver of the increase in total soil C content during 1901–2000, followed by CO2fertilization and N deposition. The climate‐induced increase of soil C occurred mainly in the cool and wet period from the middle 1940s to the middle 1970s. Overall, an increase of 1°C in mean annual temperature induced heterotrophic respiration to increase by 62 Tg C a−1. In contrast to the century‐scale trend from 1901 to 2000, during the last two decades (1981–2000), CO2fertilization was the biggest driver of the increase in soil C, while climate change alone caused soil C to decrease.

Future carbon balance of China's forests under climate change and increasing CO 2

The possible response of the carbon (C) balance of China's forests to an increase in atmospheric CO 2 concentration and climate change was investigated through a series of simulations using the Integrated Terrestrial Ecosystem Carbon (InTEC) model, which explicitly represents the effects of climate, CO 2 concentration, and nitrogen deposition on future C sequestration by forests. Two climate change scenarios (CGCM2-A2 and -B2) were used to drive the model. Simulations showed that China's forests were a C sink in the 1990s, averaging 189 Tg C yr −1 (about 13% of the global total). This sink peaks around 2020 and then gradually declines to 33.5 Tg C yr −1 during 2091–2100 without climate and CO 2 changes. Effects of pure climate change of CGCM2-A2 and -B2 without allowing CO 2 effects on C assimilation in plants might reduce the average net primary productivity (NPP) of China's forests by 29% and 18% during 2091–2100, respectively. Total soil C stocks might decrease by 16% and 11% during this period. China's forests might broadly act as C sources during 2091–2100, with values of about 50 g C m −2 yr −1 under the moderate warming of CGCM2-B2 and 50–200 g C m −2 yr −1 under the warmer scenario of CGCM2-A2. An increase in CO 2 might broadly increase future C sequestration of China's forests. However, this CO 2 fertilization effect might decline with time. The CO 2 fertilization effects on NPP by the end of this century are 349.6 and 241.7 Tg C yr −1 under CGCM2-A2 and -B2 increase scenarios, respectively. These effects increase by 199.1 and 126.6 Tg C yr −1 in the first 50 years, and thereafter, by 150.5 and 115.1 Tg C yr −1 in the second 50 years under CGCM2-A2 and -B2 increase scenarios, respectively. Under a CO 2 increase without climate change, the majority of China's forests would be C sinks during 2091–2100, ranging from 0 to 100 g C m −2 yr −1. The positive effect of CO 2 fertilization on NPP and net ecosystem productivity would be exceeded by the negative effect of climate change after 2050. Under the CGCM2-A2 climate scenario and with direct CO 2 effects, China's forests may be a small C source of 7.6 Tg C yr −1 during 2091–2100. Most forests act as C sources of 0–40 g C m −2 yr −1. Under the CGCM2-B2 climate scenario and with direct CO 2 effects, China's forests might be a small C sink of 10.5 Tg C yr −1 during 2091–2100, with C sequestration of most forests ranging from 0 to 40 g C m −2 yr −1. Stand age structure plays a more dominant role in determining future C sequestration than CO 2 and climate change. The prediction of future C sequestration of China's forests is very sensitive to the Q 10 value used to estimate maintenance respiration and to soil water availability and less sensitive to N deposition scenario. The results are not yet comprehensive, as no forest disturbance data were available or predicted after 2001. However, the results indicate a range of possible responses of the C balance of China's forests to various scenarios of increase in CO 2 and climate change. These results could be useful for assessing measures to mitigate climate change through reforestation.