Mean Annual Net Ecosystem Productivity Research Articles

Spatio-temporal dynamics of terrestrial Net ecosystem productivity in the ASEAN from 2001 to 2020 based on remote sensing and improved CASA model

The Association of Southeast Asian Nations (ASEAN) has attracted considerable attention for its rich land ecosystem productivity. However, recent excessive human disturbances may have triggered significant transformations. Consequently, understanding the spatio-temporal dynamics of ecosystem productivity in this region is crucial for strengthening regional ecological sustainability and implementing climate change mitigation strategies. In this study, we harnessed multi-source remote sensing data, an improved CASA model, Theil-Sen Median estimators, Mann-Kendall tests, and Hurst exponent to estimate the net ecosystem productivity (NEP) of the ASEAN region and simulate its spatiotemporal dynamics change. The results showed that the mean annual NEP from 2001 to 2020 in ASEAN was 396.83gC/m2, exhibiting a gradual downward trend. From a spatiotemporal tendency perspective, the areas of NEP decline in the ASEAN region far outweighed the areas of increase, with 60.03% of the total area showing a decrease in NEP, 33.14% showing a boost, and 6.83% showing no significant change. In terms of change persistence, 79.3% of NEP demonstrated anti-persistence, indicating a stronger trend of NEP change reversal. Coupling analysis results indicated that the areas with a future increasing trend of NEP constituted 36.11% of the total area, while areas with no significant future change accounted for 0.08%, areas with a future decreasing trend included 25.21%, and 38.6% of the region exhibited uncertain future trends. Researchers should continually monitor areas with decreasing NEP and uncertain future trends. This study, driven by remote sensing data and combined with the improved CASA model, has successfully estimated the Net Ecosystem Productivity (NEP) at both site and regional scales. The analysis results of NEP in the ASEAN region have enhanced our comprehension of the spatio-temporal patterns of vegetation carbon balance changes in ASEAN, providing data support for formulating “low-carbon economy” strategies and promoting the sustainable development of regional ecosystems.

Vegetation restoration in dryland with shrub serves as a carbon sink: Evidence from a 13‐year observation at the Tengger Desert of Northern China

AbstractDryland area accounts for approximately 40% of worldwide land area, which plays a significant role in regulating the carbon sequestration capacity of land. Vegetation restoration in drylands adopted to prevent land degradation, and may also serve as a carbon sink in the earlier stage. However, the persistence of the carbon sink for the revegetated ecosystem in drylands is still unknown. Can the well‐established restoration vegetation in dryland areas serve as a carbon sink in long‐run? To address this question, we investigated the carbon sequestration capacity of planted vegetation in dryland areas with 13 years of observation (2009–2021) for established vegetation restoration, which began in 1989. We found that the revegetation area serves as a carbon sink in all years. The mean annual net ecosystem productivity (NEP) is 91.61 ± 36.17 gC m−2 yr−1 (mean ± standard deviation). Annual NEP showed a significant increasing trend over the study period with a rate of 5.65 gC m−2 yr−1 yr−1 (p < 0.05). The increase in spring temperature, the earlier start of net carbon uptake and the longer duration of net carbon uptake contribute to the gradual trend of NEP. The amount of annual NEP is predominantly determined by summer precipitation. Meanwhile, our results revealed that the increase in net carbon uptake by revegetation did not lead to excessive consumption of water resources. Our results have suggested that appropriate vegetation restoration in arid areas can increase ecosystem carbon sequestration over longer timescales and mitigate climate change, with relatively low environmental consequences and risks. Considering the vast area of degraded land in the global drylands, the carbon sequestration effect of this model should be given more attention.

A multi-year study of ecosystem production and its relation to biophysical factors over a temperate peatland

Peatlands are among the largest stocks of soil carbon, which can be stored for thousands of years under well-wetted conditions. The main goals of the study were to assess annual and seasonal CO2 balances of a temperate peatland and the main biophysical factors affecting these CO2 fluxes. The studied peatland was usually a CO2 sink with a mean annual net ecosystem production (NEP) of 110±83 gCO2-C·m−2·yr−1 and extreme balances in 2006 (-17 gCO2-C·m−2·yr−1) and 2011 (194 gCO2-C·m−2·yr−1). Annual fluxes were not significantly correlated with biophysical variables, unlike seasonal data. Furthermore, the average air temperature in spring and summer was related to NEP at r2=0.65 and r2=0.61, respectively (warmer spring increased NEP while hot summer decreased NEP in these seasons). A decrease in daytime measured NEP during the summer period (June-August) was observed when TA exceeded 25 °C or VPD was above 15 hPa, respectively, due to the growing Reco and possibly plant photorespiration. These findings suggest a negative impact of ongoing global warming on temperate peatland CO2 balances.

Carbon exchange of forest plantations: global patterns and biophysical drivers

Plantation has large carbon sequestration potential, and it plays an important role in mitigating global warming. However, the responses of carbon fluxes to biophysical factors across plantation sites are still not clear. We synthesized carbon flux data measured by the eddy covariance method over the plantations at the global scale to explore the shifts of carbon exchange with latitude, discuss the link of carbon fluxes with biophysical variables, and compare the difference in carbon sequestration between needleleaf and broadleaf plantations. Annual net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (ER) across all plantations were 353±27, 1762±60 and 1385±46 g C m−2 yr−1, respectively. The mean annual NEP of needleleaf biomes was similar to that of broadleaf biomes. GPP and ER were 48% and 64% larger at needleleaf forests than at broadleaf forests. Annual NEP, GPP and ER decreased with the increase of latitude significantly. At the half-hourly scale, the increase of diffuse radiation enhanced carbon assimilation of needleleaf and broadleaf biomes remarkably. On the annual scale, both GPP and NEP were sensitive to mean annual temperature (MAT) and leaf area index (LAI). Annual precipitation was the dominant factor regulating the variability of GPP and ER, and it explained 34% and 47% of the variation in GPP and ER, respectively. The extension in growing season length (GSL) and rising soil water content (SWC) enhanced ecosystem photosynthesis and respiration of planted forests. However, the effect of GSL and SWC on net carbon uptake was not remarkable. These results highlight the importance of biophysical factors in regulating carbon dynamics of the plantations, and contribute to understanding of the differences in carbon sequestration between needleleaf and broadleaf biomes.

Multi-year eddy covariance measurements of net ecosystem exchange in tropical dry deciduous forest of India

Tropical deciduous forests are unique in terms of their geographical distribution, strong seasonality and their contribution to global carbon dynamics, but yet their carbon sequestration potential is poorly sampled. In the current study, we report the carbon balance of 65-year-old tropical dry deciduous forest in central India using the multi-year eddy covariance measurements from November 2011 to May 2019. Over the study period, the forest site was observed to be a net sink of atmospheric CO2 with a mean annual net ecosystem productivity (NEP) of 524 (± 40; ±1 SD across different years) g C m−2 yr−1 with a 233 (±15) day growing season. The NEP was partitioned into gross primary productivity (GPP) of 3358 (± 167) g C m−2 yr−1 and annual carbon loss due to respiration and decomposition (Reco) of 2834 (± 157) g C m−2 yr−1. The ecosystem showed a strong seasonality as a source of carbon during the leaf-off season (March to June), and as a carbon sink during the rest of the year with significant sequestration during the winter season (October to December). The intra-annual analysis suggested that CO2 flux is primarily controlled by canopy greenness with mean Reco/GPP ratio varying from 1.90 (± 0.12) to 0.79 (± 0.04) during leaf-off to leaf-on seasons. The monthly C fluxes in the growing season are found to be strongly correlated to the environmental variables rather than in the leaf-off season, while variability in monthly ecosystem respiration was better explained by air temperature during the leaf-off season than the growing season. Further, the inter-annual variability of NEP was mainly dependent on growing season length and mean annual temperature. The variations in annual GPP and Reco were directly dependent on the increase in mean annual temperature. The eddy covariance-based NEP was complemented by close independent biometric estimates, imparting confidence in our measurements. In conclusion, this tropical dry deciduous forest site is a substantial carbon sink and would add crucial information regarding carbon budgeting of tropical forests in global carbon balance models.

Reexamine China’s terrestrial ecosystem carbon balance under land use-type and climate change

Land use and climate change can strongly affect the terrestrial ecosystem carbon balance. However, there is a lack of clarity for existing studies that investigate total carbon balance for China’s terrestrial carbon balance (carbon storage and Net Ecosystem Productivity (NEP)). In this study, based on large data and the improved NEP model, we examined land use and climate change during 2000–2015 in China, calculated terrestrial ecosystem carbon storage change caused by land use change, and carbon sink/source variation under climate change; we found that during 2000–2015, 3.05 % of China’s land area had land use type changes and caused 32.97 Tg of carbon storage loss, consisting of 10.4 Tg from vegetation carbon storage and 22.57 Tg from soil organic carbon (SOC) loss. Built-up land occupying ecological land was the most obvious land transfer type, especially for grassland degeneration. Both temperature and precipitation showed decreasing trends throughout China. Mean annual NEP showed a carbon sink value of 41.73 g C.m−2.yr−1, and the NEPs of carbon sinks were mostly distributed in South and Midland China and partly in Northeast China. There were obvious regional differences and the carbon balance showed that North China and Northwest China were regions of net carbon sources. The other four regions were net carbon sinks. Land use changes caused carbon storage loss in all regions, NEPs in North China and Northwest China were carbon sources, while in the other regions were carbon sinks. All NEPs exhibited an increasing trend during 2000–2015, except for Mid-South China. Finally, according to regional carbon balance characters, different policy implications were drawn which can serve for the formulation of territorial spatial planning. Land exploitation should be limited and environmental conservation is needed in North and Northwest China. The temperature in Southwest China has been increasing continuously and merits attention. The control of built-up land expansion in other regions should be strengthened.

The Impact of Seasonal and Annual Climate Variations on the Carbon Uptake Capacity of a Deciduous Forest Within the Great Lakes Region of Canada.

In eastern North America, many deciduous forest ecosystems grow at the northernmost extent of their geographical ranges, where climate change could aid or impede their growth. This region experiences frequent extreme weather conditions, allowing us to study the response of these forests to environmental conditions, reflective of future climates. Here we determined the impact of seasonal and annual climate variations and extreme weather events on the carbon (C) uptake capacity of an oak‐dominated forest in southern Ontario, Canada, from 2012 to 2016. We found that changes in meteorology during late May to mid‐July were key in determining the C sink strength of the forest, impacting the seasonal and annual variability of net ecosystem productivity (NEP). Overall, higher temperatures and dry conditions reduced ecosystem respiration (RE) much more than gross ecosystem productivity (GEP), leading to higher NEP. Variability in NEP was primarily driven by changes in RE, rather than GEP. The mean annual GEP, RE, and NEP values at our site during the study were 1,343 ± 85, 1,171 ± 139, and 206 ± 92 g C m−2 yr−1, respectively. The forest was a C sink even in years that experienced heat and water stresses. Mean annual NEP at our site was within the range of NEP (69–459 g C m−2 yr−1) observed in similar North American forests from 2012 to 2016. The growth and C sequestration capabilities of our oak‐dominated forest were not adversely impacted by changes in environmental conditions and extreme weather events experienced over the study period.

Response of carbon and water fluxes to meteorological and phenological variability in two eastern North American forests of similar age but contrasting species composition – a multiyear comparison

Abstract. The annual carbon and water dynamics of two eastern North American temperate forests were compared over a 6-year period from 2012 to 2017. The geographic location, forest age, soil, and climate were similar between the two stands; however, stand composition varied in terms of tree leaf-retention and shape strategy: one stand was a deciduous broadleaf forest, while the other was an evergreen needleleaf forest. The 6-year mean annual net ecosystem productivity (NEP) of the coniferous forest was slightly higher and more variable (218±109 g C m−2 yr−1) compared to that of the deciduous forest NEP (200±83 g C m−2 yr−1). Similarly, the 6-year mean annual evapotranspiration (ET) of the coniferous forest was higher (442±33 mm yr−1) than that of the deciduous forest (388±34 mm yr−1), but with similar interannual variability. Summer meteorology greatly impacted the carbon and water fluxes in both stands; however, the degree of response varied among the two stands. In general, warm temperatures caused higher ecosystem respiration (RE), resulting in reduced annual NEP values – an impact that was more pronounced at the deciduous broadleaf forest compared to the evergreen needleleaf forest. However, during warm and dry years, the evergreen forest had largely reduced annual NEP values compared to the deciduous forest. Variability in annual ET at both forests was related most to the variability in annual air temperature (Ta), with the largest annual ET observed in the warmest years in the deciduous forest. Additionally, ET was sensitive to prolonged dry periods that reduced ET at both stands, although the reduction at the coniferous forest was relatively larger than that of the deciduous forest. If prolonged periods (weeks to months) of increased Ta and reduced precipitation are to be expected under future climates during summer months in the study region, our findings suggest that the deciduous broadleaf forest will likely remain an annual carbon sink, while the carbon sink–source status of the coniferous forest remains uncertain.

中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

在2017年1月1日-2017年12月31日期间，采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间，该森林生态系统在生长季5-10月份为碳汇，非生长季各月为碳源，净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1，生态系统呼吸为529.9 g C m^-2a^-1，总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21，P < 0.001），其温度敏感性系数（Temperature sensitivity coefficient，Q₁₀）为2.2。如果排除夜间通量观测的误差，处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放，可能导致全年生态系统呼吸量较低。在生长季5-10月份，各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型，初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化，范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份，较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用；生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用，表明生长末期降水过多影响森林的碳吸收。;Oak forests comprise the largest forest area in central China and are the potential carbon sink, while we know little about the carbon dioxide flux of oak forests in the transitional zone from subtropics to warm temperate, China. Using an open-path eddy covariance system and micro-climate instruments, the CO₂ flux, photosynthetic active radiation (PAR), air temperature, soil temperature and precipitation were simultaneously observed in a natural oak (Quercus aliena) forest at Baotianman National Nature Reserve. Based on the data sets during January to December 2017, dynamic change of CO₂ flux at different temporal scales and its underlying mechanism were analyzed. The results indicated that the diurnal and seasonal variations of CO₂ fluxes showed an obvious single peak pattern. The oak forest ecosystem was a carbon sink during the growing season (May-October), while a carbon source occurred during the non-growing season. Net carbon sequestration and emissions peaked in July and April, respectively. Mean annual net ecosystem productivity (NEP), ecosystem respiration (Re) and gross ecosystem productivity (GEP) were 569.4, 529.9 and 1099.3 g C m^-2 year^-1, respectively. The relationship between net ecosystem carbon exchange (NEE) measured at half-hour interval during night and soil temperature at depth of 5 cm can be expressed by an exponential equation (R²=0.21, P < 0.001), with its temperature sensitivity coefficient (Q₁₀) of 2.2. A low temperature at night and in the non-growing season at high elevation resulted in lower Re throughout the year. The relationship between NEE and PAR at daytime could be well expressed by a rectangular hyperbolic equation during growing seasons. Monthly initial light use efficiency, maximum photosynthetic capacity and daytime Re were 0.06-0.12 μmol CO₂ μmol^-1 photon, 0.44-1.47 mg CO₂ m^-2s^-1 and 0.07-0.19 mg CO₂ m^-2s^-1, respectively. Both higher vapour pressure deficit in July and August and higher soil moisture at the end of the growing season inhibited carbon uptake of the oak forest, indicating a negative effect of increased precipitation on carbon sequestration at the end of the growing season.

The Carbon Balance of the Southeastern U.S. Forest Sector as Driven by Recent Disturbance Trends

AbstractThis study documents annual carbon stocks and fluxes from 1986 to 2010 at 30‐m resolution across southeastern U.S. forests, analyzing trends and regional greenhouse gas exchange. We used forest inventory data to guide a carbon cycle model representing postdisturbance carbon dynamics for diverse forest and site conditions. We mapped carbon stocks and fluxes with stand age inferred from spaceborne disturbance monitoring and biomass. We assessed the fate of harvested biomass with a wood products model. We found that pine forests experienced the largest biomass removals, with paper products leading all end‐uses. Averaging across all SE forestlands, mean annual net ecosystem productivity decreased from 116 gC · m−2 · year−1 in 1986 to 71 gC · m−2 · year−1 in 2007, and 85 gC · m−2 · year−1 in 2010, equating to a range of 25 to 41 Tg C/year (mean of 34 Tg C/year). Interannual variability in forest‐atmosphere carbon exchange is dominated by the extent of harvesting, with removals ranging from 23 to 56 Tg C/year. Region‐wide live biomass stocks varied little over time, averaging 5.0 kg C/m2 for aboveground biomass or 1,780 TgC, with average annual biomass growth balanced by harvest removals. However, net biome productivity exhibited large interannual variability, spanning a sink of 16 Tg C/year in 1986 to a source of −30 Tg C/year in the year of peak harvest. Two thirds of harvest removals are emitted within 50 years, 8% as methane, causing the forest sector to act as a large CO2‐equivalent source. Uncertainties are estimated at ± 25%.

Cross-scale controls on carbon emissions from boreal forest megafires.

Climate warming and drying is associated with increased wildfire disturbance and the emergence of megafires in North American boreal forests. Changes to the fire regime are expected to strongly increase combustion emissions of carbon (C) which could alter regional C balance and positively feedback to climate warming. In order to accurately estimate C emissions and thereby better predict future climate feedbacks, there is a need to understand the major sources of heterogeneity that impact C emissions at different scales. Here, we examined 211 field plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories (NWT), Canada after an unprecedentedly large area burned in 2014. We assessed both aboveground and soil organic layer (SOL) combustion, with the goal of determining the major drivers in total C emissions, as well as to develop a high spatial resolution model to scale emissions in a relatively understudied region of the boreal forest. On average, 3.35kgCm-2 was combusted and almost 90% of this was from SOL combustion. Our results indicate that black spruce stands located at landscape positions with intermediate drainage contribute the most to C emissions. Indices associated with fire weather and date of burn did not impact emissions, which we attribute to the extreme fire weather over a short period of time. Using these results, we estimated a total of 94.3TgC emitted from 2.85Mha of burned area across the entire 2014 NWT fire complex, which offsets almost 50% of mean annual net ecosystem production in terrestrial ecosystems of Canada. Our study also highlights the need for fine-scale estimates of burned area that represent small water bodies and regionally specific calibrations of combustion that account for spatial heterogeneity in order to accurately model emissions at the continental scale.

Carbon accumulation in loblolly pine plantations is increased by fertilization across a soil moisture availability gradient

Silvicultural practices, particularly fertilization, may counteract or accentuate the effects of climate change on carbon cycling in planted pine ecosystems, but few studies have empirically assessed the potential effects. In the southeastern United States, we established a factorial throughfall reduction (D) × fertilization (F) experiment in 2012 in four loblolly pine (Pinus taeda L.) plantations encompassing the climatic range of the species in Florida (FL), Georgia (GA), Oklahoma (OK), and Virginia (VA). Net primary productivity (NPP) was estimated from tree inventories for four consecutive years, and net ecosystem productivity (NEP) as NPP minus heterotrophic respiration (RH). Soil respiration (RS) was measured biweekly-monthly for at least one year at each site and simultaneous measurements of RS & RH were taken five to eight times through the year for at least one year during the experiment. Reducing throughfall by 30% decreased available soil water at the surface and for the 0–90 cm soil profile. Fertilization increased NPP at all sites and D decreased NPP (to a lesser extent) at the GA and OK sites. The F + D treatment did not affect NPP. Mean annual NPP under F ranged from 10.01 ± 0.21 MgC·ha−1·yr−1 at VA (mean ± SE) to 17.20 ± 0.50 MgC·ha−1·yr−1 at FL, while the lowest levels were under the D treatment, ranging from 8.63 ± 0.21 MgC·ha−1·yr−1 at VA to 14.97 ± 0.50 MgC·ha−1·yr−1 at FL. RS and RH were, in general, decreased by F and D with differential responses among sites, leading to NEP increases under F. Throughfall reduction increased NEP at FL and VA due to a negative effect on RH and no effect on NPP. Mean annual NEP ranged from 1.63 ± 0.59 MgC·ha−1·yr−1 in the control at OK to 8.18 ± 0.82 MgC·ha−1·yr−1 under F + D at GA. These results suggest that fertilization will increase NEP under a wide range of climatic conditions including reduced precipitation, but either NPP or RH could be the primary driver because F can increase stand growth, as well as suppress RS and RH. Moreover, D and F never significantly interacted for an annual C flux, potentially simplifying estimates of how fertilization and drought will affect C cycling in these ecosystems.

Carbon fluxes and interannual drivers in a temperate forest ecosystem assessed through comparison of top-down and bottom-up approaches

Despite decades of research, gaining a comprehensive understanding of carbon (C) cycling in forests remains a considerable challenge. Uncertainties stem from persistent methodological limitations and the difficulty of resolving top-down estimates of ecosystem C exchange with bottom-up measurements of individual pools and fluxes. To address this, we derived estimates and associated uncertainties of ecosystem C fluxes for a 100–125 year old mixed temperate forest stand at the Bartlett Experimental Forest, New Hampshire, USA, using three different approaches: (1) tower-based eddy covariance, (2) a biometric approach involving C flux measurements of individual ecosystem subcomponents, and (3) an inventory approach involving changes in major C stocks over time. Our analysis made use of 13 years of data, collected over the period from 2004 to 2016.Estimates of mean annual net ecosystem production (NEP) ranged from 120 to 133 g C m−2, demonstrating strong agreement among methods and suggesting that this aging forest acts as a moderate C sink. The use of multiple approaches to measure C fluxes and their uncertainties helped place constraints on difficult-to-measure processes such as aboveground contributions to ecosystem respiration and belowground allocation to mycorrhizal fungal biomass (which was estimated at 20% of net primary production).Analysis of interannual variability in C fluxes revealed a decoupling between annual wood growth and either current year or lagged NEP or GPP, suggesting that source limitation (C supply) is likely not controlling rates of wood production, at least on an interannual scale. Results also demonstrated a strong association between the maximum rate of C uptake during the growing season (Amax) and the length of the vernal window, defined as the period of time between soil thaw and the onset of photosynthesis. This suggests an important, but poorly understood, influence of winter and spring climate on mid-summer canopy physiology. Efforts to resolve the mechanisms responsible should be prioritized in light of ongoing and predicted changes in climate for the northeastern U.S. region, particularly during the winter and winter-spring transition period.

Water response of ecosystem respiration regulates future projection of net ecosystem productivity in a semiarid grassland

Recent evidences show that terrestrial biogeochemical models have large uncertainty in estimating climate-change effect on grassland net ecosystem productivity (NEP), which is defined as the difference between gross ecosystem photosynthesis (GEP) and respiration (ER). It remains unclear that whether GEP or ER limits the model capability to simulate NEP responses to climate change in semiarid grasslands. Given the surrogate CENTURY-type model is widely used for Earth system modeling, we investigated two of them (i.e., DAYCENT and TECO models) and examined which processes dominate their ability to capture the responses of NEP to experimental climate changes in a temperate steppe of northern China. During the simulation from 2006 to 2008, the two models captured the observed mean annual NEP in the control plots when they were validated by the observations from an adjacent eddy-flux tower. However, they failed to capture the treatment effects of experimental warming and increased precipitation on NEP because of the poor estimations of ER responses. DAYCENT model simulated a higher precipitation effect on ER (37.83%) and TECO model overestimated the warming effect on ER by 8.18%. The simulation of treatment effects on ER and therefore NEP can be improved by an optimized parameterization of the water-related decay functions for soil organic carbon (C). The simulated cumulative loss of total ecosystem C stock during 2010–2100 were decreased when the TECO model used experiment-fitted parameters (0.72 kg C m−2) instead of using the initial validation with eddy-flux data (0.96 kg C m−2). The ecosystem shifted from C sink to source at threshold of 435 mm of annual total precipitation. Our findings indicate that future projection of C cycle in semiarid grasslands could be improved by better understanding of water response of ecosystem respiratory processes.

Observed high and persistent carbon uptake by Moso bamboo forests and its response to environmental drivers

Moso bamboo (Phyllostachys edulis) is widely distributed in subtropical regions, especially in China. However, the capacity of Moso bamboo forests to sequester carbon remains poorly quantified at the ecosystem level. To our knowledge, this study reports the first use of the eddy covariance technique to measure and quantify the net ecosystem production (NEP) of Moso bamboo forests from 2011 to 2015. The mean annual NEP was 602.7±59.7gCm−2, which is far higher than that of the other forest types in subtropical China. The total NEP of China’s Moso bamboo forests was roughly estimated at 0.027PgCyr−1, which accounts for 15–36% of the NEP of all forests of China. In Moso bamboo, the explosive growth pattern, high photosynthetic rate, and sufficient and synchronous water and heat availability, coupled with high atmospheric deposition of nitrogen and phosphorus contribute to the regulation of NEP dynamics and its carbon sequestration efficiency. An extreme drought that occurred in the July and August of 2013 significantly reduced the NEP of Moso bamboo forest by 60–78%. Our findings suggest that the carbon uptake of Moso bamboo forests is much higher than previously thought and that the large potential of these forests in mitigating climate change should not be ignored.

How do disturbances and climate effects on carbon and water fluxes differ between multi-aged and even-aged coniferous forests?

Disturbances and climatic changes significantly affect forest ecosystem productivity, water use efficiency (WUE) and carbon (C) flux dynamics. A deep understanding of terrestrial feedbacks to such effects and recovery mechanisms in forests across contrasting climatic regimes is essential to predict future regional/global C and water budgets, which are also closely related to the potential forest management decisions. However, the resilience of multi-aged and even-aged forests to disturbances has been debated for >60years because of technical measurement constraints. Here we evaluated 62site-years of eddy covariance measurements of net ecosystem production (NEP), evapotranspiration (ET), the estimates of gross primary productivity (GPP), ecosystem respiration (Re) and ecosystem-level WUE, as well as the relationships with environmental controls in three chronosequences of multi- and even-aged coniferous forests covering the Mediterranean, temperate and boreal regions. Age-specific dynamics in multi-year mean annual NEP and WUE revealed that forest age is a key variable that determines the sign and magnitude of recovering forest C source-sink strength from disturbances. However, the trends of annual NEP and WUE across succession stages between two stand structures differed substantially. The successional patterns of NEP exhibited an inverted-U trend with age at the two even-aged chronosequences, whereas NEP of the multi-aged chronosequence increased steadily through time. Meanwhile, site-level WUE of even-aged forests decreased gradually from young to mature, whereas an apparent increase occurred for the same forest age in multi-aged stands. Compared with even-aged forests, multi-aged forests sequestered more CO2 with forest age and maintained a relatively higher WUE in the later succession periods. With regard to the available flux measurements in this study, these behaviors are independent of tree species, stand ages and climate conditions. We also found that distinctly different environmental factors controlled forest C and water fluxes under three climatic regimes. Typical weather events such as temperature anomalies or drying-wetting cycles severely affected forest functions. Particularly, a summer drought in the boreal forest resulted in an increased NEP owing to a considerable decrease in Re, but at the cost of greater water loss from deeper groundwater resources. These findings will provide important implications for forest management strategies to mitigate global climate change.

CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America.

Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by measured CO2 exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000mm, annual temperatures of 2-25°C, and records of 3-10years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from -350 to +330 gCm-2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (Reco ) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and Reco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO2 exchange may be up to 3-5 times larger than current estimates.

Integrating peatlands into the coupled Canadian Land Surface Scheme (CLASS) v3.6 and the Canadian Terrestrial Ecosystem Model (CTEM) v2.0

Abstract. Peatlands, which contain large carbon stocks that must be accounted for in the global carbon budget, are poorly represented in many earth system models. We integrated peatlands into the coupled Canadian Land Surface Scheme (CLASS) and the Canadian Terrestrial Ecosystem Model (CTEM), which together simulate the fluxes of water, energy, and CO2 at the land surface–atmosphere boundary in the family of Canadian Earth system models (CanESMs). New components and algorithms were added to represent the unique features of peatlands, such as their characteristic ground floor vegetation (mosses), the slow decomposition of carbon in the water-logged soils and the interaction between the water, energy, and carbon cycles. This paper presents the modifications introduced into the CLASS–CTEM modelling framework together with site-level evaluations of the model performance for simulated water, energy and carbon fluxes at eight different peatland sites. The simulated daily gross primary production (GPP) and ecosystem respiration are well correlated with observations, with values of the Pearson correlation coefficient higher than 0.8 and 0.75 respectively. The simulated mean annual net ecosystem production at the eight test sites is 87 g C m−2 yr−1, which is 22 g C m−2 yr−1 higher than the observed annual mean. The general peatland model compares well with other site-level and regional-level models for peatlands, and is able to represent bogs and fens under a range of climatic and geographical conditions.

Carbon budget in a Mediterranean peach orchard under different management practices

The soil organic carbon (SOC) content of many Mediterranean soils is low (∼1%) and this hinders both economic and ecologic progress. The climate in much of the Mediterranean region (low annual precipitation, cool wet winters, hot dry summers), combined with traditional agricultural practices has a major impact on the carbon (C) cycle. To increase our knowledge of C fluxes in Mediterranean agro-ecosystems, this paper examines the effects on the soil and biome C budgets of a peach (Prunus persica L. Batsch) orchard, seven years after adopting sustainable management practices (Smng). The result is compared with the continued use of locally conventional management practices (Cmng). Sustainable management involved zero-tillage, weed mowing, retention of above-ground residues and the import of organic amendments, while Cmng involved tillage, removal of pruning residues and the application of mineral fertilisers. The annual net ecosystem production (NEP) was determined through field measurements of soil respiration (Li-6400, LI-COR, USA) and above- and below-ground biomass sampling. The mean annual NEP was close to 320 and 475gCm−2yr−1 in the Cmng and Smng plots, respectively. As managed ecosystems, anthropogenic C imports/exports and related changes of soil C pool were then accounted for through the net ecosystem C balance (NECB). The NECB approximated 90gCm−2yr−1 for Cmng and 730gCm−2yr−1 for Smng. This result highlights the critical role of appropriate management of the variable components on sustaining ecosystem resilience, including the management of pruning residues, the import of organic materials, and the maintenance of a cover crop. Over a 7-year study period, C stock (SOC and litter) increased at a mean rate of ∼145gCm−2yr−1 in the Smng plot while it increased at only ∼7.5gCm−2yr−1 in the Cmng plot. Whole-tree standing biomass was measured by tree excavation revealing that the C sequestered over the 14-year lifetime of the orchard was close to 25tCha−1. This study provides information on C stock variation (soil+biome) and on annual net atmospheric C removal (NEP) in a cultivated peach orchard under Mediterranean climate conditions.

Evaluating the impacts of climate variability and disturbance regimes on the historic carbon budget of a forest landscape

Landscape-level understanding of forest carbon (C) dynamics is required to quantify the net contribution of forest biomes to the global C cycle and to help forest managers to understand the impacts of forest management activities on the C sequestration in forests. In this study, the effects of interannual climate variability, carbon dioxide (CO2) fertilization, and disturbance regimes on the C dynamics of an old-growth Pacific Northwest temperate conifer forest landscape (2500ha) were studied from 1920 to 2005, using a process-based land surface model, known as the Carbon and Nitrogen coupled Canadian Land Surface Scheme (CN-CLASS). The model was parameterized with ecological, forest inventory and historical land-use data, and run using historical meteorological observations. Before performing landscape level simulations, model results were evaluated against eddy covariance flux tower observations. Simulated mean annual net ecosystem productivity (NEP) over the flux tower footprint area was 340gCm−2yr−1 from 1998 to 2005, while the measured value was 293±20.5gCm−2yr−1. When two anomalous weather years, corresponding to El Niño (1998) and La Niña (1999) events, were excluded while performing statistical analysis, measured and simulated fluxes were highly, but negatively, correlated to both annual mean air temperature and annual precipitation (R2=0.69 and 0.60, respectively). Interannual variability of simulated and measured NEP over the flux footprint area, calculated as deviations of the respective annual NEP values, was 143gCm−2yr−1 and 61gCm−2yr−1, respectively. On the landscape-level, prior to disturbance in 1920, simulated C fluxes indicated that the forest landscape was close to C neutral, with annual net biome productivity (NBP) of 0.8gCm−2yr−1. However, during the intense disturbance period from 1938 to 1944, landscape-level NBP reached about −5083gCm−2yr−1. Then from 1951 to 1997, when there were no major disturbance events, NBP gradually recovered to about 365gCm−2yr−1. At the end of the study period, in 2005, the landscape again became C source, due to harvesting of second growth stands that occurred from late 1990s to 2000s.The regression of age-detrended variations in the simulated annual C fluxes to mean daily maximum air temperature over the peak growing season (July–September), during an undisturbed period from 1963 to 1984, indicated that summer temperature was the dominant climatic control on landscape-level C fluxes. Higher temperatures caused a decrease in gross primary productivity at almost twice the rate of increase in ecosystem respiration (i.e. 27gCm−2yr−1C−1 versus 15.7gCm−2yr−1C−1, respectively). A sensitivity analysis to evaluate the impacts of climate variability and disturbances showed that the relative effect of disturbance on carbon stocks was greater than the effect on carbon fluxes. Overall CO2 fertilization effects were minor. Disturbance type and severity, represented by the standard deviation in NBP, as described in the model, determined the magnitude of the simulated C losses to the atmosphere. This study enhances our understanding of the impacts of future climate change and forest management on landscape-level C dynamics in forests.