Regional and seasonal partitioning of water and temperature controls on global land carbon uptake variability

Rise in wetland carbon uptake linked to increased potential evapotranspiration

Precisely assessing wetland net ecosystem productivity (NEP) is important for accurately evaluating global carbon budgets. However, constrained by the quality of observational data and insufficient understanding of driving mechanisms, assessments of China's wetland NEP still have considerable uncertainties. Therefore, this study assessed continuous observations from 30 eddy covariance flux towers across various wetland types in China and applied the random forest (RF) model to simulate the spatiotemporal dynamics of China's wetland NEP. The results showed that from 1982 to 2020, China's wetlands represented a net C-CO2 sink overall, with an average NEP of 21.61 ± 0.04 mg C m-2 h-1 and annual net C-CO2 absorption of 56.23 Tg C. Riverine and coastal wetlands had the highest NEP, while freshwater marshes had the lowest. From 1982 to 2020, the wetland NEP in China exhibited a significant increasing trend. Further analysis indicated that potential evapotranspiration (PET) is the main driving factor behind the significant increase in NEP in China's wetlands, with a clear threshold effect: NEP rises with PET up to a certain point (e.g., 160 mm), after which it declines. This study accurately quantified the spatiotemporal dynamics of China's wetland NEP and revealed the critical impact of PET on NEP, thus providing a new perspective for performing wetland carbon cycle research and formulating climate change mitigation strategies.

Nonlinear spatiotemporal variability of gross primary production in China's terrestrial ecosystems under water energy constraints

Gross primary production (GPP) plays a crucial role in carbon cycling and ecosystem productivity, yet its variability is significantly influenced by climatic factors. This study investigates the spatiotemporal variability of GPP in China's terrestrial ecosystems, with a focus on water and energy limitations. It aims to clarify the relationship between GPP and climatic variables across different regimes. The results reveal nonlinear turning points in more than half of the country's regions, most turning points occurred in the 1980s and 2000s. Water-related factors predominantly influenced GPP changes, while energy limitations contributed to GPP decline in certain areas. Over the past decade, ecological restoration efforts focused on vegetation greening have enhanced habitat quality in China. However, future climate warming is projected to shift more regions into energy-limited conditions, leading to negative GPP sensitivity to temperature and SSRD, and potentially accelerating GPP degradation. These findings underscore the need for targeted management strategies in energy-limited regions to mitigate the adverse impacts of climate change on ecosystem productivity. This study provides critical insights into the interplay between climatic factors and GPP dynamics, offering a valuable framework for future research and policy development aimed at sustaining ecosystem services under changing climatic conditions.

Growth Rate and Not Growing Season Explains the Increased Productivity of Masson Pine in Mixed Stands

Increased tree species diversity can promote forest production by reducing intra-specific competition and promoting an efficient unitization of resources. However, questions remain on whether and how mixed stands affect the dynamics of intra-annual xylem formation in trees, especially in subtropical forests. In this study, we randomly selected 18 trees from a monoculture of 63-year-old Masson pine (Pinus massoniana) growing in pure stands and mixed them with 39-year-old Castanopsis hystrix in Pinxiang, southern China. A total of 828 microcores were collected biweekly throughout the growing season from 2022 to 2023 to monitor the intra-annual xylem formation. Cell production started in early March and ended in late December and lasted about 281 to 284 days. Xylem phenology was similar between mixed and pure stands. During both seasons, the Masson pine in mixed stands showed higher xylem production and growth rates than those in pure stands. The Masson pine in mixed stands produced 45-51 cells in 2022 (growth rate of 0.22 cells day-1) and 35-41 cells in 2023 (0.17 cells day-1). Growth rate, and not growth seasons, determined the superior xylem growth in the mixed stands. Our study shows that after 39 years of management, Masson pine and C. hystrix unevenly aged mixed stands have a significant positive mixing effect on Masson pine xylem cell production, which demonstrates that monitoring intra-annual xylem growth dynamics can be an important tool to evaluate the effect of species composition and reveal the mechanisms to promote tree growth behind the mixing effect.

Sensitivity of land carbon sinks to the three major oscillations in the Northern Hemisphere

The Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Pacific-North American Pattern (PNA) cause climate variability in the Northern Hemisphere (NH), which affects the carbon cycle of terrestrial ecosystems. Based on a dynamic global vegetation model, we analysed the impacts of the AO, NAO and PNA on changes in terrestrial climate and carbon cycle dynamics from 1980 to 2017. The positive AO (pAO), positive NAO (pNAO), and positive PNA (pPNA) mainly led to warmer and more humid conditions in the North Asia (NA) and Europe (EUR), whereas the negative AO (nAO), negative NAO (nNAO), and negative PNA (nPNA) resulted in colder and drier conditions. Furthermore, the nAO, nNAO, and nPNA increased the carbon sinks of terrestrial ecosystems, whereas the pAO, pNAO, and pPNA reduced the carbon sinks, especially in EUR. We also quantified the direct impacts of the oscillations in the concurrent season and their legacy impacts from the preceding season separately. Increased AO and NAO indices increased the carbon sinks in the East Asia (EA) and EUR, whereas an increased PNA index reduced the carbon sinks in most parts of the NH. With respect to legacy impacts, increased AO and PNA indices enhanced the carbon sinks in the Central-Western Asia and North Africa (CWN), Temperate North America (TNA) and Boreal North America (BNA), whereas an increased NAO index strengthened the carbon source capacity in the CWN, EUR, TNA, BNA. These results provide a framework for conducting further research on the mechanisms of interannual variability of the terrestrial carbon cycle.

Assessment of the potential for carbon sink enhancement in the overlapping ecological project areas of China

Ecological engineering can significantly improve ecosystem carbon sequestration. However, few studies have projected the carbon sink trends in regions where ecological engineering projects overlap and have not considered the different climate change conditions and land use scenarios. Using the ensemble empirical mode decomposition method and machine learning algorithms (enhanced boosted regression trees), the aims of this study to elucidate the stability of carbon sinks and their driving mechanisms in areas where ecological projects overlap and to predict the potential enhancement in carbon sinks under varying climate and human activity scenarios. The findings revealed that: (1) The carbon sinks clearly and steadily increased in regions where five ecological projects were implemented from 1982 to 2019. In contrast, the carbon sinks did not significantly increase in regions with two or three ecological projects. (2) As the number of ecological projects increased, the impact of human activities on the carbon sinks gradually decreased. In eastern China, rapid economic development and significant interference from human activities hindered the growth of carbon sinks. In contrast, in western China, the warming and humidification trend of the climate, large-scale afforestation, and other ecological projects have significantly improved carbon sinks. (3) The regions with five overlapping ecological projects exhibited the greatest enhancement and stability of carbon sinks under different scenarios. Compared with the SSP585 scenario, under the SSP126 scenario, the carbon sinks increased, and their stability was greater. Achieving carbon neutrality requires major ecological projects to account for the limitations imposed by climatic conditions. Instead of isolated projects or the implementation of single restoration measures, a comprehensive approach that uses the synergistic effects of combined ecological strategies is recommended.

Enhanced global carbon cycle sensitivity to tropical temperature linked to internal climate variability

The sensitivity of atmospheric CO2 growth rate to tropical temperature (γT) has almost doubled between 1959 and 2011, a trend that has been linked to increasing drought in the tropics. However, γT has declined since then. Understanding whether these variations in γT reflect forced changes or internal climate variability in the carbon cycle is crucial for future climate projections. We show that doubling sensitivity events can arise in simulations by Earth system models with perturbed initial conditions but are likely explained by internal climate variability. We show that the doubling sensitivity event is associated with the occurrence of a few, but very strong, El Niño events, such as 1982/83 and 1997/98. Such extreme events result in concurrent carbon release by tropical and extratropical ecosystems, increasing the variance of the global land carbon sink and its apparent sensitivity to tropical temperature. Our results imply that the doubling sensitivity does not necessarily indicate a change in carbon cycle response to climate change.

Modeling the carbon dynamics of ecosystem in a typical permafrost area

Climate change poses mounting threats to fragile alpine ecosystem worldwide. Quantifying changes in carbon stocks in response to the shifting climate was important for developing climate change mitigation and adaptation strategies. This study utilized a process-based land model (Community Land Model 5.0) to analyze spatiotemporal variations in vegetation carbon stock (VCS) and soil organic carbon stock (SOCS) across a typical permafrost area - Qinghai Province, China, from 2000 to 2018. Multiple potential factors influencing carbon stocks dynamics were analyzed, including climate, vegetation, soil hydrothermal status, and soil properties. The results indicated that provincial vegetation carbon storage was 0.22 PgC (0.32 kg/m2) and soil organic carbon pool was 9.12 PgC (13.03 kg/m2). VCS showed a mild increase while SOCS exhibited fluctuating uptrends during this period. Higher carbon stocks were observed in forest (21.74 kg/m2) and alpine meadow (18.08 kg/m2) compared to alpine steppes (9.63 kg/m2). Over 90 % of the carbon was stored in the 0-30 cm topsoil layer. The contribution rates of soil carbon in the 30-60 cm and 60-100 cm soil layers were significantly small, despite increasing stocks across all depths. Solar radiation, temperature, and NDVI emerged as primary influential factors for overall carbon stocks, exhibiting noticeable spatial variability. For SOCS at different depths, the normalized differential vegetation index (NDVI) was the foremost predictor of landscape-level carbon distributions, which explained 52.8 % of SOCS variability in shallow layers (0-30 cm) but dropped to just 12.97 % at the depth of 30-60 cm. However, the dominance of NDVI diminished along the soil depth gradients, superseded by radiation and precipitation. Additionally, with an increase in soil depth, the influence of inherent soil properties also increased. This simulation provided crucial insights for landscape-scale carbon responses to climate change, and offered valuable reference for other climate change-sensitive areas in terms of ecosystem carbon management.

A meta-analysis reveals increases in soil organic carbon following the restoration and recovery of croplands in Southwest China

In China, the Grain for Green Program (GGP) is an ambitious project to convert croplands into natural vegetation, but exactly how changes in vegetation translate into changes in soil organic carbon remains less clear. Here we conducted a meta-analysis using 734 observations to explore the effects of land recovery on soil organic carbon and nutrients in four provinces in Southwest China. Following GGP, the soil organic carbon content (SOCc) and soil organic carbon stock (SOCs) increased by 33.73% and 22.39%, respectively, compared with the surrounding croplands. Similarly, soil nitrogen increased, while phosphorus decreased. Outcomes were heterogeneous, but depended on variations in soil and environmental characteristics. Both the regional land use and cover change indicated by the landscape type transfer matrix and net primary production from 2000 to 2020 further confirmed that the GGP promoted the forest area and regional mean net primary production. Our findings suggest that the GGP could enhance soil and vegetation carbon sequestration in Southwest China and help to develop a carbon-neutral strategy.