Focus on Nature-based Solutions Toward Sustainability

Climate exerts both short-term and long-term impacts on the ecosystem carbon assimilation. However, the main climatic drivers for the variability of gross primary productivity (GPP) remain unclear across various timescales and vegetation types. Here, we combine the state-of-the-art machine learning algorithms with a well-established explanatory method to explore the impacts of climatic factors on long-term GPP variability at global FLUXNET sites across four timescales and six plant functional types. Results show that diffuse shortwave radiation (SWdif) dominates GPP variability at the sub-daily (half-hourly to three hourly) timescales especially for the tree species, and acts as the secondary contributor after air temperature at the daily or longer timescales. Attribution analyses further showed that the main effects of SWdif are much higher than their interactive effects with other climatic factors in regulating the GPP variability. By identifying the main climatic drivers, this study improves the understanding of the climate-driven GPP variability and provides important implications for the future projection of ecosystem carbon assimilation under global climate change.

Over the past decades, ecological restoration initiatives in China have made great progress in restoring degraded forests and increasing vegetation coverage, yet the carbon sequestration effects of these initiatives in the context of climate change are not clear. In this study, we assessed the effects of vegetation restoration on gross primary production (GPP) in China's forestry engineering areas, where large-scale vegetation restoration programmes were launched, during 2001–2020 by disentangling the respective roles of land cover change (LCC), CO₂ fertilization, and climate changes using a two-leaf light use efficiency model. We found that LCC attributed by the vegetation restoration dominantly accelerated the increase of GPP in seven out of the eight areas, and CO₂ fertilization played a near-equivalent role in all areas. By contrast, the changes in different climate factors contributed to GPP variations diversely. The solar radiation variation greatly inhibited the vegetation GPP over time in seven out of these areas, and the changes in air temperature and vapor pressure deficit regulated GPP inter-annual variations without clear trends in all areas. This study advances our understanding of the contribution of China's afforestation on its forest GPP in a changing climate, which may help to better manage forests to tackle the challenge of the climate crisis in the future.

Terrestrial ecosystems play a pivotal role in the global carbon sequestration process, and their photosynthetic capacity is highly susceptible to fluctuations in climate conditions. In 2022, the Yangtze River Basin (YRB) in China experienced an extensive and severe compounded heat and drought event. Compared with the past two decades, our results revealed that the temperature increased by approximately 0.78 ± 0.45 °C and precipitation decreased by about 45.20 ± 30.10 mm from July to October 2022 over the whole YRB. Region I (west from the Sichuan Basin and east to the easternmost of the basin) experienced a more severe temperature increase (0.98 ± 0.35 °C) and precipitation decrease (−60.27 ± 23.75 mm) compared to the other regions in the YRB. Changes in temperature and precipitation resulted in an increase of 0.14 ± 0.06 kPa in vapor pressure deficit (VPD) and a decrease of 5.28 ± 2.09 m³ m⁻³ in soil moisture, ultimately leading to a total loss of 26.12 ± 16.09 Tg C (about −6.08% compared to the 2001–2021 mean) in gross primary productivity (GPP) of July to October in 2022. It is noteworthy that broadleaf forests, which comprise 12.03% of the natural vegetation in region I, contributed only 6.46% of the GPP loss between July and October compared to other vegetation types, showing greater resistance to this climate event. Our findings from multiple linear regressions highlight that high temperatures and reduced soil moisture together contribute up to 94% photosynthesis loss in July–October in natural vegetation in region I, while the contribution of reduced VPD is minimal. In the future, we will further explore the impacts of compound heat and drought events on the coupled carbon and water cycles across different ecosystems, in order to better understand the ecosystem response mechanisms to extreme climates.

The international community, through treaties such as the Paris agreement, aims to limit climate change to well below 2 °C, which implies reaching carbon neutrality around the second half of the century. In the current calculations underpinning the various roadmaps toward carbon neutrality, a major component is a steady or even expanding terrestrial carbon sink, supported by an increase of global forest biomass. However, recent research has challenged this view. Here we developed a framework that assesses the potential global equilibrium of forest biomass under different climate change scenarios. Results show that under global warming carbon storage potential in forest aboveground biomass gradually shifts to higher latitudes and the intensity of the disturbance regimes increases significantly almost everywhere. CO₂ fertilization stands out as the most uncertain process, with different methods of estimation leading to diverging results by almost 155 PgC of above ground biomass at equilibrium. Overall, assuming that the sum of human pressures (e.g. wood extraction) does not change over time, that total forest cover does not change significantly and that the trend in CO₂ fertilisation as it is currently estimated from satellite proxy observations remains, results show that we have reached (or are very close to reaching) the peak of global forest carbon storage. In the short term, where increased disturbance regimes are assumed to act quicker than increased forest growth potential, global forests might instead act as a carbon source, that will require even more effort in decarbonization than previously estimated. Therefore, the potential of forests as a nature-based solution to mitigate climate change brings higher uncertainties and risks than previously thought.

Crop harvested carbon (HC) is one of the most important components of the carbon cycle in cropland ecosystems, with a significant impact on the carbon budget of croplands. China is one of the most important crop producers, however, it is still unknown on the spatial and temporal variations of HC. This study collected statistical data on crop production at the province and county levels in China for all ten crop types from 1981 to 2020 and analyzed the magnitude and long-term trend of harvested crop carbon. Our results found a substantial increase of HC in cropland from 0.185 Gt C yr⁻¹ in 1981 to 0.423 Gt C yr⁻¹ in 2020 at a rate of 0.006 Gt C yr⁻¹. The results also highlighted that the average annual carbon sink removal from crop harvesting in China from 1981 to 2020 was 0.32 Gt C yr⁻¹, which was comparable to the net carbon sink of the entire terrestrial ecosystems in China. This study further generated a gridded dataset of HC from 2001 to 2019 in China by using jointly the statistical crop production and distribution maps of cropland. In addition, a model-data comparison was carried out using the dataset and results from seven state-of-the-art terrestrial ecosystem models, revealing substantial disparities in HC simulations in China compared to the dataset generated in the study. This study emphasized the increased importance of HC for estimating cropland carbon budget, and the produced dataset is expected to contribute to carbon budget estimation for cropland ecosystems and the entire China.