Chronic warming and dry soils limit carbon uptake and growth despite a longer growing season in beech and oak

Prolonged warming and drought reduce canopy-level net carbon uptake in beech and oak saplings despite photosynthetic and respiratory acclimation

Tree net carbon (C) uptake may decrease under global warming, as higher temperatures constrain photosynthesis while simultaneously increasing respiration. Thermal acclimation might mitigate this negative effect, but its capacity to do so under concurrent soil drought remains uncertain. Using a 5-yr open-top chamber experiment, we determined acclimation of leaf-level photosynthesis (thermal optimum Topt and rate Aopt) and respiration (rate at 25°C R25 and thermal sensitivity Q10) to chronic +5°C warming, soil drought, and their combination in beech (Fagus sylvatica L.) and oak (Quercus pubescens Willd.) saplings. Process-based modeling was used to evaluate acclimation impacts on canopy-level net C uptake (Atot). Prolonged warming increased Topt by 3.03-2.66°C, but only by 1.58-0.31°C when combined with soil drought, and slightly reduced R25 and Q10. By contrast, drought reduced Topt (-1.93°C in oak), Aopt (c. 50%), and slightly reduced R25 and Q10 (in beech). Mainly because of reduced leaf area, Atot decreased by 47-84% with warming (in beech) and drought, but without additive effects when combined. Our results suggest that, despite photosynthetic and respiratory acclimation to warming and soil drought, canopy-level net C uptake will decline in a persistently hotter and drier climate, primarily due to the prevalent impact of leaf area reduction.

The influence of agricultural drought on carbon emissions across the four sub-regions of China

Vegetation is crucial in carbon sequestration, as it stores soil carbon and biomass. However, agricultural droughts significantly reduce vegetation growth, directly impacting the amount of carbon sequestered through photosynthesis. This study investigates the effects of agricultural drought on carbon emissions across four sub-regions of China, Northwest China, North China, the Qinghai-Tibet region, and South China, from 2001 to 2020. Three remote sensing-based drought indices, the Moisture Anomaly Index (MAI), Vegetation Anomaly Index (VAI), and Temperature Anomaly Index (TAI) were used for drought monitoring. Advanced statistical techniques were employed to explore the relationship between these indices and carbon emissions, including auto-correlation and spatial cross-correlation. The results indicate that temporal variations between carbon emissions and agricultural drought indices exhibit distinct regional patterns. Among the indices, VAI demonstrated the strongest correlation with carbon emissions, with values ranging from r = 0.56 to 0.76. Carbon emissions varied significantly across regions, with the highest recorded in North China, followed by South China, Northwest China, and Qinghai-Tibet regions. Spatial cross-correlation analysis revealed that the highest positive correlation (r > 0.5) between carbon emissions and drought indices was observed in South China, whereas a moderate correlation was found between MAI and carbon emissions in Northwest China. The correlation between VAI and carbon emissions ranged from r = -0.6 to > 0.8. TAI exhibited a positive correlation with carbon emissions in South China, whereas negative correlations were observed in Northwest China and northeast North China. These findings provide valuable insights for mitigating drought-induced carbon emissions and promoting sustainable land management practices.