Vegetation Productivity Responses to Compound and Individual Climatic Stressors in Drylands

Abstract

Dryland ecosystems are a critical component of the terrestrial carbon cycle and host a significant portion of the world's biodiversity. Under anthropogenic warming, these water-limited ecosystems are increasingly threatened by the more frequent and severe heatwaves and often co-occurring droughts (known as compound hot-dry extremes). However, the respective impacts of heat and water stress on ecosystems functioning in drylands during compound extremes remain unexplored. Using satellite-observed solar-induced chlorophyll fluorescence as a proxy for vegetation productivity, our study examined its response to variations of soil moisture and air temperature by separating conditions where the two climatic factors are coupled (hot and dry) or decoupled (hot but not dry, dry but not hot). The decoupled conditions confirm soil moisture as the predominant control of dryland ecosystem productivity. However, temperature acts as a significant modulator of the moisture control particularly under wet or dry extremes: high temperatures amplify both the adverse effect of dry extremes and the positive effect of wet extremes. The productivity sensitivity to temperature and water varies strongly across regions and vegetation types, with deeper-rooted vegetation exhibiting stronger resistance to hot-dry extremes. With more frequent occurrence of hot-dry extremes under global warming, this trend leads to more negative productivity anomalies and leaves a footprint on the trajectory of productivity changes in drylands. This study highlights the additional heat-water stress on dryland ecosystems by compound hot-dry extremes superimposed on that from anthropogenic warming and drying, which adversely impact photosynthetic activities and increasingly counteract the beneficial CO₂ fertilization effect.