Hot drought reshapes the relationship between tree species diversity and forest resilience

Abstract

Climate warming is increasing the occurrence of hot droughts, in which water deficits coincide with anomalously high temperatures, posing growing risks to forest ecosystems and carbon sequestration. Yet whether tree species diversity (TSD) affects forest resilience differently under drought-only and hot drought remains unclear. Here, we conducted a global assessment of TSD effects on NDVI-based forest resilience following drought-only and hot drought events. By integrating satellite-based estimates of resilience with global tree species richness data and environmental factors across ten major forest biomes, we found that global forests exhibited reduced post-drought growth under both drought conditions, with more spatially extensive reductions under hot drought. Importantly, the TSD-resilience relationship differed markedly between the two drought types. TSD effects on forest resilience were widely negative under drought-only conditions but were predominantly positive under hot drought. Hot drought also amplified both positive and negative TSD effects relative to drought-only events. Interaction analyses further showed that, under drought-only conditions, TSD effects were broadly modulated by mean annual temperature and vapor pressure deficit, whereas under hot drought, mean annual temperature was the only environmental variable showing widespread modulation of TSD effects. In addition, resilience was more sensitive to the number of tree species under hot drought and in species-poor forests. Our findings highlight that anomalously high temperature accompanying drought reshapes both the direction and magnitude of TSD effects on forest resilience and identify low-richness regions as strategic targets where management interventions may deliver greater ecological benefits.