Bioclimatic controls on compound dry-hot thresholds that govern dryland grassland ecosystem stability

The intensifying frequency and severity of compound moisture–temperature extremes pose a profound threat to ecosystem stability. This is especially the case for drylands, which are facing these compound events at rates increasing at least twice as much as they do for humid regions, both in terms of event frequency and intensity. Identifying “tipping” compound dry-hot thresholds at which vegetation survival is currently threatened is therefore critical to anticipate large-scale ecosystem collapse in water-scarce regions.

In this study, we built a copula-based probabilistic framework to investigate the responses of 132 grassland sites to compound dry-hot events of varying intensity between 2000–2022. The sites considered comprise 299 species and span an area of 6.6 million km² in north China, with climates ranging from hyper-arid to dry sub-humid. At each site, our framework allowed us to examine the likelihood for dry-hot conditions to pose a threat to the vegetation. That is, we established site-specific “eco-risk probabilities” relating compound dry-hot intensity thresholds (defined by the standardized soil moisture and heatwaves index, CMHI) to significant impacts on vegetation structure. We further investigated the relationship between eco-risk probabilities, “tipping” dry-hot thresholds, and both longer-term ecosystem pedoclimatic conditions and underlying biotic factors like species richness.

We found >64% of the surveyed drylands area to have experienced an increase in eco-risk with intensifying compound dry-hot events between 2000-2022. “Tipping” thresholds for compound dry-hot events spanned the full breath of the CMHI index, from -2.84 (extremely severe dry-hot event) to -0.16 (very dry-hot event), indicating that different grassland ecosystems show very different levels of vulnerability. Among the multiple pedo-climatic and biotic factors considered as possible explainers for site-specific “tipping” dry-hot thresholds, continued warming emerges as the primary driver. Notably, a relatively higher species’ phylogenetic diversity greatly helps grasslands resist compound dry-hot extremes.

Our results confirm previous findings showing that dryland ecosystem stability is under an acute risk with rising temperatures; however, enhancing plant phylogenetic diversity may help mitigate the escalating threat faced by these ecosystems.