Biodiversity and Mass Effects Co-regulate Urban Forest Drought Resistance of 116 Cities in Yunnan Province, China

Climate change induced severe droughts present substantial challenges to urban forests, encompassing all trees growing within cities. While previous studies have highlighted the role of species richness in enhancing drought resistance in natural ecosystems, its impact within urban forests remains less understood. To address this key question, this study compiled extensive inventories of tree species richness in 1,115 urban forest patches across 116 cities with diverse ecoregions in Yunnan—China’s most biodiverse province. By using satellite-derived enhanced vegetation index (EVI) as proxy of urban forest growth and the standardized precipitation evapotranspiration index (SPEI) to differentiate drought timing, frequency, and intensity, we systematically analyzed the impact of species richness on drought resistance, considering vegetation characteristics, climate variables, human activities, and their interactions. We reveal that species richness and forest cover co-regulate drought resistance in urban settings. However, a consistent and significant positive effect of species richness is observed primarily in warm, higher-income regions during infrequent dry-season droughts (p < 0.01). Urban forests characterized by a higher proportion of non-native and evergreen species show higher drought resistance. These results underscore the importance of effective management in urban forests to enhance their resilience against drought.