Julen Astigarraga

Climate change, characterized by an increase in extreme climatic events, poses a significant threat to forest productivity worldwide, potentially compromising its critical role in regulating the global carbon cycle. The response of forest productivity to compound events, such as hotter droughts, can be strongly modulated by functional traits. Forests dominated by acquisitive tree species (prioritizing resource acquisition) may be more vulnerable than forests dominated by conservative species (prioritizing efficiency and stress tolerance). Meanwhile, functional diversity could enhance ecosystem resistance. Although projections indicate that hotter droughts will increasingly hamper forest productivity, most studies focus solely on drought intensity, neglecting duration and frequency. To assess the effects of hotter droughts, we analyzed how their intensity, frequency, and duration influence forest productivity over time, and evaluated the role of functional identity and diversity in modulating forest responses. We analyzed forest productivity from 1985 to 2018 in a Mediterranean region of central Spain using generalized linear mixed models. Our estimates were based on biomass data resulting from integrating LiDAR, multispectral imagery, and forest inventory records from 748 plots. Our results indicated that the negative effects of the increased vapor pressure deficit and drought dimensions on forest productivity have intensified over time. Under extreme drought conditions, forests with higher functional diversity and dominated by moderately conservative species experienced smaller declines in productivity, whereas forests dominated by highly conservative species showed increasingly negative responses. This result may reflect repeated drought effects resulting in long stomatal closure periods and carbon starvation. Our findings highlight the increasing and negative impact of hotter droughts over time on Mediterranean forests. Altogether, the stabilizing role of functional diversity and identity in drought tolerance traits, such as cavitation resistance, should be considered in models addressing long-term climate change impacts on forest mitigation potential.