Effects of niche marginality on hotter-drought tree mortality in angiosperms and gymnosperms

Aim: Current observations of global tree mortality events associated with drought raise concerns about climate change risks to forests' dynamics and function. It is unclear which forests are more susceptible to pulses of mortality under further changing climates. We examined whether tree mortality related to hotter droughts is predominantly occurring in edge or core populations in niche space and assessed whether mortality patterns are consistent with species' drought tolerances. Location: Global. Time Period: 1970–2020. Major Taxa Studied: Angiosperm and gymnosperm trees. Methods: We estimated species' climatic niches on the basis of global occurrences and annual time series of climate. We computed the distance to the niche core of drought mortality sites (982 observations, 44 tree species) and compared it with null models of randomly distributed tree mortality. We assessed how loss of xylem conductance and hydraulic safety margin related to populations' position in niche space and to the degree of climate anomaly during mortality. Results: In the year of mortality, 64.3% of the sites were closer to the species' niche edge than the niche core. However, when considering long-term climate averages, both marginal and central populations experienced drought-associated pulses of mortality. Overall, tree mortality was related to populations' shifts towards the edge of species' climatic niches and, in angiosperm mortality sites, species' drought tolerances were correlated with the intensity of climatic anomalies. In gymnosperm sites, marginal populations were more affected and other processes, such as heat-induced stress or cumulative drought effects, may have affected mortality. Main Conclusions: Both marginal and central populations, even for highly drought-tolerant species, are vulnerable to climate change. Climate anomaly magnitude, cumulative drought effects, plant physiological limits and species niche geometry help explain range-wide patterns of hotter-drought-associated tree mortality.