Summary 1Although climatic variability is a strong driving force for forest dynamics, drought-induced mortality has generally received much less attention than other types of disturbance. 2 In 1998-99 northern Patagonia was affected by one of the most severe droughts of the 20th century, coinciding with a strong La Nina event, and this caused high mortality of Nothofagus dombeyi (coihue), the dominant tree species in Nahuel Huapi National Park. 3 Factors involved in determining this mortality of N. dombeyi were examined at both patch and tree level. Radial growth characteristics of killed trees and survivors were compared by dendrochronological analyses. Relationships between growth and climate were investigated using response function analysis. 4 At the tree scale, individuals with variable growth were more prone to die from drought than trees with more regular growth. Juveniles whose growth patterns showed sensitivity to climate were particularly likely to die. However, among both killed trees and survivors, older trees were less sensitive to climate. 5 Mean growth rate was a good predictor of mortality in adult trees, showing that trees with slower growth rate were more susceptible to drought. Susceptible trees may have been negatively affected by the drought that affected northern Patagonia in 1956-57. 6 These results underscore the importance of considering drought-induced tree mortality as a non-random mechanism influenced by site, previous stress/disturbance history, ontogeny, vigour, climatic sensitivity and physiology. Spatial patterns of extensive full and partial crown dieback, which are evident in many temperate forests worldwide, may reflect the superposition of these predisposing factors on strong / repeated interannual fluctuations of climate.

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.

Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.