Elevated mortality rates in tree populations is one of the widely recognized consequences of climate change, specifically of severe drought conditions coupled with high temperature. Many of the widely publicized case studies from North America have involved high mortality rates in low-diversity forests, often composed of gymnosperms. This study examined the effects of a drought event that caused moderate levels of mortality across a wide range of ecosystems. The Texas Drought of 2011 killed approximately 6% of all trees in Texas across a wide range of ecosystems and climate zones, affecting numerous trees species. Here we focus on quantifying patterns of tree mortality in central Texas in an effort to extract more general patterns of tree mortality under global-change-type drought. Using multivariate analyses, we examined the effects of species, tree size, and community characteristics such as tree density and species composition. We also examined the effects of environmental drivers and predisposing site factors on mortality rates.
Results/Conclusions
We confirmed the significance of high temperature on tree mortality and also showed that local temperature variation at the county level significantly affected mortality. In addition, aspect and the water storage capacity of the rhizosphere affected mortality. Oak species (Quercus virginiana, Q. fusiformis, Q. sinuata and Q. stellata) were among the tree species with the highest mortality rates, but also the woody encroacher Ashe juniper (Juniperus asheii). In juniper and oak populations, mortality rates were more strongly affected by intraspecific rather than interspecific competition. In contrast to the findings of other studies, tree size was not a consistent predictor of mortality risk across species. In Yaupon holly (Ilex vomitoria) and oak species, trees that had died were significantly smaller than trees that survived the drought, however, dead juniper trees were significantly larger. Size effects diminished with increasing temperature suggesting that tree size modulates the individual experience of water stress, but heat stress affects all trees regardless of size. We use these findings to develop a general framework for predicting drought-related mortality risk across species and communities.