Widespread increase of tree mortality triggered by an exceptional drought in east Texas, USA

Weimin Xi, Biological and Health Sciences, Texas A&M University-Kingsville, Kingsville, TX, Mukti Subedi, Department of Natural Resources Management, Texas Tech University, TX, Zhiping Liu and Ming Yan, School of Biological Sciences, Shanxi Normal University, Linfen, China

Background/Question/Methods
Severe drought increasingly induced higher tree mortality worldwide and significantly alters forest structure, species composition, biomass dynamics, ecosystem services, carbon fluxes, and energy interactions. However, the impacts of drought regime coupled with all other associated disturbances such as hurricanes, wildfire under a changing climate are not fully explored. In 2011, Texas experienced the most severe drought in its 100-year history, resulting in the death of more than 300 million trees. In east Texas, an estimated 60 million trees were dead. This study aimed to further our understanding of drought-triggered mortality using ~1800 field plots data from the Forest Inventory and Analysis (FIA) program over 20 years (2001-2019). In this study, the forest trees in eastern Texas were divided according to different regions and types to further understand their response to drought. Plots that were affected by all other factors except with a clear indication of drought were filtered out. Density independent factors (drought severity and length) and density-dependent factors (tree density, base area, and stand age) were selected to explore drought-triggered tree mortality. Generalized nonlinear mixed models (GNMMs) were used to examine trends in tree mortality and understory recruitment rates and the relative contribution of influencing factors.
Results/Conclusions
We found widespread, varied, and long-lasting increase of tree mortality across east Texas forests triggered by the exceptional drought coupled with tree death from wildfire and insects. Our results showed that Pine trees were largely affected by the severe drought as dominant species, and small and shorter trees were more susceptible to drought. Tree mortality in the plantation stands was lower than that of trees from the natural forest plots. Overall, the effect of drought on tree death is greater than that of competition, and the effect of drought severity and drought length on tree death is similar. Annual mortality rates before and after drought showed a significant negative correlation with drought severity and length of the drought. We concluded that (1) Among six tree species groups, Pinus trees experienced higher impacts than others; (2) The effect of drought was more pronounced on small-sized and shorter trees; (3) drought triggered mortality in natural forests is more significant than that of plantation forests; (4) Compared with the competition, drought severity and length together was the leading cause of trees death.