Abstract: To explore the effects of multiple time-scales, climatic and stand factors on tree mortality in forests, we examined the changes in annual and inventory-cycle tree mortality patterns across 264 forest inventory plots in four national forests of eastern Texas. These data were obtained from the Forest Inventory and Analysis (FIA) Program and the plots had been individually surveyed in four inventory cycles over the past 20 years. The generalized linear mixed effects model (GLMM) was used to explore the effects of climatic factors (drought severity, duration of drought, mean annual temperature, and mean annual precipitation), tree size (diameter at breast height) and stand factors (basal area, stand density, and stand age) on tree mortality. The results showed that tree mortality rates increased by 151% in the particular year with severe drought and by 123% during exceptional inventory cycle during the inventory cycle with severe drought. The major cause of death was weather (exceptional drought and large hurricanes). Both drought severity as measured by standardized precipitation evapotranspiration index (SPEI) and the duration of drought had significant negative effects, whereas annual precipitation had a significant positive effect on tree survival. Tree basal area had a significant negative effect, while tree size, stand age and stand density had significant positive effects on tree survival. Trees with larger size (DBH) were more vulnerable to drought than smaller ones. During the exceptional drought, tree mortality rate of pine species (2.1%) was lower than that of hardwood species (3.9%), while tree mortality in the natural forests (3.0%) was higher than that in the pine plantations (1.9%). Our results suggested that it was essential to consider the relative importance of both intrinsic (tree size) and extrinsic (stand factors and climatic factors) factors in analyzing tree mortality.

Key words: tree mortality, GLMM, exceptional drought, SPEI