Stand-scale tree mortality factors differ by site and species following drought in southwestern mixed conifer forests

Impacts of drought on tree mortality in high-elevation mixed-conifer forests of southwestern U.S. are poorly understood. A recent extended and severe drought in the region provided an opportunity to investigate the patterns and factors associated with tree mortality in this forest type. Specifically, we quantified mortality that occurred between 1995 and 2008 of four tree species, white fir (Abies concolor), limber pine (Pinus flexilis), trembling aspen (Populus tremuloides), and Douglas-fir (Pseudotsuga menziesii), in mixed-conifer forests over three sites in northern Arizona within 84, 0.02 ha plots. We found: (1) varied but substantial tree mortality (4-56% by basal area) in most species between 1996 and 2006 in association with recent severe and prolonged drought; (2) tree mortality differed among sites and species with aspen and white fir having the most mortality (>30% by basal area); (3) relationships between tree mortality and most climatic factors (e.g. temperature, precipitation, Palmer Drought Severity Index) were lagged 1-4 yr; (4) bark beetle attack and intraspecific tree basal area were consistently and positively related to tree mortality for most species and sites, whereas topographic and other stand characteristics were less consistently related to mortality. Results show that aspen, Douglas-fir, and white fir were more vulnerable to recent drought-associated mortality than limber pine. Associations between tree mortality and intraspecific basal area support further evaluation of treatments that reduce intraspecific competition within stands to lower risks of tree mortality in southwestern mixed conifer forests.