Relationship of stand characteristics to drought-induced mortality in three Southwestern pinon-juniper woodlands

Extreme drought conditions accompanied by rising temperatures have characterized the American Southwest during the past decade, causing widespread tree mortality in pinon-juniper woodlands. Pinon pine (Pinus edulis Engelm.) mortality is linked primarily to outbreaks of the pinyon ips (Ips confusus (Leconte)) precipitated by drought conditions. Although we searched extensively, no biotic agent was identified as responsible for death in Juniperus L. spp. in this study; hence this mortality was due to direct drought stress. Here we examine the relationship between tree abundance and patterns of mortality in three size classes (seedling/sapling, pre-reproductive, reproductive) during the recent extended drought in three regions: southwest Colorado, northern New Mexico, and northern Arizona. Pinon mortality varied from 32% to 65%, and juniper mortality from 3% to 10% across the three sites. In all sites, the greatest pinon mortality was in the larger, presumably older, trees. Using logistic regression models, we examined the influence of tree density and basal area on bark beetle infestations (pinon) and direct drought impacts (juniper). In contrast to research carried out early in the drought cycle by other researchers in Arizona, we did not find evidence for greater mortality of pinon and juniper trees in increasingly high density or basal area conditions. We conclude that the severity of this regional drought has masked density-dependent patterns visible in less severe drought conditions. With climate projections for the American Southwest suggesting increases in aridity and rising temperatures, it is critical that we expand our understanding of stress responses expected in widespread pinon-juniper woodlands.