Warming temperatures and decreasing soil moisture are increasing tree mortality in mature Douglas-fir forests of western Oregon, USA

Abstract

Temporal patterns of tree mortality were determined for naturally regenerated Douglas-fir-dominated (Pseudotsuga menziesii (Mirb.) Franco) conifer stands, aged from about 125 to 500+ years, along an elevational gradient in western Oregon, USA based on periodic mortality surveys. Tree mortality increased from 2000 to 2023, a prolonged period of drought and warming. Soil moisture, air temperature, and vapor pressure deficit (VPD) were the key climatic predictors of tree mortality for all sites, species, and canopy classes based on logistic regression modeling. Increasing trends in annual probability of mortality were associated with decreasing summer available soil water and increasing summer temperature and associated VPD. These trends were most pronounced at mid-elevation (∼600 m) sites west of the Cascade crest, and they were higher for small diameter trees in suppressed and intermediate canopy class positions compared with co-dominant and dominant trees for all sites and species. Competition exacerbated the climate effects on mortality of small trees at drier sites. Insect-induced mortality was evident in large Douglas-fir following prolonged drought and high summer temperatures. In recent decades, long-term soil drying associated with warming temperatures appears to underlie tree mortality in these mesic forests of western Oregon.