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

Temporal patterns of tree mortality were determined for mature, naturally regenerated Douglas-fir-dominated (Pseudotsuga menziesii var menziesiiPseudotsuga menziesii (Mirb.) Franco) conifer stands 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 significantly higher for 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 temperatures.  In recent decades, long-term soil drying associated with warming temperatures appears to underlie tree mortality.  With continued climate warming in the coming decades, tree mortality from drought compounded by other climate-related disturbances will continue to increase in these mesic forests of western Oregon.  

Impact/Purpose

Droughts and high summer temperatures in the Pacific Northwest (PNW) of the United States are causing tree mortality and increasing tree die off from wildfire and biological disturbance agents (BDAs, i.e., pests and pathogens) but the role of climate are not well understood.  Our understanding of the effects of drought measured in terms of water availability on tree mortality is limited due to a lack of soil moisture data.  None of the common precipitation-based measures of drought including the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI) have been shown to reliably capture the onset and severity of drought in the PNW, making climate change projections of tree mortality from drought and forest disturbances difficult.  PESD scientists examined the role of soil moisture in tree mortality from climate and BDAs in the PNW based on long-term soil moisture data from a network of monitored field sites in Oregon established by the United States Environmental Protection Agency in the late 1990s.  PESD scientists are first to show that: 1) the 2000-2021 drought in the PNW was the driest 22-year period in past 700 years when soil moisture levels decreased by ~80% at some low- to mid-elevation sites in western Oregon; and 2)  the increase in the probability of tree mortality in the PNW is most strongly associated with decreasing water availability and increasing summer temperature in recent decades..  Our work is important for filling in the gaps of knowledge in understanding the complex interactions of climate and forest disturbance agents on conifer forests in the PNW under climate change scenarios.  Because the climate is predicted to continue warming in the 21st century, climate-induced tree mortality in the PNW is expected to intensify in frequency and magnitude at low- to higher-elevations where water is a primary limiting factor of tree growth and mortality.  Hotter droughts in combination with climate stress are predicted to increase forest vulnerability to wildfires and BDAs in western forests.  Understanding the important role of soil moisture in tree mortality in the west is critical for assessing risk and managing our limited resources to mitigate the socioeconomic and ecological impacts of climate change to the environment and public health.

Citation

Cline, S., E. Lee, Ron Waschmann, Mike Bollman, AND P. Beedlow. Warming temperatures and decreasing soil moisture are increasing tree mortality in mature Douglas-fir forests of western Oregon, USA. Elsevier Science Ltd, New York, NY, 372:110681, (2025). [DOI: 10.1016/j.agrformet.2025.110681]

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