Douglas-fir (Pseudotsuga menziesii) dominated forests of the Pacific Northwest are an integral part of the region, and provide numerous ecosystem services. These services are particularly important in the Cedar River Municipal Watershed (CRMW), which covers >364 km2 and provides critical ecosystem services by supplying clean drinking water to >1.4 million people in the greater Seattle area. Previous research has estimated the annual mortality of second growth Douglas-fir in western Washington to be <2%. However, recent records from within the CRMW suggest that annual mortality is >30%, which could be due to a rapid switch from suppression mortality to agent-based mortality. The Douglas-fir beetle (Dendroctonus pseudotsugae) and plant pathogens are key agents of mortality in mature or physiologically-stressed Douglas-fir trees, but their contribution to the current rates of mortality in the CRMW is unknown. We sought to quantify the individual and interacting roles of Douglas fir beetle and plant pathogens as drivers of Douglas fir mortality in the CRMW through field measurements in stands experiencing different rates of mortality. In parallel, we also used seven years of field-collected data on Douglas-fir beetle flight dynamics to measure the effect of recent temperature conditions on its seasonality.
Stand characteristics and mortality agent presence vary as Douglas-fir stand mortality levels increase. Stands exhibiting high mortality (those with ≥ 30% standing dead Douglas-fir) had the lowest proportion of trees with evidence of pitch streaming relative to medium (15-30% standing dead Douglas-fir) and low mortality stands (<15% standing dead Douglas-fir). However, these high mortality stands also had an intermediate proportion of frass presence, pitch streaming, and root rot in living trees, which suggests that these are remaining as a source of biotic agents to nearby stands. Stands with medium mortality had the greatest proportion of beetle frass presence, evidence of pitch streaming, and root rot infection within living trees, while stands exhibiting low mortality had the greatest proportion of living Douglas-fir without root rot infection and beetle frass presence. Douglas-fir beetle flight dynamics suggested a bimodal pattern with the largest population peak typically occurring in early spring when overwintering adults emerge en masse, and a second smaller but noisier peak linked to development of overwintering larvae. A novel temporal and spatial interaction between bark beetles and root rot pathogens could be altering forest dynamics in the CRMW, leading to increases in biotic agent-based tree mortality and disrupting ecosystem services.