Citation:

Lee, E., P. Beedlow, S. Cline, AND D. Shaw. Unprecedented Douglas-fir mortality from drought and Swiss needle cast at Tillamook, Oregon. 2023 Swiss Needle Cast Cooperative annual meeting, Corvallis, OR, November 30, 2023.

Impact/Purpose:

Droughts and biological disturbance agents (BDAs, i.e., pests and pathogens) in the Pacific Northwest (PNW) of the United States are causing tree mortality but the disturbance-climate relationships are not well understood (Agne et al. 2018).  In recent decades, significant tree mortality has been linked to prolonged drought in western North America (Allen et al., 2010; Halofsky et al., 2020; Anderegg et al., 2021).  However, our understanding of drought’s influence on forest susceptibility to BDAs is limited (Krawchuk and Moritz, 2011; Kolb et al 2016) due to a lack of soil moisture data (Cosh et al., 2021).  We address the role of climate in tree growth and Douglas-fir mortality from drought and Swiss needle cast (SNC) for a 90-year-old stand at Tillamook, OR.  We use long-term data from a network of monitored field sites in the Douglas-fir region of western Oregon established by the United States Environmental Protection Agency in the late 1990s.  Our findings indicate that tree growth, decline and mortality in this stand are most strongly associated with decreasing available soil water (ASW) and increasing SNC severity in recent decades.  Identifying and understanding the role of soil moisture in forest disturbances in the PNW is critical for assessing risk and effectively managing limited resources and tools available to mitigate the socioeconomic and ecological impacts of climate change to the environment and public health.  

Description:

Droughts and biological disturbance agents (BDAs, i.e., pests and pathogens) in the Pacific Northwest (PNW) of the United States are causing tree mortality but the disturbance-climate relationships are not well understood (Agne et al., 2018).  In recent decades, significant tree mortality has been linked to prolonged drought in western North America (Allen et al., 2010; Halofsky et al., 2020; Anderegg et al., 2021).  However, our understanding of drought’s influence on forest susceptibility to BDAs is limited (Krawchuk and Moritz, 2011; Kolb et al 2016) due to a lack of soil moisture data (Cosh et al., 2021).  PESD scientists examined the role of soil moisture in tree growth and tree mortality from drought and Swiss needle cast (SNC) for a 90-year-old stand at Tillamook, Oregon where SNC is most severe in North America.  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; 2)  the mortality of Douglas-fir from SNC in combination with drought at Tillamook, starting in 2015, was unprecedented in North America; 3) the risk of Douglas-fir mortality from drought and SNC is increasing with decreasing soil moisture and increasing SNC severity in coastal forests and plantations.  Our work is important for filling in the gaps of knowledge in understanding the complex interactions of drought, wildfires, and biological disturbance agents on conifer forests in the PNW under climate change scenarios.  Because the climate is predicted to continue warming in the 21st century, wildfire and BDA activities in the PNW are expected to intensify in frequency and magnitude at low- to higher-elevations where water is a primary limiting factor of tree growth and/or temperature is a primary limiting factor of BDA suitability.  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 forest disturbances by fire and BDAs 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.

Record Details: