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Large influence of soil moisture on wildfires, biological disturbance agents, and tree growth in Pacific Northwest coniferous forests.
Citation:
Lee, E.Henry, P. Beedlow, Steve (he/him/his) Cline, C. Welk, James Markwiese, AND R. Waschmann. Large influence of soil moisture on wildfires, biological disturbance agents, and tree growth in Pacific Northwest coniferous forests. Post-fire Symposium at Oregon State University, Corvallis, OR, February 07 - 08, 2023.
Impact/Purpose:
The megadrought in southwestern United States in the period 2000-2021 was unprecedented in the last 1200 years and climate models predict the extended drought conditions will occur more frequently as the region continues to warm in the 21st century. These changes are already affecting sensitive forested ecosystems, as evidenced by the demise of 129 million trees in California during the unprecedented 2012-2016 drought and increased frequency and extent of wildland fires in western forests. Progressively increasing temperatures and seasonal shifts causing reductions in snowpack and low summer stream flows in Oregon, USA suggest that the recent observed levels of tree mortality from drought, fire, and biological disturbance agents in California could foreshadow future events for forests in Oregon. PESD scientists examined the role of soil moisture in tree growth, fire, and tree mortality from BDAs in the Pacific Northwest based on long-term soil moisture 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. PESD scientists were first to show that: 1) soil moisture levels in 2000-2021 were the driest 22-year period in past 700 years in Oregon and are declining most rapidly at low elevations in and adjacent to the Willamette Valley; 2) forested ecosystems in Oregon are becoming more water limited than energy limited in recent decades as indicated by tree growth rates declining in response to decreasing available soil water across the region; 3) the increasing trend in annual area burned in biomass-rich forests in western Oregon and Washington are most strongly associated with soil moisture availability; 3) while more area is burned in the semi-arid, biomass-poor ecoregions in eastern Oregon, the risk of fire and future tree mortality from drought and BDAs is increasing faster in the mesic, biomass-rich forests in western Oregon where ASW decreased by as much as 84% from 1998 to 2020. Our work is important for filling in the gaps of knowledge in understanding the role of soil moisture in forest disturbances by fire and BDAs in the PNW and 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.
Description:
Droughts 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 disturbance-climate relationships are not well understood (Agne et al. 2018). Significant tree mortality has been linked to prolonged drought in recent decades 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 wildfires and 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 soil moisture in tree growth, fire, and tree mortality from BDAs in the PNW based on 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. We examine the relationships between climate and area burned by fire to compare the fire-climate relationships between biomass-rich and biomass-poor ecoregions in the PNW. Our findings indicate that increased rates of disturbance by fire and insects in the PNW and tree growth decline are most strongly associated with decreasing available soil water (ASW) in recent decades, and the area burned westside in 2020 was most strongly associated with antecedent climatic conditions of anomalously low ASW. Understanding the important role of soil moisture in forest disturbances by fire and BDAs in the PNW 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.