Large influence of soil moisture on wildfires, biological disturbance agents, and tree growth in Pacific Northwest coniferous forests
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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.