Citation:

Shaw, D., P. Beedlow, E. Lee, D. Woodruff, G. Meigs, S. Calkins, M. Reilly, A. Merschel, S. Cline, AND R. Comeleo. The complexity of biological disturbance agents, fuels heterogeneity, and fire in coniferous forests of the western United States. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science Ltd, New York, NY, 585:120572, (2022). https://doi.org/10.1016/j.foreco.2022.120572

Impact/Purpose:

Changing climate and drought are affecting both wildfire and biological disturbance agents (BDAs, i.e., insects, pathogens, and parasitic plants) across the western United States due to increased growing season temperature, decreased growing season precipitation, increased length of the growing season, warmer winters, reduced snow accumulation, increasing extreme events, and the influence of drier fuels conditions occurring for a longer duration and earlier in the season. Drought can exacerbate the effects of BDAs on primary productivity and survival, resulting in increased tree mortality, risk of wildfire frequency and intensity, erosion and sedimentation, most notably in the Pacific Southwest. Traditionally, BDAs were thought to increase the likelihood and severity of wildfire by adversely affecting forest health. However, recent research indicates that BDAs do not necessarily increase, and in some cases can reduce, fire outcomes. PESD scientists in collaboration with USFS scientists and forest health specialists at the Department of Forestry, Oregon State University (OSU) investigated a wide range of BDAs in coniferous forests of the western continental USA (the West), and their influences on forest composition, structure, fuels and fire. PESD, USFS and OSU scientists show that: 1) the spatiotemporal patterns of BDAs life history and impacts on forests vary among BDAs from episodic, landscape-scale (bark beetles, defoliators), to chronic, localized effects (dwarf mistletoe, root rots); 2) native BDAs create stand and landscape heterogeneity that may accentuate or dampen fire effects depending on the agent and time scale at which they operate; and 3) climate change in combination with hotter droughts and management effects increased the influence of some BDAs (e.g., bark beetles) on tree decline, mortality, and fuels. Thus, BDAs play complex roles in fuels and fire patterns across the West that cannot be categorized simply as increased or decreased impacts on fire risks and outcomes. We speculate that the role of BDAs on wildfire may be important at small spatial scales but diminished at large spatial scales as severe fire weather becomes increasingly common and widespread in the West. Our work is important to wildfire managers and other stakeholders in the West where wildfire and BDAs activities are increasing in range, magnitude and severity under a changing climate, resulting in decreasing air and water quality.

Description:

Forest biological disturbance agents (BDAs) are insects, pathogens, and parasitic plants that cause tree decline and mortality. Traditionally, BDAs were thought to increase the likelihood and severity of fire by adversely affecting forest health. However, there are a wide range of BDAs in coniferous forests of the western continental USA (western US), and their influences on forest composition, structure and, consequently, fire outcomes are not fully understood. Recent research indicates that BDAs do not necessarily increase, and in some cases can reduce, the likelihood and severity of fire. We review BDA groups and the existing literature of their effects on fuels and fire in the western US, and ask, 1) How do forest BDAs influence fuels and fire in the western US? and 2) Do BDAs increase fire risk? The spatiotemporal aspects of BDA life history and impacts on forests vary among BDAs from episodic, landscape-scale (bark beetles, defoliators), to chronic, localized effects (dwarf mistletoes, root rots). In general, native BDAs create stand and landscape heterogeneity that can either accentuate or dampen fire effects, depending on the agent and time scale at which they operate. The episodic nature of the two most prevalent and well-studied native BDA groups, bark beetles and defoliators, leads to distinct time-since-outbreak changes in fuels, which cause variation for fire outcomes, depending on whether or when fires occur. Bark beetle epidemics increase fire risk over short time scales but may dampen fire effects over time as live fuels recover and dead fuels decompose. Defoliators typically have less direct effects on fuels and fire than bark beetles, and they can dampen fire risk. The chronic-localized effects of dwarf mistletoes and root diseases are more site-specific and vary spatially. Native root diseases and dwarf mistletoes alter stand-level fuels slowly and create fine-scale heterogeneity. Invasive, non-native mortality agents, such as Phytophthora ramorum (cause of sudden oak death) have unique, long-term impacts on ecosystems and fire. In addition to BDAs, fire suppression and management history have affected forest health throughout the West western US by altering species composition to favor shade-tolerant tree species that are more susceptible to BDAs and drought, increasing tree density, decreasing average tree size, and homogenizing dry forests. Climate change has resulted in longer, hotter growing seasons and hotter droughts, which has, in combination with management effects, increased the influence of some BDAs (e.g., bark beetles) on tree decline, mortality, and fuels. Thus, BDAs play complex roles in fuels and fire patterns across the West that cannot be categorized simply as positive or negative impacts on fire risks and ecological outcomes.

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