Forest Disturbance Agents and Fire; What’s the Connection?
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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 wildfire by adversely affecting forest health. However, recent research indicates that BDAs do not necessarily increase, and in some cases can reduce, fire outcomes. There are a wide range of BDAs in coniferous forests of the western continental USA (the West), and their influences on forest composition, structure and, consequently, fire are not fully understood. We review BDA groups and the existing literature of their effects on fuels and fire in the West. The spatio-temporal aspects of BDA life history and impacts on forests vary among BDAs from episodic, landscape-scale (bark beetles, defoliators), to chronic, localized effects (dwarf mistletoe, root rots). 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 mistletoe and root diseases are more site specific and vary spatially. Native root diseases and dwarf mistletoe alter stand level fuels slowly and create fine scale heterogeneity. Invasive, non-native mortality agents, such as Phyphthora ramorum (cause of sudden oak death) have unique, long-term impacts on ecosystems and fire. In general, 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. In addition to BDAs, fire suppression and management history have affected forest health throughout the West by 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 increased or decreased impacts on fire risks and outcomes.