Shaw David C, Beedlow Peter A, Lee E Henry, Woodruff David R, Meigs Garrett W, Calkins Stephen J, Reilly Matthew J, Merschel Andrew G, Cline Steven P, Comeleo Randy L
Department of Forest Engineering, Resources, and Management, Oregon State University, 216 Peavy Forest Science Complex, 3100 SW Jefferson Way, Corvallis, OR 97331, USA.
U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA.
For Ecol Manage. 2022 Dec 1;525:1-27. doi: 10.1016/j.foreco.2022.120572.
Forest biological disturbance agents (BDAs) are insects, pathogens, and parasitic plants that affect tree decline, mortality, and forest ecosystems processes. BDAs are commonly thought to increase the likelihood and severity of fire by converting live standing trees to more flammable, dead and downed fuel. However, recent research indicates that BDAs do not necessarily increase, and can reduce, the likelihood or severity of fire. This has led to confusion regarding the role of BDAs in influencing fuels and fire in fire-prone western United States forests. Here, we review the existing literature on BDAs and their effects on fuels and fire in the western US and develop a conceptual framework to better understand the complex relationships between BDAs, fuels and fire. We ask: 1) What are the major BDA groups in western US forests that affect fuels? and 2) How do BDA-affected fuels influence fire risk and outcomes? The conceptual framework is rooted in the spatiotemporal aspects of BDA life histories, which drive forest impacts, fuel characteristics and if ignited, fire outcomes. Life histories vary among BDAs from episodic, landscape-scale outbreaks (bark beetles, defoliators), to chronic, localized disturbance effects (dwarf mistletoes, root rots). Generally, BDAs convert aboveground live biomass to dead biomass, decreasing canopy fuels and increasing surface fuels. However, the rate of conversion varies with time-since-event and among BDAs and forest types, resulting in a wide range of effects on the amount of dead fuels at any given time and place, which interacts with the structure and composition of the stand before and subsequent to BDA events. A major influence on fuels may be that BDAs have emerged as dominant agents of forest heterogeneity creation. Because BDAs play complex roles in fuels and fire heterogeneity across the western US which are further complicated by interactions with climate change, drought, and forest management (fire suppression), their impacts on fuels, fire and ecological consequences cannot be categorized simply as positive or negative but need to be evaluated within the context of BDA life histories and ecosystem dynamics.
森林生物干扰因子（BDAs）是影响树木衰退、死亡以及森林生态系统过程的昆虫、病原体和寄生植物。人们普遍认为，BDAs通过将活立木转化为更易燃的枯立木和倒木燃料，增加了火灾发生的可能性和严重程度。然而，最近的研究表明，BDAs不一定会增加，甚至可能降低火灾发生的可能性或严重程度。这导致了人们对BDAs在美国西部易发生火灾的森林中影响燃料和火灾方面所起作用的困惑。在此，我们回顾了关于BDAs及其对美国西部燃料和火灾影响的现有文献，并建立了一个概念框架，以更好地理解BDAs、燃料和火灾之间的复杂关系。我们提出以下问题：1）美国西部森林中影响燃料的主要BDAs群体有哪些？2）受BDAs影响的燃料如何影响火灾风险和结果？该概念框架基于BDAs生活史的时空特征，这些特征驱动着森林影响、燃料特性以及起火后的火灾结果。BDAs的生活史各不相同，从偶发性的、景观尺度的爆发（树皮甲虫、食叶害虫）到慢性的、局部的干扰效应（矮槲寄生、根腐病）。一般来说，BDAs将地上活生物量转化为死生物量，减少树冠层燃料并增加地表燃料。然而，转化速率随事件发生后的时间以及不同的BDAs和森林类型而变化，导致在任何给定时间和地点对死燃料量产生广泛影响，这与BDAs事件前后林分的结构和组成相互作用。对燃料的一个主要影响可能是，BDAs已成为创造森林异质性的主要因素。由于BDAs在美国西部的燃料和火灾异质性中发挥着复杂作用，而与气候变化、干旱和森林管理（火灾扑救）的相互作用又使其进一步复杂化，它们对燃料、火灾和生态后果的影响不能简单地归类为正面或负面，而需要在BDAs生活史和生态系统动态的背景下进行评估。
