Abstract
AbstractIncreasing forest fuel aridity with climate change may be expanding mid‐to‐high‐elevation forests' vulnerability to large, severe, and frequent wildfire. Long‐lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper‐montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post‐fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (<12‐yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post‐fire conifer regeneration was generally very poor among plots that experienced either a single high‐severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre‐fire dominant, shade‐tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed post‐fire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low‐then‐high severity, or primarily fire‐adapted pines, if high‐then‐low severity. Our findings suggest that these forests, affected by expansive high‐severity and/or short‐interval wildfire, may transition into a patchy, low‐density, pine‐dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire‐adapted tree species, lower tree densities, and more non‐forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid‐to‐high‐elevation forests may begin resembling their drier, lower‐elevation mixed‐conifer counterparts in structure and composition.
Highlights
Wildfire is the most pervasive natural disturbance agent shaping temperate forest ecosystems
Study areas The fires in this study were in mid-to-high-elevation upper-montane and subalpine forests on two volcanic mountains of the Central Cascade Range of the Pacific Northwest (PNW), Mt
We identified burn severity in our study areas using the relativized delta normalized burn ratio (RdNBR), which was classified into discrete burn severity groups following the spectral value thresholds outlined by Miller and Thode (2007)
Summary
Wildfire is the most pervasive natural disturbance agent shaping temperate forest ecosystems. Mid-to-high-elevation forests, to absorb stress and recover following recently observed and predicted increases in fire frequency, severity, and size (Cansler and McKenzie 2014, Dennison et al 2014, Abatzoglou et al 2017, McKenzie and Littell 2017, Reilly et al 2017) These forests have historically experienced relatively infrequent fire (Agee 1993, Tepley et al 2013) and so are dominated by shade-tolerant, obligate seeding conifer species (Abies, Tsuga, Picea, genera) that require long fire-free intervals and/or proximate live seed sources to re-establish (Franklin and Hemstrom 1981, Agee 1993). Infrequent wildfire in these cool and moist forest types has limited the number of natural experiments available to study; little research has yet to empirically evaluate these concerns
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