Wildfire Interactions With Fuel Treatments in Sierra Nevada
Forests: Consequences for Forest Structure and Understory
Plant Diversity
Jens T. Stevens,* Graduate Group in Ecology, University of California, Davis, CA 95616, Department of Plant Sciences,
University of California, Davis, CA 95616; Hugh D. Safford, USDA Forest Service, Pacific Southwest Region, Vallejo, CA
94592, Department of Environmental Science and Policy, University of California, Davis, CA 95616; Susan Harrison,
Department of Environmental Science and Policy, University of California, Davis, CA 95616; Andrew M. Latimer,
Department of Plant Sciences, University of California, Davis, CA 95616
Abstract—Wildfires in the Ponderosa pine (Pinus ponderosa Laws.) and mixed-conifer forest belt of the Sierra
Nevada Mountains in California are increasing in size and severity. These increases are due to forest structural changes and accumulated fuel loads associated with past management practices after a century of
fire exclusion. The most common management strategy in these forests is to implement fuel treatments to
reduce canopy fuel loads and decrease subsequent wildfire severity. We present results from a large statewide study of fuel treatment-wildfire interactions, with specific focus on the ecological effects of these two
disturbances on forest structure and understory plant diversity. We sampled forest vegetation plots from
twelve wildfires that burned into previously implemented fuel treatments, as well as adjacent unburned fuel
treatments and controls. We show that wildfire severity is dramatically reduced by fuel treatments, with tree
mortality decreasing by up to 75%, and that the degree of structural change caused by wildfire is much less in
treated stands than in untreated, high tree density stands. Effects of fuel treatments on patterns of shrub and
tree regeneration in response to wildfire are contingent upon whether stands had previously been treated,
with post-wildfire shrub recruitment highest in untreated stands and post-wildfire tree recruitment highest in
treated stands. We found strong evidence that the extent of forest canopy disturbance affects the diversity
and composition of the understory plant community. Intermediate levels of forest disturbance, which we
largely found in treated forests that subsequently burn in wildfire, had the highest stand-level understory
plant diversity, relative to unburned forests or untreated forests that burned at high-severity. We used information on species’ evolutionary histories and traits to demonstrate that the proportion of the understory
flora with evolutionary origins in mesic climates decreases with increasing fire severity, and that increased
water stress in open-canopy forests following high-severity fire may filter understory species composition on
the basis of leaf traits.
In: Keane, Robert E.; Jolly, Matt; Parsons, Russell; Riley, Karin. 2015. Proceedings
of the large wildland fires conference; May 19-23, 2014; Missoula, MT. Proc.
RMRS-P-73. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky
Mountain Research Station. 345 p.
USDA Forest Service Proceedings RMRS-P-73. 2015.
The content of this paper reflects the views of the authors, who are responsible for the
facts and accuracy of the information presented herein.
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