Influence of Recent Bark Beetle Outbreak on Fire Severity and Post-Fire Tree Regeneration in Montane Douglas-fir Forests

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Ecological Society of America

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Understanding how disturbances interact to shape ecosystems is a key challenge in ecology. In forests of western North America, the degree to which recent bark beetle outbreaks and subsequent fires may be linked (e.g. outbreak severity affects fire severity) and/or whether these two disturbances produce compound effects on post-fire succession is of widespread interest. These interactions remain unresolved, largely because field data from actual wildfires following beetle outbreaks are lacking. We studied the 2008 Gunbarrel Fire, which burned 27,200 ha in Douglas-fir (Pseudotsuga menziesii) forests that experienced a bark beetle outbreak 4-13 years pre-fire ("gray stage," after trees have died and needles have dropped), to determine whether outbreak severity influenced subsequent fire severity and post-fire tree regeneration. In 85 sample plots we recorded pre-fire stand structure and outbreak severity; multiple measures of canopy and forest-floor fire severity; and post-fire tree seedling density. Pre-fire outbreak severity was not related to any measure of fire severity except for mean bole scorch, which declined slightly with increasing outbreak severity. Instead, fire severity varied with topography and burning conditions (proxy for weather at time of fire). Post-fire Douglas-fir regeneration was low, with tree seedlings absent in 65% of plots. Tree seedlings were abundant in plots of low fire severity that also had experienced low outbreak severity (mean = 1,690 ha-1), suggesting a dual filter on tree regeneration. Although bark beetles and fire collectively reduced live basal area to < 5% and increased snag density to > 2,000% of pre-outbreak levels, the lack of relationship between beetle outbreak and fire severity suggests these disturbances were not linked. Nonetheless, effects on post-fire tree regeneration suggest compound disturbance interactions that contribute to the structural heterogeneity characteristic of mid/lower montane forests.