Vegetation Response to Prescribed Fire in Douglas-fir Forests, Olympic National Park

Fire exclusion for more than a century in Olympic National Park changed old-growth Douglas-fir forest stand structure and species composition from an open structure of fire-adapted species to a more crowded, complex structure formed by fire-avoiding species. A previous study identified an historical mean fire return interval of 21 yr for these forests in the eastern Olympics prior to fire exclusion, with frequent, small surface fires maintaining an open forest. We tested whether Douglas-fir/salal forests would support low intensity prescribed fires, and monitored community responses for 3 yr post-fire in a randomized complete block ANOVA. Compared to pre-fire values, total fuels, 1000-hr fuels, 1-hr fuels, duff depth, total tree density, tree species density, sapling density, understory cover, and understory frequency of five prominent species were significantly lower one month post-fire. Differences in tree basal area, Douglas-fir sapling density, and western redcedar tree density were not significant after the fire. Lower sapling density was an important result. Salal, a known resprouter, was the only understory species to return to at least 50% of pre-fire cover within 3 yr. This first use of prescribed fire in Olympic National Park demonstrated that Douglas-fir/salal forests would support low intensity surface fire. Although community structure changed significantly immediately after fire, the tree canopy was little affected, and the understory will eventually recover to pre-fire values. The data from this study will contribute to a fire management plan that will incorporate prescribed fire with fire suppression, non-suppressed fires, and other active management to maintain forest health in the eastern Olympic Mountains.