Forest vegetation and fuel dynamics following stand-replacing wildfire, re-burn, and postfire management in the Siskiyou Mountains, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xp68kj50g

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  • Following high-severity fire, forest succession may take alternate pathways depending on the pattern of the fire and any secondary disturbances during early stand development, with lasting consequences for ecosystem function. The objectives of this research were to quantify: (1) early postfire regeneration as influenced by the spatial pattern of a landscape-scale wildfire, and (2) the influence of secondary disturbances during early succession, specifically recurrent high-severity fire and postfire logging. Four years after the 200,000-hectare Biscuit Fire, patch-scale conifer seedling density was 80% Douglas-fir (Pseudotsuga menziesii) and ranged from 127-6494 stems/ha. Median density (1603 stems/ha) was ~12 times the prefire overstory density of 134 stems/ha. Due to the mixed-severity fire mosaic, ~58% of high-severity area was ≤ 200 m from patches of live trees, and ~81% was ≤ 400 m. Median conifer density exceeded 1000 stems/ha out to 400 m distance before declining rapidly at larger distances. Following a short-interval (15-year) repeat fire, vegetation communities contained nearly all species found in mature/old-growth stands and after a single fire, indicating high community persistence through two severe fires. The short-interval burn had the highest species richness and plant cover with additions of disturbance-adapted forbs and low shrubs, likely associated with a propagule bank that developed between fires. Sprouting capacity of hardwoods and shrubs was unaltered by recurrent fire, and conifer regeneration density (median 1495 stems/ha) was similar to single burn stands. Three to four years after fire, high-intensity postfire logging resulted in a greater proportion (28%) of felled biomass left on site as slash compared to moderate-intensity logging (14%), because tree felling was less selective. However, despite bole removal and slash generation, there was no significant difference in surface woody fuels among logged and unlogged treatments at 17-18 years after fire. Fuel profiles and fire modeling both suggested that regenerating vegetation rather than residual dead wood is the main driver of early seral fire hazard, with low fire potentials 3-4 years postfire and very high potentials at 17-18 years. Postfire logging increased fire potentials within this context due to generation of slash, with differences fading in ~15-20 years. The results from this research underscore the importance of the mixed-severity context in which stand-replacing fire often occurs, both for providing seed sources within a fire and for highly variable disturbances that interact with succession and plant adaptation.
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