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Fire is the dominant disturbance process in western U.S. forests, and although effects of fire in upland forests are relatively well-studied, there is little information about fire effects on riparian forests, critical areas of the landscape for both habitat and water quality. This dissertation examines different aspects of fire effects in riparian areas of two recent fires in Oregon, the Biscuit Fire in southwestern Oregon and the B&B Complex Fire in the Cascade Mountain Range. In the first of three studies, I compared riparian fire severity to that in uplands and investigated factors influencing riparian fire severity in both fire areas. In a second study, the relationships among ground-based indices of fire severity in riparian areas, and the relationships between ground-based and remotely-sensed indices of fire severity, were examined. In a third study, I investigated patterns in post-fire riparian plant community regeneration in the same areas.
I found that understory fire severity was significantly lower in riparian areas compared to adjacent uplands, suggesting a decoupling of understory fire effects in riparian areas versus uplands. However, there were no differences in overstory fire severity between riparian areas and uplands in either fire. Understory and overstory fire severity indices were found to be weakly related, suggesting that there are limitations in the use of both types of fire severity indices. However, both overstory and understory fire severity in riparian areas were most strongly predicted by upland fire severity. Riparian fuel properties were also strong predictors of riparian fire severity. Patterns in post-fire riparian regeneration were influenced, at a coarse spatial scale, by factors associated with position in a watershed (headwater versus main stem channels) in the Biscuit Fire and by elevation/plant association in the B&B Complex Fire. At a finer spatial scale, differences in species composition and microsite conditions between deciduous hardwood- and conifer-dominated communities, and understory fire severity, influenced patterns of post-fire regeneration. Results of these studies suggest that management practices that reduce upland fire severity may also reduce riparian fire severity. Results also suggest that post-fire riparian regeneration efforts be tailored to site-specific vegetation conditions of complex riparian environments. |
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