Abstract:
Fire exclusion has been associated with structural and compositional changes
in many upland forests of the western United States, but little is known about the
impacts on riparian forests, portions of the landscape protected for habitat and water
quality. For this study, I characterized the historic disturbance and tree recruitment
processes of riparian forests in southwestern Oregon. I identified the ways in which
riparian forest structure and composition have changed over the past 100 to 150 years,
with a particular focus on the effects of fire exclusion. I used dendro-ecological
methods to analyze tree ring data collected in riparian forest stands of two vegetation
types in the Rogue River basin of southwestern Oregon: the mesic 'Mixed Conifer' zone (Pinus-Pseudotsuga-Calocedrus-Abies) and the more xeric 'Interior Valley' zone (Pinus-Quercus-Pseudotsuga). My results suggest riparian forests in both vegetation
zones developed with frequent disturbance by fire and that, since fire has been
excluded, these forests have entered a new successional trajectory. In the Mixed
Conifer zone, tree recruitment in the pre-settlement period appears to have been driven
by the interaction between climatic fluctuations and mixed-severity fires, which
historically maintained multi-aged stands of shade-intolerant species. Patches of highseverity
fire within the mixed-severity matrix created large canopy gaps (>30 m in
diameter) in which new cohorts of Douglas-fir (Pseudotsuga menziesii) could
establish within existing stands and perpetuate Douglas-fir overstory dominance. Tree
recruitment appears to have been closely linked Since fire exclusion became effective,
there has been an increase in tree density and a shift toward the establishment of white
fir (Abies concolor), a fire-sensitive, shade-tolerant species. In the Interior Valley
zone, riparian forests were most likely shaped by a low-severity fire regime. Frequent
fires likely killed most tree seedlings and maintained open savannas or woodlands
with shade-intolerant hardwoods and scattered, open-grown conifers. Fire exclusion in
the Interior Valley zone has resulted in exceptionally high survival rates in Douglas-fir
recruited during the late 19th and early 20th centuries. This led to the establishment of
a closed-canopy conifer forest with fewer shade-intolerant hardwoods, which are an
important source of food and habitat for wildlife. My findings suggest future
management guidelines for riparian forests in fire-prone forests of the Pacific
Northwest should consider the substantial and potentially undesirable ecological
effects of the current hands-off management policy in riparian forests. Future
proposals to attempt restoration of pre-settlement forest structure and composition in
upland forests should also consider including restoration treatments within adjoining
riparian forests.
