Abstract:
In response to concerns about excessive stand densities and high-severity wildfires, land
managers in the western United States are carrying out extensive programs of fuel reduction
thinning. How will these sudden reductions in canopy cover and associated changes in
habitat affect native and exotic herbaceous vegetation and canopy species regeneration?
Where thinning treatments have both fuel-reduction and ecological restoration objectives, to
what extent are these goals compatible? I compared vegetation and abiotic site
characteristics between thinned and unthinned chaparral and oak communities of
southwestern Oregon where landscape-scale fuel-reduction thinning is occurring. I sampled
paired thinned and unthinned transects, established four to seven years post-treatment. I also
contrasted impacts of manual vs. mechanical thinning methods, and examined differences in
herbaceous composition and responses among canopy community types. Thinning
treatments had significant impacts on site conditions and herbaceous cover and composition.
Herbaceous cover increased on thinned sites, but species richness and diversity did not
change. The strongest differences were decreases in perennial species cover and a near
doubling of cover by annual species on thinned sites. While overall proportions of natives and
exotics did not change, there were changes at the functional group level. Exotic annual grass
cover and the proportion of herbaceous community cover composed of these species
increased, while cover by native perennial grasses and regeneration of oaks did not. Cover
and proportion of native annual forbs increased more than all other functional groups, while
exotic annual forbs and native perennial forbs declined. Herbaceous communities at thinned
sites had an early-post-disturbance type of composition dominated by native annual forbs and
exotic annual grasses four to seven years after treatments. Re-establishment of native shrubs
was sparse in thinned areas, likely due to a lack of fire-stimulated germination. Sites that
varied in canopy species composition also supported different herbaceous communities in the
absence of thinning, and appeared to respond differently to fuel reduction treatments. Manual
and mechanical treatment impacts on abiotic site conditions differed, but their overall impacts on vegetation across canopy community types did not. Treatment type differences in
herbaceous responses to thinning were found within some canopy vegetation groups, but
sample sizes were small. Results suggest that fuel-reduction thinning may have some
unintended negative impacts on oak and chaparral communities of southwestern Oregon.
Although treatments have altered fuel conditions, thinning does not appear to have achieved
restoration goals and may have substantially changed the composition and regeneration of
native perennial communities including canopy species. Continued research and expanded
monitoring that account for differences between canopy vegetation communities and
treatment types are needed to inform adaptive management in these ecosystems.
