- Wetland prairies of the Willamette Valley, among the rarest of Oregon’s ecosystems, are
threatened by invasion of woody species and non-native pest species. Because fire has been
important in maintaining Willamette Valley prairies for at least 1000 years, prescribed burning is
a top choice of managers for preventing encroachment of woody species. However, the effects
of prescribed burning on present day wetland prairie communities, with their mix of native and
non-native species, are not clear. Moreover, because strict governmental requirements for smoke
management hinder prescribed burning, natural-area managers are considering alternatives to
fire, like mowing and hand removal of woody plants.
The present study explored the ecological interactions of a wetland prairie by
experimentally testing the responses of burning, hand-removal of woody species, and mowing
with removal of cut material on woody plants and key native and non-native grasses and forbs.
We evaluated these perturbations for their effectiveness in meeting management objectives of
reducing abundance of woody species, reducing or preventing an increase in abundance of nonnative
pest species, and increasing or at least maintaining native species’ abundance. After three
treatments periods (1994, 1996, and 1998) the following patterns emerged:
• Woody species Burning and hand-removal consistently caused the greatest reduction
in cover of woody species. Mowing with removal of cut material did not reduce the
abundance of woody species compared to the control. As woody plant cover
decreased, plant mortality increased, indicating that treatments influenced woody
plant cover at least partially through changes in survival.
• Native herbaceous species All management treatments consistently failed to
significantly increase the cover of Deschampsia cespitosa, while burning decreased
the number of inflorescences. As a group, native graminoid flowering consistently
showed no significant responses to the management treatments. Although burning
and hand-removal significantly increased the cover of native forbs as group after the
second treatment period in 1997, native forbs showed no significant responses to
treatments in 1999.
• Non-native herbaceous species Response of non-native forb cover to management
treatments has been inconsistent during the five year study period with no significant differences in 1995, significant reductions by burning and hand-removal in 1997, and
increases with mowing in 1997. Mowing consistently increased flowering of nonnative
graminoids as a group.
Overall no treatment was clearly superior in fulfilling restoration objectives. Burning
was effective in reducing woody cover and did not promote non-native herbaceous, but it
reduced the flowering the dominant native grass, Deschampsia cespitosa. Hand-removal woody
species was also effective at reducing woody cover and sometimes promoted the abundance of
some native species at different times, but it also at times promoted non-native herbaceous
species. Because mowing with removal of cut material was ineffective in reducing woody cover
and tended to promote non-native herbaceous species, this treatment is not a recommended