- 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 four
treatments periods (1994, 1996, 1998, and 2000) the following patterns emerged:
• Woody species Burning and hand-removal of woody plants every two years consistently
decreased the cover of woody species, even with a gradual increase in the controls to over
double 1994 levels. The change in woody cover after mowing with removal of cut material
was not significantly different from that in 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 None of the treatments had a significant effect after four
treatment periods on the proportional change in cover or in number of inflorescences of
Deschampsia cespitosa, although the change in number of inflorescences was consistently
less than that in the control, including a significant decrease with burning in 1997. As a
group, native graminoid species showed no significant differences in flowering response to
treatments during any of the four treatment periods. However, burning and mowing
consistently promoted flowering of Juncus tenuis. The cover of native forbs as a group was
not significantly different from that of the control, with the exception of 1997, where cover
increased with burning and hand-removal. Even though native forbs as a group have
generally not shown significant differences, individual native forb species have shown
positive responses to treatments: Sidalcea cusickii, spp. purpurea, Eriophyllum lanatum, and
• Non-native herbaceous species Both hand-removal and mowing promoted flowering of
non-native graminoids as a group. In contrast, burning maintained levels of flowering of
non-native graminoids similar to that of the controls. Cover of non-native forbs as a group
showed no significant treatment differences except for 1997, in which significant reductions
in cover occurred after burning and hand-removal.
The results to date show that prescribed burning is the best option of the three tested
management treatments for achieving the restoration goals for native wetland prairies.
Because mowing with removal of cut material was ineffective in reducing woody cover and
tended to promote non-native herbaceous species, this treatment should be avoided.