Prescribed Fire and the Response of Woody Species in Willamette Valley Wetland Prairies

Abstract

A single fall‐season prescribed burn and two consecutive fall‐season prescribed burns were conducted in 1988 and 1989 to quantify the effects of fire intensity and frequency on woody species in two Willamette Valley wet prairies. Fuel biomass, fuel properties, fire behavior, biomass consumption, and changes in woody species density and height were documented before and after the burns. Before burning, Rosa nutkana (Nootka rose) was the most abundant woody species at both sites. In 1988, burns were significantly more intense, produced more heat per unit area at Fisher Butte than at Rose Prairie, and were fairly complete, with 37% and 35% woody biomass consumption, respectively. In 1989, burns were spotty; only 11% of woody biomass was consumed at Fisher Butte. The one‐burn treatment did not affect R. nutkana density in 1 × 15 m transects at either site; the two‐burn treatment significantly increased its density in three of the five communities sampled. Burning significantly reduced height growth by eliminating taller individuals while stimulating sprouts from belowground tissues. Redistribution of R. nutkana and trees into shorter height classes indicates the possibility of reducing the visual dominance of woody species if periodic burns were conducted in native Willamette Valley prairie. The low‐intensity burns in this study were sufficient to remove the woody shoots under 3 m tall but did not reduce their capacity to regenerate and did not reduce the density of woody species. After decades of fire suppression, two burns were not sufficient to reestablish the desired balance between native herbaceous and woody species or to reduce the presence of introduced woody species. A long‐term commitment to the reintroduction of fire as a management tool will be required to maintain native wetland prairies in the Willamette Valley.