Vegetation composition and response to fire of native Willamette Valley wetland prairies Public Deposited

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  • Fires set by Native Americans were important in shaping and maintaining Willamette Valley prairie plant communities. With fire exclusion after Euro-American settlement in the 1840's
  • Fires set by Native Americans were important in shaping and maintaining Willamette Valley prairie plant communities. With fire exclusion after Euro-American settlement in the 1840's, composition of the remaining prairies shifted toward shrub and tree dominance with a concomitant decline in endemic prairie plant populations. I hypothesize that by restoring fire as an ecosystem process native prairie flora would be enhanced by invigorating vegetative growth of natives, by controlling exotic species, and by controlling successional invasion ofwoody species. Additionally, I hypothesize that native prairie composition might be better enhanced by multiple burns compared to a single burn. Initial vegetation composition, fuels, fire behavior, and subsequent changes in plant composition following fire were quantified in a total of five plant communities at Rose Prairie and Fisher Butte study sites near Eugene, Oregon.Three treatments were assessed over three years (unburned control, and once burned, and twice burned). A total of 205 species were observed. Species richness was dominated bynative forbs in all five plant communities. In contrast, vegetative cover was dominated by native perennial graminoids in all communities except one where exotic perennial graminoids dominated. Rosa nutkana was the most prevalent woody species in all communities. Initial preburn R. nutkana densities ranged from 30,519 to 50,963 plants ha -1 at Rose Prairie and from 17,156 to 53,067 ha -1 at Fisher Butte. Fisher Butte had more tree encroachment relative to Rose Prairie. Most trees at both sites were seedlings and saplings, ranging between 3 to 70 cm in height. Total tree density initially ranged from 0 to 568 trees ha -1 and from 430 to 1,071 ha-1 at Fisher Butte where Fraxinus latifolia was invading the wet Rosa Juncus community. Total above ground biomass of Willamette Valley wetland prairies wasgreater than that of many other North American grasslands ranging from 6,958 to12,038 kg ha-'. The first season following one burn, above ground biomass decreased significantly in one community (-25.4%) and increased significantly in another (+ 68%).Response to burning was often inconsistent and varied greatly by species among plant communities. Native as well as exotic species increased in frequency or cover with burning. Eight native perennials and three native annuals established or significantly increased frequency (p = 0.1) in burned areas in two or more plant communities. Four exotic perennials and one exotic annual increased significantly in frequency following fire. Although frequency of Deschampsia cespitosa increased in burn treatments, cover declined significantly in the first postfire year following one burn, indicating a shift from fewer large plants to a greater number of smaller plants immediately following burning.Significant increases in vegetative cover occurred in burned areas in two Rose Prairie communities and in one Fisher Butte community, due primarily to changes in cover of perennial graminoids. Total cover of exotic species increased following one or two burns in all but one community. Significant increases in exotic forb cover occurred in two communities. After two burns, relative frequency of native perennial forbs increasedsignificantly in three communities. Relative frequency of exotic perennial graminoids declined significantly in burn treatments in all but one community where they increased in the control treatment. Native annual forbs increased in burns in all but one community.Increases of woody biomass, frequency, and cover following burning suggest that Willamette Valley wetland prairie woody plants are highly resilient to burning and quickly recover and increase following fire. However, height of woody plants significantly declined following burning leading to an improved prairie structure. In addition, the number of Fraxinus latifolia seedlings and saplings were significantly reduced in burn treatments in the Rosa Juncus community. Fire behavior and fuel conditions in relation to vegetation response are required to develop optimal frequency and intensity of burn prescriptions. Information on the response of native, woody, exotic plants, and aboriginal food plants will assist land managers in determining optimal fire frequency for burning Willamette Valley wetland prairies to maintain and enhance desired species. Thisresearch initiates a long-term assessment of fire effects on Willamette Valley vegetation to determine optimal burn prescriptions for wetland prairies.
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