Graduate Thesis Or Dissertation


The Response of Ventenata dubia to Prescribed Fire and Ungulate Grazing on the Pacific Northwest Bunchgrass Prairie Public Deposited

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  • Ventenata dubia L. (ventenata) is an introduced, winter annual grass that has recently been raising concerns across the Pacific Northwest Bunchgrass Prairie and the Palouse Prairie. It is well established now in pasturelands, croplands, and a variety of ecosystems including grasslands, sagebrush steppe, ponderosa pine forests and woodlands. Ventenata is quickly raising concerns about impacts on agricultural production, wildlife habitat, plant community composition, and watershed services. Even with this wide range of habitats and long list of concerns for managers, very little of its basic ecology and dynamics are known or just beginning to be studied. Prescribed fire and targeted herbivory are two management tools that are often used to control exotic annual grasses, but may also exacerbate populations. My objective for this thesis was to examine how these two disturbance factors influence and/or exacerbate ventenata invasion in the Pacific Northwest Bunchgrass prairie (PNB) using two different studies. In the first study, I explored how ventenata responded to prescribed fires, large ungulate herbivory (cattle and elk), and the interactions between them over the last 10 years on the PNB prairie. I used monitoring data (2008, 2016, and 2018) from experimental prescribed fire and grazing study plots established in 2004 and burned in the fall of 2006 and 2016. Using a randomized block repeated measures mixed linear model ANOVA I found ventenata was significantly increasing overtime in all response variables (p=0.0011, p=0.0001, p=0.0007). I found no significant differences (P<0.05) in ventenata with cattle grazing alone except for frequency (p = 0.03) in 2016, 10 years after the first prescribed fire. However, there was some evidence that grazing (livestock grazing combined with elk herbivory) increased ventenata frequency (P = 0.11) across all years, especially if those sites are not also managed with prescribed fire. For my second study, I resurveyed a subset of plots established in 2004 to compare ventenata cover and standing crop between paddocks that were grazed by cattle and elk to those where cattle had been excluded for 12 years. The study design was a randomized complete block design with four blocks and two treatment levels: 1) grazed (1.3-1.6 ha/AUM), and 2) fenced to exclude cattle, but open to elk (excluded). Using a linear mixed model ANOVA I found no significant differences (P<0.05) between grazed and excluded treatments. However, three out of the four blocks had much higher cover and standing crop in grazed areas, with, one block demonstrating the opposite pattern. Thus there is some evidence that combined herbivory by cattle and elk may lead to more ventenata, even though the effect was not captured when all blocks were analyzed. My thesis represents some of the first experimental and observational research involving ventenata and prescribed fire and ungulate grazing. Although more work still needs to be done, the findings contained in this research will help add to the much needed science for managing this species.
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