Graduate Thesis Or Dissertation


Forb Community Response to Various Disturbances in the Pacific Northwest Bunchgrass Prairie Public Deposited

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  • Observed declines in native pollinator species worldwide has generated concern and focused research into the disturbances, past and present, which may have contributed to these losses. In grasslands, for example, habitat degradation and fragmentation from historical and current crop production and livestock grazing has left only a few remnants of the native systems. The remaining remnants face further modification by altered fire regimes and invasive species. One of the most threatened and understudied of the fragmented temperate grasslands is the Pacific Northwest Bunchgrass Prairie (PNB) located in the northwest region of the United States. The overarching objective for this thesis, accomplished through two research studies, was to examine how a variety of different disturbances affect the floral resources in the PNB. In the first study, we explored the individual and interactive effects of three disturbances (prescribed fire, grazing, and invasive species) on the floral resources within the largest remaining remnant of the PNB prairie, known as the Zumwalt Prairie. Specifically, we addressed how prescribed fire, grazing, and their interactions impact forb community composition, abundance, richness, and diversity over 12 years. We also examined the response of forb cover, richness, and diversity to the increasing cover of the newest invasive species threat to the PNB, Ventenata dubia. Our results indicated that prescribed fire has more influence on forb community composition over time than moderate grazing alone or no livestock grazing and that a ten-year fire return interval does not significantly influence forb species richness or diversity in this system. The indicator species for each of the treatments illustrated that maintaining disturbances like prescribed fire and herbivory may help support certain forbs that are important to pollinators and for culturally significant plants of the region’s indigenous peoples. We found significant negative correlations between forb cover and richness once the increase in V. dubia cover ranged between 20-37%, suggesting a threshold for impacts on the forb community that should be monitored. In the second study, we explored the legacy effect on forb communities where historical cultivation sites were abandoned and seeded with exotic pasture grasses (seeded old fields) on the PNB. Specifically, we evaluated differences in forb community, abundance, richness, and diversity between these sites (treatments), tested for indicator species within each treatment, and included comparisons over the course of two consecutive growing seasons. Native prairie sites contained more abundant, rich, and diverse floral resources than seeded old fields across growing seasons and in both sampling years. Our findings add to the evidence that the legacy of cultivation and reseeding disturbances can persist long after these land uses end, especially on forb communities. Further, our findings suggest this legacy may decrease habitat quality for pollinators in seeded old fields, especially in August. Differences in indicator species between sites and over the growing season also identified important areas for future research to maintain culturally important forb species which may help enhance floral resources for pollinators, especially in the fall. The findings in this study highlight a need for more research into how pollinators respond to these differences and how active restoration may be needed to conserve forb communities for pollinators and culturally significant forb species for indigenous people in the region. Both of these studies are the first to address questions about how these disturbances (prescribed fire, livestock grazing, invasion by V. dubia, and cultivation legacies) affect the composition, abundance, richness, and diversity of forb communities on the PNB. The results from this research can help land managers, mainly in the PNB ecosystem, and also adds to the information needed to help conserve and promote floral resources for native pollinators. More research needs to focus upon understanding the continuum of these habitats, as they are all extremely important for conservation of native pollinators and maintenance of all ecosystem services.
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  • Foundation for Food and Agriculture (ID: 549031) with support from the Oregon Agricultural Research Foundation.
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