Graduate Thesis Or Dissertation
 

Drivers and Impacts of a Recent Annual Grass Invasion: Ventenata dubia and Fire in the Inland Northwest

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  • Biological invasions threaten native biodiversity, alter ecosystem function, and are a major cause of economic losses across the planet. The most impactful invaders alter disturbance regimes and initiate state shifts to outside the historical range of variability of the ecosystem. Concern for ecological and economic losses has prompted a rapid expansion of invasion ecology research. However, the continual arrival of new invaders with unknown ecological impacts demands further research to help close the ever-growing knowledge gap. In the Pacific Northwest, a recently introduced, rapidly spreading Eurasian annual grass, Ventenata dubia (ventenata) is poised to alter fire behavior and ecosystem function across forest-mosaic landscapes of the Inland Northwest, USA. This dissertation aims to: 1) determine the biotic and abiotic factors associated with the V. dubia invasion, 2) characterize the relationship between invasion and plant community diversity in burned and unburned areas, 3) examine how biotic and environmental factors interact to influence community invasion resistance, and 4) evaluate the influence of V. dubia on fuel characteristics and fire behavior at multiple scales. I used field data, statistical analyses, and landscape fire simulations to determine the drivers and impacts of the V. dubia invasion at community and landscape-scales in the Blue Mountains Ecoregion of the Inland Northwest. In Chapter 2, I identified V. dubia’s unique niche in forested ecosystems of the region, including historically invasion and fire-resistant dwarf shrublands imbedded within the larger forested landscape. I demonstrated that V. dubia expands invasion impacts in these ecosystems rather than occurring in areas already impacted by other invasive annual grasses (Bromus tectorum and Taeniatherum caput-medusae), increasing the overall invasion footprint. Chapter 2 also examined the relationship between V. dubia and plant community diversity with and without fire. I found that V. dubia was weakly related to community diversity in unburned areas but was strongly negatively related to diversity and abundance of functionally similar species in burned areas. These results suggest that V. dubia may fill an otherwise seemingly unoccupied niche in unburned areas but may outcompete functionally similar species for post-fire resources. In Chapter 3, I explored interacting drivers of community invasion resistance using an in-situ manipulation experiment across three vegetation types. I found that community biomass and some traits (specific leaf area, fine-to-total root volume, and height) may confer invasion resistance of existing communities to V. dubia. However, this was only the case in the most productive wet meadow vegetation types. I found no evidence that biomass or community trait composition contributed to invasion resistance in less productive and more stressful low sage-steppe or scab-flat vegetation types, indicating that environmental and biotic factors interact to influence invasion resistance. To assess the potential influence of V. dubia invasion on fire behavior across the region, I evaluated the influence of V. dubia on fuels and fire in Chapter 4 using a novel application of the landscape-scale Large Fire Simulator, FSim. I show that invasion increased fire spread, burn probabilities, and fire intensity across forest-mosaic landscapes by increasing fuels and fire occurrence in invaded non-forested areas adjacent to fuel rich forests. Overall, this dissertation provides some of the first documentation of V. dubia’s niche and invasion dynamics in forested landscapes, and characterizes how this invasion differs from other problematic species in this region. My work demonstrates that V. dubia may initiate a grass-fire cycle in historically fire- and invasion-resistant scabland ecosystems and that annual grass invasion can have substantial impacts on fire behavior in uninvaded forests – ecosystems thought to be resistant to annual grass impacts. Together, these chapters provide valuable information from the invasion front to aid the management of this rapidly spreading species.
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Related Items
  • Landscape Fire and Conservation Science Research Group
  • Northwest Fire Science - Ventenata Invasion in the Blue Mountains
  • Claire Tortorelli's research website
  • Chapter 3 manuscript - Community invasion resistance is influenced by interactions between plant traits and site productivity
  • Chapter 2 manuscript - Expanding the invasion footprint: Ventenata dubia and relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA
  • Related manuscript - The invasive annual grass Ventenata dubia is insensitive to experimental removal of above-ground resident biomass across a productivity gradient
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  • This work was supported by funding from the Joint Fire Science Program, Oregon Native Plant Society, Association of Fire Ecology, and the National Science Foundation.
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