Succession in a post-fire world: Bunchgrass seedling dynamics after wildfire in sagebrush steppe ecosystems
AuthorGicklhorn, Jeffrey M.
AdvisorNewingham, Beth A.
Natural Resources and Environmental Science
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Native plant communities experience a constant cycle of disturbance and recovery, and many disturbance regimes are expected to increase in frequency and severity with global change. Altered disturbance regimes can lead to drastic changes in plant community structure and shifts to alternate states. Ecosystem restoration plays a key role in attempting to return those communities to the appropriate successional trajectory. The Great Basin ecoregion of North America has experienced increasing frequency and size of wildfires coupled with increasing non-native annual grass establishment and widespread domestic livestock grazing. Native bunchgrasses are commonly seeded as restoration treatments after wildfire to stabilize soils and limit annual grass establishment; however, seedings often fail. Appropriate post-fire livestock management plays an essential role in increasing long-term restoration treatment efficacy.My first chapter examined changing post-fire plant community dynamics over time in the absence of disturbance over two years on two seeded Wyoming big sagebrush sites. Plant community dynamics examined included community composition by functional group, bunchgrass spatial relationships, and factors affecting seedling bunchgrass growth and survival. Seeded functional groups increased with time, suggesting seedings were effective at altering plant community composition. Bunchgrass spatial relationships initially reflected artificial structure associated with drill seeding; however, spatial patterns shifted over time to reflect plant-plant interactions occurring. Bunchgrass seedling growth and survival were negatively affected by increasing neighbor density, and species differed in their responses in year one but not in year two.My second chapter examined the interaction between post-fire plant community structure and timing of initial post fire defoliation over two years on the same sites. We altered plant community structure using removal treatments, and implemented defoliation treatments starting in the first fall after fire. Seedling removal delayed senescence and decreased bunchgrass cover and density, while adult removal did not have consistent effects. Spring defoliation shortened senescence, and decreased inflorescence production, leaf production, stem length, and total bunchgrass foliar cover. Fall defoliation exhibited mixed effects; however, fall year-two defoliation exhibited fewer negative effects as compared to fall year-one. Seedling removal and spring defoliation interacted to produce the most negative effects, suggesting that defoliating when seedling density is low may be unwise. General management recommendations include: 1) promoting bunchgrass seedling growing conditions the first year after fire, 2) avoiding spring defoliation all together and delaying fall defoliation until at least the second year after. If initial seedling density is low, delaying livestock further or implement additional restoration treatments. We acknowledge intrinsic differences across sites, and the need for informed and broad management recommendations; however, a site-specific approach is recommended rather than a one-size-fits-all strategy. Lastly, a conservative approach to reintroducing livestock is appropriate when one is uncertain about possible negative effects on restored species.