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Examining contingencies of the plant community–grazing relationship at multiple scales across North America
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- Author / Creator
- Grenke, Jessica SJ
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Defoliation is a globally ubiquitous driver of plant community structure and function. However, plant community responses to defoliation remain challenging to predict as they are highly context-dependent. Following defoliation, plant community diversity and production may increase, decrease, or display no change depending on climate, plant neighbors, and the disturbance’s timing, frequency, and location. In this thesis, I disentangle the role of these three core contingencies in mediating the plant community–defoliation relationship across a broad spectrum of scales in time and space. First, I test the assumption that defoliation instigates higher plant production and diversity when applied using high-intensity, short-duration holistic grazing management across the northern great plains. I find holistic grazing management supports fewer vascular plant species at both local and landscape scales despite no difference in plant community composition and variability. However, holistically managed ranches showed markedly higher plant community production than their regionally typical counterparts, with 20% higher aboveground biomass, 26% higher litter mass, and equivalent amounts of surface root biomass. Second, I examined how varying the proportion and patchiness of defoliation disturbances impacted community-wide competition and production using experimental mesocosms. I found that defoliation intensity strongly interacted with the proportion of the plant community disturbed to influence whole-community production and competition. Increased defoliation intensity negatively affected mesocosm production (aboveground, belowground, and total biomass) when all individuals were defoliated but positively affected production where half the mesocosm was defoliated. Finally, I used a meta-analysis of grazing studies across North America to examine how the consequences of grazing exclusion on plant community production and richness vary depending on the weather at the sampling time, climate, year, and study location. Study location and sampling time mediated the effects of grazing exclusion on plant community production and richness. Plant richness, not plant abundance, responded to grazing exclusion differently depending on mean annual precipitation, with less positive effects of exclusion found when precipitation was higher. Data collection within an unusually wet year detected less positive effects of grazing exclusion. Our results demonstrate that climate and weather drive the effects of grazing exclusion on plant communities and that these effects are growing more deleterious toward plant abundance over time. Overall, the results of this thesis promote a new understanding of how grazing management, plant-plant interactions, and study location in time and space modify the plant community–defoliation relationship.
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- Graduation date
- Fall 2023
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.