Date of Award:

12-2026

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Wildland Resources

Committee Chair(s)

Peter Adler

Committee

Peter Adler

Committee

Noelle Beckman

Committee

James Lutz

Committee

Moria Robinson

Committee

Kari Veblen

Abstract

Plant species require resources, such as water, light, and nutrients, and develop characteristics to compete for these limited resources. Multiple plant species can coexist in a given community if they vary their characteristics to acquire resources at different temporal or spatial scales. For example, neighboring plants coexist when they take up water in the soil at different times of the year or soil depths. Alternatively, neighboring plants with similar characteristics may compete strongly for a limited resource. However, when a plant population is growing or shrinking, it can be difficult to determine if population changes are due to resource limitation in the environment (e.g., not enough resources regardless of other plant species) or competition (e.g., other species are consuming all the resources). Furthermore, certain plant species, like invasive species, can outcompete others in the community, leading to reduced community diversity. This is problematic because reduced plant community diversity leads to reduced ecosystem functions, such as pollination, carbon capture, and nutrient cycling. Understanding how plants compete for resources and coexist can improve our ability to manage areas threatened by invasive plant species.

Using simulations and data from real plant communities, I explored the effects of environmental conditions and competition on plant demography (i.e., survival, growth, fecundity). My simulation study showed that ignoring environmental variables can lead to incorrect conclusions about how plants coexist in a community. I then tested how soil temperature and competition interact and affect an invasive species in eastern Idaho. I found that under cold conditions, the invasive species cheatgrass established best in bare ground, where the soil temperatures are slightly warmer and more favorable for establishment. Finally, I used a large dataset on an invasive species across western North America to identify how plant communities resist invasion. Resistance to invasion is highest in plant communities with limited resources and species that can tolerate stressful conditions. Overall, my dissertation highlights how we can improve plant competition models and makes general conclusions about how invasive species outcompete other species in a community.

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