Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Nichole N. Barger

Second Advisor

William D. Bowman

Third Advisor

Katharine N. Suding

Fourth Advisor

Timothy R. Seastedt

Fifth Advisor

Daniel F. Doak

Abstract

Understanding the key factors that determine community structure is a central goal of ecology. While in plant community ecology there is wide consensus of the primary importance of dispersal limitations and abiotic filters in determining community structure, ecologists are just beginning to understand how biotic interactions restrict or enhance the performance and abundance of certain species within communities. Biological soil crusts (‘biocrusts’) – surface-dwelling soil biotic communities comprised of cyanobacteria, algae, lichens, bryophytes, and fungi – coexist with vascular plants in dryland ecosystems worldwide. Where they occur, biocrusts modify soil resource availability (e.g., water, nutrients) and other soil characteristics that may strongly influence the plants with which they coexist. Yet, general patterns underlying biocrust-plant interaction outcomes have remained uncertain. Using a combination of greenhouse experiments, observational field data, and analytical techniques including meta-analysis, I examine how biocrusts influence dryland plant species and communities. I experimentally demonstrate that, in general, biocrusts inhibit the germination of individual plant species, but enhance plant growth. Using meta-analysis, I quantify the relative importance of key biocrust, plant, and environmental explanatory variables in determining outcomes of plant responses to biocrusts globally. I find that biocrust composition and plant functional traits including lifeform, photosynthetic pathway, and presence of nitrogen-fixing symbionts can be used to predict plant responses to biocrusts. Additionally, I show that biocrusts likely contribute to the biotic resistance of plant communities through inhibition of exotic over native plant species during recruitment. Collectively, these results suggest that by increasing or decreasing the performance of certain plant functional types within communities, biocrusts may mediate plant community structure and diversity. Finally, I test this assumption using an observational field study. I show that biocrust spatial heterogeneity is positively associated with plant species richness via niche-based processes, suggesting biocrusts may stabilize plant species coexistence at community scale. Across this work, I find that biocrusts can have strong impacts on plant species performance and plant community structure. Generally, this work has implications for understanding the potential of plant-soil interactions to influence plant community assembly and diversity and suggests the importance of biocrusts in supporting plant diversity and invasibility patterns in dryland ecosystems.

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