Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Wildland Resources

Committee Chair(s)

Andrew Kulmatiski


Andrew Kulmatiski


Jeanette Norton


Karen H. Beard


Peter Adler


Linda Kinkel


Plants compete for the same basic nutrient and water resources. According to the competitive exclusion principle, when a substantial overlap in resource pools exists, the best competitor for resources should drive all other species to extinction. The ability for plants to coexist in violation of the competitive exclusion principle is the “biodiversity paradox”. Coexistence is actually beneficial for plants: as species diversity increases, you typically see increases in plant biomass production (known as the biodiversity-productivity relationship). The mechanisms behind coexistence and the biodiversity-productivity relationship remain an ecological mystery. One hypothesis is that plants obtain water and nutrients from different places in the soil, which reduces competition and results in plants coexisting and thriving by exploiting more spaces in the soil. Another hypothesis is that plants alter the soil in which they grow to their own detriment by accumulating species-specific soil pathogens or reducing soil nutrient levels. These plant-altered soils reduce the growth of species that are becoming too dominant in a plant community, creating a plant-soil feedback (PSF) effect that maintains biodiversity and increases productivity. I explored the role of PSFs and niche partitioning in coexistence and the biodiversity-productivity relationship. I investigated 1) how PSFs affect the biodiversity-productivity relationship in controlled greenhouse experiments, 2) whether greenhouse experiments are the best method to measure the role of PSFs in biodiverse communities in the field, 3) how PSFs affect the biodiversity-productivity relationship in diverse plant communities in the field, and 4) how partitioning of soil nitrogen and soil water affect coexistence and plant productivity.

Greenhouse experimentation suggested PSFs influence productivity and the biodiversity-productivity relationship, but PSFs when measured in the greenhouse were not correlated with PSFs that were measured in the field. This implies PSFs should be measured in the field when trying to predict coexistence or the biodiversity-productivity relationship as observed in the field. Our ability to predict coexistence and productivity in the field was slightly improved by the inclusion of PSFs. However, partitioning of soil water and soil nitrogen was strongly correlated with landscape productivity in the same system, indicating that PSFs are not the dominant mechanism of these phenomena.