Plant community composition, soil community structure, and ecosystem function

Non-indigenous plants threaten the structure and function of many native ecosystems throughout the world. In the Mediterranean-type ecosystems of southern California, changes in plant community structure are apparent following exotic plant invasions. Native plant communities are often simplified, and once converted to exotic plant communities, may exhibit long-term patterns of stability. However, we generally know little about the functional differences between exotic and native species, and virtually nothing with respect to how these groups interact with soil microorganisms. I used a series of ecological experiments to test whether soil communities associating with native and exotic plant communities differ in their ecological effects. These effects include plant growth attributes and soil physical properties such as aggregate stability. Part of this investigation used a mesocosm system, where manipulated plant communities could interact with soil mutualists such as arbuscular mycorrhizal fungi. In a soil feedback experiment that used these soil communities, I found that an herbaceous native grassland species grew best in soil derived from native plant communities, whereas a weedy exotic species was inhibited by this same treatment. In another experiment that used various field-collected and commercial inoculants, I report strong positive effects on soil aggregate stability due to mycorrhizal fungi, an important component of soil biota. Moreover, I found that soil communities associating with native host plants produce a higher percentage of water-stable soil aggregates than do soil communities associating with exotic host plants. The soil communities associating with native host plants also resulted in enhanced mycorrhizal root colonization in the California sagebrush seedlings used for this experiment. Finally, a second year mesocosm experiment tested the relative importance of biotic properties such as species richness and a community's vegetative history on ecosystem invasibility. I used path analysis and an a priori causal model to predict the growth of two native and two exotic species used to invade model plant communities. All four test species grew best in native communities, and two biotic mediator variables (arbuscular mycorrhizal structure and plant productivity) strongly influenced the overall result. Species richness affected only plant productivity, and did not influence invader performance.