Invasive Plant-Soil Feedbacks and Ecosystem Resistance and Resilience: A Comparison of Three Vegetation Types in California

Ecosystem processes are strongly dependant on plant-soil feedbacks. The invasion of exotic plant species can result in the introduction of novel traits capable of de-coupling native plant-soil feedbacks and leading to altered nutrient cycling and availability and microbial community composition. In general the degree to which an invading species will impact the system it invades is dependant on how much it differs from plant species native to that system. However, there are examples in which invasion of an exotic plant similar to natives has led to significant alterations of ecosystem processes. The objective of this work was to examine the impacts of a single suite of exotic annual plants invading three very different vegetation types in southern California, grasslands, coastal sage scrub and chaparral. I predicted that invasion of exotic annuals would have greater impacts on shrubland systems than grasslands due to the greater disparity in plant traits between the exotic annuals and the shrub species. Comparisons of invasion impacts, however must consider factors other than vegetation type such as soil parent material, pH and soil moisture as mechanisms by which a native system may be more or less resistant to changes associated with invasion and recover following native vegetation re-establishment. In order to examine vegetation type resistance to invasion and soil resilience of these systems, I analyzed soils for total carbon and nitrogen, extractable phosphorus and nitrogen, nitrogen mineralization, soil respiration and microbial community composition using phospholipid fatty acid analysis. Long and short-term restorations were sampled concurrently with the invaded and native vegetation types to assess resilience of soils. Regardless of vegetation type nitrate was reduced by invasion, seasonality of sampling was a greater driver of microbial community composition than invasion or restoration and abiotic factors proved to be important to microbial species composition and soil nutrient availability. Individual chemical pools and functional groups of microbes responded differently in each vegetation type. However, the degree to which invading species differed from natives did not predict the level to which invasion would impact the system.