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Invasion in the Chaparral: Uncovering Soil Microbial and Plant Physiological Mechanisms


Global change contributes to drastic shifts in vegetation composition resulting in changes in ecosystem processes across the world. One important example is plant invasion, which often leads to vegetation community type conversion, such as conversion from native shrubland to invasive grassland. Chaparral, California’s most pervasive vegetation type, has recently undergone invasion. The shift from evergreen chaparral shrubs to invasive grassland will have cascading effects on ecosystem services. The overarching goal of this research is to understand water use, root and fungal dynamics of invaded chaparral communities that may inform restoration efforts. My first chapter examines how differences in root development relate to soil-water dynamics between a chaparral shrub and an invasive grass. I explored above- and below-ground strategies in concert of a native chaparral shrub and an invasive grass species in southern California using soil moisture sensors, manual minirhizotron imagery, stable isotopes, sap flux sensors and normalized difference vegetation index (NDVI). I found that the invasive grass species depleted soil moisture and produced longer roots earlier than the native shrub. Depletion of soil moisture earlier by E. calycina suggests that invasive grasses could accelerate the onset of the summer drought in chaparral systems, assuring their persistence following invasion. My second chapter examines how invasion and nitrogen deposition structure composition of fungal communities. I found that invasive grasses had a lower richness and relative abundance of symbiotic fungi compared to native shrubs. My third chapter explores how invasive-conditioned soils affect the growth of chaparral shrub seedlings and associated fungal communities and I found that native inoculation produced a more diverse fungal symbiont community. Chapter four aims to detect if invasive grass water-use strategies are detrimental to shrub seedling success, and found that invasive removal positively affected the establishment of native shrub seedlings and seedling mortality, which increased alongside invasive cover. Overall, my dissertation demonstrates that competition between invasive and native plants as well as shifts in fungal communities contributes to invasive grass persistence and shows how joining tools and perspectives from diverse fields can provide a holistic look at system responses to change.

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