Canopy Phenology and the Coexistence of Invasive Species in a Walnut Woodland Understory

Environmental heterogeneity of resources is thought to have important functions in the mechanisms that influence species coexistence. This thesis research explores how environmental heterogeneity created by canopy trees may influence coexistence of annual plants in the understory. Southern California black walnut (Juglans californica S. Watson) provides a unique opportunity for studying heterogeneity because the vegetative phenology of the walnut canopy may influence the interaction among species in the understory by providing shade and by leaching the allelochemical juglone into the soil. Two common species that are found in the walnut woodlands are black mustard (Brassica nigra) and Italian thistle (Carduus pycnocephalus). I hypothesized that the walnut canopy is either 1) leading to environmental sorting of the two species into their preferred habitats, allowing coexistence, or 2) that the walnut canopy is equalizing the competitive differences among species, also leading to coexistence. The research approach included transect observations, a competition field experiment, one mesocosm germination experiment, and two mesocosm competition experiments. The purpose of the transect observations was to determine the abundance of species in the understory and to quantify environmental variables. The mesocosm germination experiment was conducted in order to determine the effects of light and juglone on the germination rates of the two invasive species. The field and mesocosm competition experiments were conducted in order to determine the effects of light availability on the competitive outcome between the two species. When grown outside the canopy of walnuts, black mustard was competitively superior to Italian thistle. Both species exhibited the greatest rates of CO2 uptake and biomass accumulation when grown in full sun without interspecific v competition. However, black mustard competition significantly inhibited Italian thistle growth. However, the walnut canopy caused environmental heterogeneity in resources, which allowed Italian thistle to experience reduced competition from black mustard in the walnut understory. In dark shade treatments, both species exhibited reduced CO2 uptake and biomass. However, Italian thistle was able to tolerate dark shade, whereas black mustard growth was significantly inhibited in this treatment. This study provides an example of the effects that a dominant canopy tree can have in ameliorating interspecific competition in the understory. Continuing to explore how competitive outcomes are mediated by environmental and biotic factors will yield more insight into how plant communities are structured and change through time and space.