Non-native plant invasions have been cited as a cause of decline of numerous plant communities, including Southern California walnut woodlands. These woodlands are dominated by Juglans californica, the California black walnut, which is a rare, endemic, allelopathic tree. Barriers to native community assembly in walnut woodlands include abiotic filters, such as light and water availability, and biotic filters, including competition with invasive plant species, and allelopathy, due to the chemical juglone. Here we present two experiments designed to assess how these abiotic and biotic environmental filters affect the establishment, growth, and reproduction of native and non-native annual plant species, with a focus on developing a trait-based restoration approach for this ecosystem. A laboratory experiment was conducted to observe the effects of five juglone concentrations (from 0 to 0.5mM) on the germination of several native and invasive species. A field experiment was conducted to examine native and non-native annual recruitment with respect to microclimate and competition. Competition treatments included native species only, invasive species only, and both native and invasive species, all nested within canopy and open plot locations. Leaf size, phenology/timing of germination, plant stress, and growth rate were measured in each plot.
The germination rate of invasive mustard species, Brassica nigra and Hirschfeldia incana, was significantly inhibited by juglone concentrations greater than or equal to 0.01mM (t=-9.9, p<0.001); whereas the germination rates for the native annual species Amsinckia intermedia and Phacelia distans were inhibited at higher concentrations greater than or equal to 0.05mM (t=-5.1, p<0.001). The native annual Deinandra fasciculata was the only species tested whose germination was significantly improved by juglone concentrations of 0.05mM and 0.1mM (t=5.9, p<0.001). In the field experiment, germination rates for all species between canopy and open locations were similar. Native Amsinckia intermedia and invasive Brassica nigra displayed similar phenology, being the first species to germinate. Both had the highest germination rates across all canopy and seeding treatments. Preliminary functional trait results showed a significant reduction in leaf size in both species when grown in exposed areas, compared to areas under the walnut canopy (β = -25.0, p<0.001). Taken together, results show that native, compared to invasive, species tolerate higher concentrations of juglone during germination, and that the microclimate under walnut canopies may facilitate the growth of early-germinating species. Measurements will be taken and analyzed through the entire growing season in 2017.