Spatio-temporal heterogeneity of environmental conditions allows for the coexistence of competing plant species in a community. The vegetative phenology of deciduous trees leads to environmental differences that competing understory plant species experience, as the timing of leaf growth and senescence in the canopy results in high-light and low-light environments under the canopy at different times of the year. Some species of canopy trees exude allelopathic compounds as well, which can fluctuate spatially and temporally throughout the year. This study examines how the phenology and allelopathy of the deciduous tree Juglans californica (Southern California black walnut) causes spatio-temporal heterogeneity in the understory that promotes the coexistence of the invasive plant species Brassica nigra (black mustard) and Carduus pycnocephalus (Italian thistle). Field and greenhouse competition experiments were conducted in which light transmittance was manipulated using shade structures. Experimental units were seeded with three treatments of invasive species in order to account for inter- and intraspecific competition differences. The abundance, growth, and physiological performance of each species were measured. An additional greenhouse experiment was conducted to determine the germination rates of the two invasive species under different treatments of light transmittance and soil concentration of juglone, an allelopathic compound produced by Juglans spp.
Results/Conclusions
Results provide evidence that the phenology of the native tree can influence the spatial distribution of two invasive plant species. Increasing concentrations of juglone and shade significantly reduced Brassica germination. In contrast, Carduus germination was unaffected by juglone and was greater with increasing shade. Brassica had higher relative growth rates and were larger in size in full sun compared to shade treatments, whereas Carduus grew largest in the darkest treatment and formed compact rosettes in the high light treatment. Taken together, our results indicate that Carduus can exploit the high shade conditions under walnut canopies in part due to its resistance to allelopathy. Brassica is inhibited from the understory due to its adaptation to high light conditions, and likely does not germinate under walnut trees during leaf-off conditions due to its sensitivity to juglone. This study provides an example of how a dominant tree species can influence community assembly among understory species, as well as providing useful information for restoration efforts.