Effects of disturbance of biological soil crust on emergence of exotic vascular plants in California sage scrub

Invasion by nonnative plant species is shifting California sage scrub (CSS) communities from native perennial shrubland to exotic annual grassland. Biological soil crusts (BSCs) in CSS prevent soil erosion, increase water infiltration, and augment soil fertility. Disturbance of BSCs is known to increase germination of exotic plants in other abiotic stressed environments, but the effect of BSC disturbance in CSS is unknown. Using field and greenhouse experiments, I tested the hypothesis that disturbance of rugose, late-successional BSC increases emergence of exotic vascular plants in coastal CSS. At Whiting Ranch Wilderness Park in southern California, 22 plots were established and emergence of exotic and native plants was compared between disturbed and undisturbed subplots containing BSC. In a complementary greenhouse experiment, seed fate (emerged, not emerged, or missing from wind or granivory) in disturbed BSC cores was compared to seed fate in undisturbed BSC cores for three exotic and three native species. In the field, disturbance of BSC significantly increased total exotic emergence, particularly for annual grasses, and total native emergence was greater in undisturbed BSC. In the greenhouse, seed fates for all species were significantly different between disturbed and undisturbed BSC cores. All exotic species showed greater emergence in disturbed BSC and a greater percent were missing in undisturbed BSC. Native species showed varied responses including one native annual that showed the greatest emergence among all species in undisturbed BSC. Results confirm BSC as a critical ecological factor affecting exotic plant invasions in CSS.