Life in interstitial space: Biocrusts inhibit exotic but not native plant establishment in semi-arid grasslands

Exotic plant species commonly exploit disturbances more successfully than native plants. This outcome is widely attributed to the fact that disturbance reduces biotic resistance from native plant competitors. However, biocrusts, communities of mosses, lichens, and micro-organisms, are a prominent component of semi-arid grasslands occurring in the interstitial spaces between vascular plants. Biocrusts may provide an important source of biotic resistance to invaders, different from native plant competition, but poorly understood. We established a large-scale field experiment to examine how intact versus disturbed biocrusts influenced the emergence and establishment of four native and four exotic plant species in intermountain bunchgrass systems over 2 years-one wet and one dry. We also conducted a complementary greenhouse experiment to explore how differences in moisture might influence biocrust effects on germination. In the greenhouse, biocrusts inhibited the germination of both native and exotic plants in the high moisture treatment only. In field experiments, biocrusts inhibited the overall emergence of exotic seedlings in the wetter of the 2 years and inhibited the establishment of exotic seedlings in both years, but they had no overall effect on the emergence or establishment of native seedlings. We found that intact biocrusts in intermountain grasslands can suppress the establishment of some exotic plants, but have much weaker effects on natives. They also suggest that water availability may influence biocrust effects on seed germination. Synthesis. Our results indicate that intact biocrusts may provide an important source of biotic resistance to exotic plant invasions in intermountain grasslands. Furthermore, precipitation inputs may mediate biocrust effects on plant establishment.