UC San Diego

Exotic annual grasses are invading native plant communities in many areas including the western United States, and pose a significant challenge to habitat restoration. Observations in California grasslands suggest that exotic species may become active earlier in the growing season than native species, and that this distinct phenology may contribute to invasion success. We hypothesized that flexible germination cues may allow exotic annual grasses to start annual growth early each growing season and preempt resources prior to native seedling establishment, a kind of seasonal priority effect. Flexible germination cues could incur a cost, however, if they cause seeds to germinate before the onset of favorable growing conditions. To evaluate these predictions, we compared native and exotic species performance in a coastal sage scrub community under both early (off-season) and ambient (natural) rainfall timings. Exotic annual grasses germinated substantially with off- season watering, but none of the early seedlings survived until the onset of the natural rains. Exotic annual grasses that experienced off-season watering had a depleted seedbank and lower germination following the natural rains. In contrast, native species did not germinate following the off-season watering pulse, and instead emerged with the beginning of the cold natural rains. Our results suggest that phenology is an important factor influencing invasion success and invader impact. Under some conditions, pre-growing season watering could be an important restoration strategy for native plant communities in early stages of invasion by depleting the exotic seedbank and allowing for native species to establish with reduced competition