Seasonal priority effects: implications for invasion and restoration in a semi-arid system.
The timing of seasonal activity (i.e. phenology) may play an important role in plant invasions. In ecosystems characterized by seasonal rainfall, early-active exotic species may pre-empt resources and attain competitive dominance via a seasonal 'priority advantage'. Exotic annual grasses in California are often active earlier than native species, potentially because they possess greater germination plasticity. While these problematic invaders may usually benefit from having early phenology, their flexible germination cues might be manipulated as a restoration strategy to germinate seeds far in advance of favourable growing conditions, leading to a 'priority disadvantage'. We manipulated the start of the growing season in an invaded California coastal sage scrub community characterized by a Mediterranean-type climate to (i) identify whether early-season phenology confers a performance advantage and (ii) test whether rainfall timing could be manipulated to favour native species. We compared the performance of seeded native and exotic focal species under ambient rainfall timing (winter rains) vs. with a pre-growing season (late-summer) watering event. Under ambient rainfall timing, exotic annual grasses and forbs germinated earlier and reached higher levels of abundance than native species, consistent with a seasonal priority advantage. Many exotic annual grasses germinated with pre-season watering, but none survived until the onset of natural rains. Observations suggest that early-germinating seedlings suffered mortality via herbivory. The watering pulse thus depleted the exotic seedbank, fewer exotic individuals germinated with winter rains, and exotic species attained lower abundance than under the natural rainfall timing. Native species, whether annual or perennial, did not germinate with the pre-season watering pulse, suggesting they may have more constrained germination cues than the exotic species. Synthesis and applications. Our results suggest that phenology is an important factor influencing invasion success, and that this could be manipulated to favour native species. Manipulation of the start of the growing season, for example through a pre-growing season watering event, could be a successful restoration strategy for native species in some ecosystems.