Seasonal patterns of disturbance influence recruitment of the sea palm, Postelsia palmaeformis

Physical disturbance can have important effects on the distribution and abundance of sessile organisms, and temporal variation in the disturbance regime may strongly influence recruitment of species which reproduce seasonally. The sea palm Postelsia palmaeformis ij a kelp with an annual, heteromorphic life history that inhabits the mid-tidal zones of some of the most wave-exposed rocky intertidal areas on the Pacific coast. Postelsia is thought to thrive in wave beaten areas, since winter storms remove patches of the mussel Mytilus californianus, the dominant competitor, thereby freeing bare space on the rock. To evaluate the effects of temporal variation in the physical disturbance regime and the effects of mussels on Postelsia recruitment, small plots in a mussel bed surrounded by existing Postelsia were seasonally cleared of mussels. Sea palm recruitment occurred in all plots, but was greatest in the most recent winter mussel removal treatment, suggesting survival of microscopic or young stages of Postelsia may be enhanced beneath mussel beds during fall/winter. Postelsia's annual life history seems ideally suited to wave-exposed sites, where predictable winter disturbances remove mussels. This study shows that Mytilus can also have positive effects on Postelsia recruitment.