SIZE AND SURVIVAL OF INTERTIDAL PLANTS IN RESPONSE TO WAVE ACTION: A CASE STUDY WITH FUCUS GARDNERI

Hydrodynamic forces generated by breaking waves have been proposed to act as one of the primary physical mechanisms constraining the sizes to which wave‐swept intertidal plants can grow. Plants inhabiting intertidal areas are generally small relative to subtidal and terrestrial plants, and within a species, plants on wave‐exposed shores are usually smaller than those at wave‐protected sites. However, although these relationships have been well documented, there have been no manipulative field experiments demonstrating that wave forces are directly responsible for limiting plant sizes. In this study, I examined the effects of wave forces on plant sizes in the field by reciprocally transplanting Fucus gardneri (rockweed) individuals between wave‐exposed and wave‐protected intertidal sites at Fogarty Creek Point, Oregon. Mean sizes of wave‐exposed plants transplanted to protected sites increased significantly relative to exposed control transplants. Mean sizes of wave‐protected plants transplanted to exposed sites decreased significantly relative to protected control transplants. These data support but modify an accompanying mathematical model that predicts size‐dependent survivorship of Fucus under conditions of high and low wave exposure. This model incorporates the trade‐off between higher reproductive capacity and lower probability of survival at larger plant sizes. “Optimal” plant sizes predicted by the model closely match the mean observed sizes of plants collected from exposed and protected locations. However, the match between observed and predicted plant sizes occurred not via differential survivorship, but by direct size modification resulting from plant tattering.