Costs of Reproduction in an Intertidal Kelp: Patterns of Allocation and Life History Consequences

Reproductive investment of the intertidal kelp Alaria nana was manipulated experimentally to investigate costs of reproduction. Allocation patterns were examined using treatments that employed vegetative and reproductive tissue removal. Growth, survivorship, size, reproductive investment, carbon, nitrogen, and phenolic secondary metabolities were monitored in tagged experimental plants. Further experiments investigated the contribution of reproductive tissue to drag forces on the plant in intertidal environments. Carbon, nitrogen, and phenolics were all higher in reproductive tissue (sporophylls) compared to vegetative tissue (the frond) in control plants. When some vegetative tissue was removed, carbon and nitrogen in the sporophylls decreased while percent phenolics remained unchanged. Reproductive tissue removal had no effect on plant survirorship, growth, or size compared with control plants. When the entire frond was kept clipped, sporophyll number decreased, often to the point of complete attrition. Although allocation patterns in Alaria nana are consistent with reproductive costs (i.e., the sporophylls are not self—supporting), there is no evidence that reproduction imposes costs on the plant that are demographically important such as growth, size, and survivorship. Instead, both reproductive investment and percent phenolics were positively correlated with growth and size in control plants.