Patterns and sources of variation in flowering, seed supply and seedling recruitment in surfgrass phyllospadix torreyi

Abstract

Knowledge of patterns and sources of variation in adult reproduction and juvenile recruitment is essential for understanding the dynamics of populations and their capacity to recover from disturbances. In-depth knowledge of this kind is lacking for many species of seagrass. Here we examined the degree to which temporal and spatial variation in seedling density in the surfgrass Phyllospadix torreyi was explained by the local production and deposition of seeds. We did this by measuring floral density, seed production, seed supply and seedling recruitment at 5 sites near Santa Barbara, California, USA, over a 4 yr period. We found that the local density of dehisced female spadices explained nearly 41% of the variation in the number of seeds caught in seed traps. Large seed crops were common in the absence of nearby males and noticeable numbers of seeds were periodically caught in traps when female flowers were locally rare or absent, suggesting that pollen and seed dispersal occasionally occurred over distances greater than the scale of our study sites (i.e. >50 m). Overall, the summer densities of female spadices at a site explained >60% of the variation in the density of seedlings that recruited to the site the following winter. Deviations from positive associations among surfgrass flowers, seeds and seedlings appeared to reflect interannual variability in environmental conditions. Most notable was the coincidence of universally low fruit production and seedling recruitment during the 1997-1998 El Niño event. Seedling density varied independently of rates of seed deposition when all years were examined, but was highly dependent on rates of seed supply when the seedling data from the 1998 El Niño winter were omitted from the analysis. Collectively, our observations demonstrated significant coupling between adult reproduction and local recruitment in surfgrass that was interrupted by adverse growing conditions associated with the 1997-1998 El Niño.