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.

Growth and Reproduction of Ruppia maritima in the Northern Gulf of Mexico

“…Reed et al (2009) demonstrated a similar correlation between reproductive structures and seed numbers in Phyllospadix torreyi. Seagrass canopies increase particle retention (Hendriks et al, 2010), so the presence of mature plants is likely to increase the retention of seeds as well as the sediment that may retain them even after the mature plants die back.…”

Where a male is hard to find: consequences of male rarity in the surfgrass Phyllospadix torreyi

“…Female-biased sex ratios have been documented for the genus Phyllospadix (Williams 1995, Shelton 2008, 2010b, and recent work has shown that sex ratios can vary among spatially segregated populations (Addison 2009, Reed et al 2009. A variety of mechanisms may produce skewed sex ratios of flowering shoots, including spatial segregation of the sexes, non-random production of gender, or differential mortality or growth.…”

“…Most populations of Phyllospadix spp. are female-biased (see below), but seed production estimates range widely, from nearly 100% successful (Williams 1995, Reed et al 2009) to nearly 100% failed (Shelton 2008).…”

“…With single-seeded fruits, measurements of fruit and seed production are equivalent; in this paper we refer only to seed production. Flowering buds are formed in late spring, and pollination extends from early summer to early fall (Reed et al 2009). The range of pollen transport is unknown for P. torreyi; in other seagrass species with filiform pollen it is on the order of tens of meters or less (Ruckelshaus 1996, Verduin et al 1996, Ackerman 2002.…”

“…The range of pollen transport is unknown for P. torreyi; in other seagrass species with filiform pollen it is on the order of tens of meters or less (Ruckelshaus 1996, Verduin et al 1996, Ackerman 2002. Fruit development typically occurs throughout the summer, with full maturation requiring ~3 mo; peak seed dispersal is completed by mid-fall (Reed et al 2009). P. torreyi and its congeners are found along rocky shorelines of the northern Pacific Ocean.…”