Microsites play an important role for seedling survival in the seagrass Amphibolis antarctica

Abstract

The contribution of recruitment to local population dynamics of plants is limited by production of seeds or availability of suitable sites for establishment of seedlings. Viviparous seedlings of the seagrass Amphibolis antarctica have been observed in great numbers within gaps in meadows of other seagrass species. We conducted a field experiment to assess the extent to which production of viviparous seedlings or availability of suitable sites limits recruitment of A. antarctica into gaps in seagrass meadows. At each of six gaps, seedlings of A. antarctica were transplanted at two densities onto three substrata; sand, seagrass matte (exposed rhizomes and sheath material) and established seagrass meadow. Seedling survival was tracked over 2 months and seedling survival half-lives were compared among treatments. Planting density had no effect on total survivorship, suggesting that increased seedling production would not increase recruitment rates. Physical dislodgement of seedlings by wave and current forces was the primary factor impeding successful recruitment. Of the substrata tested, the best potential establishment sites were associated with the matte of dead seagrass rhizome and sheath fibre. Seedlings entangled in seagrass matte fibres required greater force to remove, and we propose that the entangling nature of matte enhances seedling survival and establishment. The force needed to dislodge seedlings from sand was constantly exceeded by the current velocities acting upon seedlings during the study, and we suggest that the 100% loss of seedlings from sand was driven by hydrodynamic forcing. Seedling loss in the seagrass meadow plots was accelerated by contact with adult seagrass, which increased the rate of seedling dislodgement through the sweeping of leaves across the substratum. Recruitment in the population of A. antarctica examined in this study was limited by the availability of suitable establishment sites, such as those provided by the seagrass matte substratum along the fringe of meadow gaps. The ability of A. antarctica seedlings to recruit to seagrass meadow gaps allows local population persistence of this species in areas dominated by other seagrass species.