Seed Rain, Transient Seed Banks, and Seedling Recruitment of Annuals on a Coastal Beach1

Abstract

Annual plants are important colonizers of the bare sandy areas that typify the pioneer zone of many coastal beaches. The spatial distribution of the potential seed rain in autumn, transient seed bank in early spring, and seedling recruitment in late spring were examined for the summer annuals in a 40 × 45 m section of beach on Staten Island, NY. The relationship of the dominant perennial, Ammophila breviligulata, to recruitment was also explored. Estimated seed rain was very high, showing species-specific distribution patterns along the shore-to-land gradient for Triplasis purpurea, Cenchrus tribuloides, and Heterotheca subaxillaris, the most abundant native annuals. Seed rain and spring recruitment for T. purpurea and C. tribuloides were greatest in regions of the beach where A. breviligulata was less abundant. Recruitment of T. purpurea and C. tribuloides, but not H. subaxillaris, was negatively associated with living A. breviligulata. Summed over all annuals, recruitment was 73.6 ± 5.2 seedlings m−2 in the absence of litter and 45.8 ± 4.3 seedlings m−2 where A. breviligulata litter occurred. Thus, the distribution and abundance of A. breviligulata was inversely related to the seed rain and recruitment of annuals. The weedy annual Mollugo verticillata was most abundant in the transient seed bank (669 ± 205 seeds m−2), but T. purpurea was also common (211 ± 32 seeds m−2) mean total density of all species was 1,082 seeds m−2. For the three natives, only a small proportion (ca. 1–30%) of the seed rain was detected in the transient seed bank. Depletion of seeds over the transition from seed bank to seedling recruitment was 77% for C. tribuloides, over 90% for T. purpurea and H. subaxillaris, and 99% for M. verticillata. Thus, only a very small fraction of the potential seed rain in autumn resulted in successful seedling recruitment the following spring. Despite the massive loss of seeds across seasons, patches of annual species maintain viable populations from year to year and provide vegetation cover on the coastal beach.