Assessing the potential for competition on a coastal beach and the significance of variable seed mass in Triplasis purpurea

In annual plant communities of coastal ecosystems, the potential exists for both intraspecific and interspecific interactions. Two native annual grass es, Triplasis purpurea and Cenchrus tribuloides, commonly co-occur along th e shore of Staten Island, New York. T. purpurea matures variable seeds with in leaf sheaths along tillers: heavier, nondispersed seeds occur at lower n odes, while lighter, dispersible seeds occur at upper nodes. Interplant dis tances recorded in the held were very low between T. urpurea individuals (9 -28 mm), but were greater between T. purpurea and C. tribuloides. However, field collections late in the growing season revealed that the presence of two C. tribuloides neighbors within a 10 cm circle around a target T. purpu rea individual significantly reduced dry mass and seed output of the target . In an interspecific competition experiment in the greenhouse at a density of two plants per pot, one C. tribuloides significantly reduced the dry ma ss and reproduction of one T. purpurea. The T. purpurea individuals that em erged from lighter, upper-node seeds of parental plants had lower dry mass and seed production than those from heavier, lower-node seeds. The per unit mass effect of C. tribuloides on T. purpurea mass was highly significant f or T. purpurea from upper-node seeds. With its larger seeds and seedlings, C. tribuloides is clearly a superior competitor. Differences in the dispers ibility of seeds from upper and lower nodes of T. purpurea, and the subsequ ent size of the seedlings emerging from them, may favor variable seed mass in the competitive environment of this coastal system.