Seed bank composition along a phosphorus gradient in the northern Florida Everglades

Seed-bank samples were collected in the northern Everglades along a phosphorus gradient with three vegetation zones (cattail with the highest phosphorus levels, mixed cattail-sawgrass (transition), and sawgrass with the lowest phosphorus levels). The size and composition of the seed banks were estimated using both a direct seed separation (seed assay) and a seedling emergence technique (seedling assay). In the seedling assay, seed-bank samples were kept under two moisture regimes, saturated soil and shallowly flooded. In the seed assay, whole seed of 21 species were found, with a mean of about 8 species per sample in the cattail zone and about 5 in the transition and sawgrass zones. On the basis of their appearance, seeds of 18 species were judged to be intact, that is, probably viable. There were about 7 species per sample with intact seed in the cattail zone, and 3 and 4 in the transition and sawgrass zones, respectively. In the seedling assay, seedlings of only II species were found. All II were found in the saturated soil treatment, and 7 were also found in the shallowly flooded treatment. in the seed assay, the mean total number of whole and intact seeds in the cattail zone was estimated to be 78,400 and 44,400 seeds m(-2), respectively. The mean total number of whole and intact seeds in the transition zone was much lower, 14,200 and 10,500 seeds m(-2) respectively, and in the sawgrass zone, it was slightly higher at 20,900 and 14,700 seeds m(-2), respectively. In the seedling assay, seed densities were much lower and were estimated to be only 3,700, 800, and 1,300 seeds m(-2), respectively, in the cattail, transition, and sawgrass zones. The seedling assay results suggest that only about 1.3% of the intact seed had germinated. Overall, the composition of the seed banks of the transition and sawgrass zones were not significantly different from one another, but they were significantly different from that of the cattail zone. Seeds of several species were restricted primarily or exclusively to cattail-dominated areas, including seeds of Typha sp.