Marsh Seed Banks of the Okefenokee Swamp: Effects of Hydrologic Regime and Nutrients

Abstract

Seedling germination from the seed banks of two marshes in the Okenfenokee Swamp, a deep macrophyte marsh and a shallow sedge marsh, was examined in the greenhouse under three hydrologic regimes and four nutrient conditions. Experimental hydrologic conditions of 3—mo duration, were constant exposure (no standing water), 1—mo exposure followed by reinundation, and constant inundation, repeated four times in 1 yr; nutrient conditions were amendments of N, P, N and P, and a control. The macrophyte marsh, whose water level fluctuates irregularly, has three guilds of species adapted to germination under different hydrologic conditions: drought—adapted, such as Rhyschospora inundata, which germinates in dry (exposed) conditions; inundation—adapted, such as Nymphaea odorata, which germinates only under water; and generalists, such as Eriocaulon compressum, which germinates in both exposed and inundated conditions. The relative abundances of these guilds fluctuate with the frequency and duration of drawdown in the habitat. Rhychospora inundata is a drought—adapted fugitive species in macrophyte marshes of the Okefenokee Swamp. It has a large population of seeds in the sediment, which germinate during a drought. Reinundation stimulates flowering and fruiting of adult plants. The sedge marsh, whose water level fluctuates regularly, has a single guild of species, dominated by Carex wateriana, which will not germinate in standing water. Nutrient content seedlings was parallel to the nutrients available. Nitrogen appears to limit marsh plant growth during drawdown, while phosphorus appears to limit a few species during inundation. As a result, most plants grow more rapidly in standing water, especially those that require drawdown for germination. The result is that this ombrotrophic, nutrient—poor wetland becomes more "lake—like," or P limited, during inundation, and more "land—like," or N limited, during low water levels.