The Ecology of Hydric Hammocks - USGS National Wetlands ...

cause they are tropical plants confinedmostly to south Flnrida- Noneoccur in abundance and only one, whitestopper, is a tree. Isolated populationsof this species occur north ofLake Okeechobee (Little 1978); one ispresent in hydric hammock in TosohatcheeState Reserve. The geographicaldistributions of several herbaceousplants common to hydric hammocks,including goldfoot fern andwild coffee, extend north from theCaribbean to central Florida.A striking example of the influenceof a species' range on the compositionof hydric hammock is cabbage palm, thepredominant tree species in many hydric hammocks. From North Carol i na tonorthern Florida and west across thepanhandle, this species is essentiallyrestricted to the coast (Figure 28).South of a line from about Cedar Keyon the gulf coast to St. Augustine onthe Atlantic, cabbage palm rangesFigure 28. Distribution of cabbage palm inFlorida (from Little 1978). Asterisks on the insetmap indicate isolated populations of cabbagepalm outside of its continuous range (from Brown1973).across Florida' s peni nsul a. North ofthis line, a few interior populationsare scattered a1 ong rivers. Hydrichammocks in the northern region, forexample in San Felasco Hammock StatePreserve (Tab1 e 5), 1 ack cabbage palm.The inland Gulf Hammock site that wesampled (Table 5) is situated at theedge of cabbage palm's coastal range.Cabbage palm was absent from much ofthe forest, but it was common, especiallyin the understory, in someparts.Numerous studies have demonstratedor inferred that patterns of vegetationin southern forested wetlands arestrongly influenced by flooding(Bedinger 1978; Huffman and Forsythe1981; Wharton et a7. 1982; Leitman eta7. 1983). Frequency, duration,depth, and timing of flooding are factorsthat affect the species compositionof a forested wetland. The effectsof flooding on these communitiesare mediated by physical and chemicalchanges in the soils and varying responsesto the alterations by plantspecies.The nature and extent of the physicaland chemical processes that followsoil inundation largely depend on theduration of submergence and on soilproperties (Ponnamperuma 1984). Whena soil is flooded or saturated, gasexchange between the soil and air isgreatly restricted. The slowing ofgas diffusion is particularly great insoils with high clay content. Oxygensupply to the soil is drastically cut,and roots and microorganisms depletethe oxygen in the soil water veryrapidly. Gases produced by soi 1metabol ism (e.g., carbon dioxide) accumulate.Anaerobic conditions inducea number of chemical changes in thesoil, many of which are detrimental toplant growth. Nitrate is replaced byammonium, often less preferred for uptakeand assimilation by plantspecies. Oxidized forms of iron, manganese,and sulfur are reduced to potentiallytoxic forms, including sulfides. Ethanol, another potent i a1toxin, is a by-product of anaerobicrespiration in most plant roots.