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

evapotranspiration and sink-hol edrainage.Probably the major outputs of waterfrom hydri c hammocks are evapotranspirationand surface runoff.Ground-water recharge is unlikely tobe significant in most hydric hammocks;recharge is minimal or nonexistentwhere Tertiary limestone and,therefore, the Floridan aquifer, lieclose to or at the surface (Conover eta7. 1984). Surface water detained inhydric hammocks exits by sheet flowand small streams. Shall ow-lyingpermeable 1 imestone tends to restrictthe development of surface drainage,1 imi ting rivers in 1 imestone terrain,such as the Suwannee of northernFlorida, to few tributaries(Stringfield and Le Grand 1966). Thegulf coastal hammocks are transectedby small, poorly-defined streams anddrainageways. Discharge of hammockdrainage to receiving areas (e-g., theestuaries of the gulf coast) is probablyspread over time as well as area.The dense vegetation and flat topographyof hydric hammocks probably slowsthe movement of water and attenuatespeak storm flows.2.5 PHYSICAL CONTROL OF DISTRIBUTION OFHYDRIC HAMMOCKSA unique combination of physicalconditions is required for the existenceof hydric hammocks. It includesflat terrain, the influence of limestone,sandy or sandy clay soil, ahigh water table, and occasional-toseasonalinundation by a mixture ofrainfall, seepage, and, sometimes,river overflow. Southeastern statesbesides Florida 1 ack the configurationof physical features needed for thedevelopment of hydric hammocks. Theflat topography of much of peninsularFlorida is rep1 aced northward by morevaried and hilly terrain. Except inparts of South Carolina's coastalplain, limestone bedrock is buried farbeneath the surface (Stringfield andLe Grand 1966). Alluvial rivers originatein the mountains and Piedmontand carry high loads of sediment tothe coastal plain. The sediments aredeposited in extensive floodplains,providing substrate for bottomlandhardwood forests, the predominant typeof foresled w~t:~iLd {;;;&J ~ V I I el ai.1982).Hydric hammocks are scarce inFlorida's panhandle west of St. Marksfor the same reasons that they arerare outside of the State. Most ofFlorida's large river systems andfloodplains, and all of the alluvialrivers, are concentrated in that region(Wharton et al. 1977). Tributarycreeks often O~taFy deep, i3drrCWravines that dissect the hilly landscapeof the highlands (Clewell 1981).In the flatter coastal lowlands, limestonedips below the surface just westof St. Marks (Figure 5).The extent and distribution of hydrichammocks within peninsularFlorida are greatly influenced by topographyand physiography. The1 argest stands, the gulf coastal hammocks,inhabit ocean-smoothed terraceswith limestone and the water tableclose to the surface. Narrower bandsof hydric hammock commonly occur betweenmixed hardwood swamp and uplandforest in northern and central Florida(Figure 14a). Where the slope between1 owl and and up1 and increases, hydri chammock thins or disappears (Figure14b). On the west bank of the upperSt. Johns River, a swath of hydrichammock about 50 km long and averaging1-2 km wide abuts freshwater marsh(Figures 15, 16). In contrast, on theeast side cf the river, hydric hammockoccurs only in small patches amongmarsh and pine fl atwoods communities.The soil types are similar, if not thesame (U.S. Soil Conservation Service1960, 1974), and the elevations of thehammocks with respect to the St. JohnsRiver suggest that the frequency ofriver flooding is alike. One differencethat may account for the more extensivehydric-hammock forest on thewest bank of the river is the largenumber of tributary streams on thatside (Figures 15, 16). We speculatethat the streams, and their riverswamps, promote the establ i shment ofhydric hammocks via their effects on1 ocal hydro1 ogy and the containment of