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By Cathy Vogelsong Reprinted from The Understory, Feb.-Mar. 2004 Florida's wetlands, including both swamps (with trees) and marshes (treeless), are extremely complex and relatively poorly understood. They once covered half of the state, but drainage and development have reduced wetlands to about 10% of Florida's land area. Swamps are still distributed throughout the state, ranging in climate from subtropical to north temperate. One classification system subdivides Florida swamps based on four environmental variables: hydroperiod, frequency of fire, depth of organic matter accumulation, and the source of the water in the habitat. Much like pine flatwoods, freshwater swamps often exist in a mosaic of ecosystems. As stated in Myers in Ewel (Ecosystems of Florida), "these environmental variables affect swamps throughout the world, but the unique combination in Florida of high fire frequency, low topography, high surficial groundwater tables, and seepage from deep groundwater aquifers has produced a collage of wetlands that is unmatched in diversity." Hydroperiod is the major environmental feature and control in swamps. Hydroperiod is the amount of time each year that there is standing water, or soils are saturated. Roughly, there is an inverse relationship between length of hydroperiod and species diversity, though other environmental factors can change this. The longer the hydroperiod--less oxygen is available and minerals like soluble iron and manganese accumulate--the fewer the number of species that can tolerate the increasingly stressful conditions. To survive, plants use a number of special adaptations to acquire and conserve oxygen, including adventitious roots, aerial roots, cypress "knees," the ability to transport oxygen downward to the roots, specialized lenticels, and thick, leathery, evergreen leaves with waxy cuticles. Flared trunk bases called buttresses help to stabilize trees in the soft ground. Still, the most flood-tolerant conifer (cypress) and hardwood (Nyssa) trees both require periods of drought for seed germination. Fires in swamps are crucial but occur infrequently, from once a decade to once a century. These slow-burning fires burn off accumulated litter and peat. This prevents the swamps from succeeding into mesic ecosystems, and limits the plant species. The source of water--from rain, shallow groundwater, deep aquifer seepage, and flowing rivers--greatly affects the nutrients in the swamp system. For example, the pond cypress in the cypress savannas of south Florida are a stunted form (called "dwarf" or "hat rack" cypress) chiefly because that ecosystem is dependent on nutrient-poor and frequently fluctuating, rainwater levels. Accumulation of litter is dependent upon the other three major variables above. Decomposition of the litter is low, especially in acid and near anaerobic conditions. Florida swamps can be divided into two major groups, river swamps and stillwater swamps, based upon their water regimes. This takes into account the source of water, flow rate, and hydroperiod. One-third of Florida swamps are river swamps, with most occurring in north Florida (temperate climate). These are the floodplain forests, divided into "whitewater" rivers that carry particulate matter and "blackwater" rivers carrying dissolved organic matter (tannic acid makes the water appear blackish). They have a short hydroperiod and a measurable flow rate at least part of the year, which increases dissolved oxygen in the water Vegetation includes the mast-producing oaks, which provide a major food source for animals. These factors make river swamps the most diverse and productive swamps in the state. Stillwater swamps have no perceptible water flow and most are fed by rainwater and shallow, acidic groundwater. A major exception to this is the hydric hammock, fed by deep groundwater seeping through limestone outcrops. Most stillwater swamps are flooded or saturated over six months a year. Plant diversity is low, with a single species sometimes dominating in areas with the longest periods of standing water. Cypress swamps, for example, occur where a depression meets a high water table above an impermeable clay layer. There are approximately 100 species of woody plants in Florida's swamps. Cypress are the most common and flood-tolerant wetland trees in Florida, and dominate in areas with fluctuating water levels. They are slow-growing, long-lived and deciduous. The bald cypress (Taxodium distichum) is found in flowing water systems or cypress strands; pond cypress (T. ascendens) dominates stillwater, low-nutrient sites, forming cypress domes. Evergreen conifers include slash and pond pines, southern red cedar (Juniperus silicicola, which has not recovered from overharvesting for the pencil industry) and Atlantic white cedar (chiefly Chamaecyparis thyoides). Cabbage palm is common and generally fire resistant. Black gum (Nyssa sylvatica) is a stillwater species; the closely related water tupelo (N. aquatica) is found in river swamps. Oaks are common in northern Florida. Sweet bay, loblolly bay and swamp bay dominate bay swamps. Red maple and pop ash are also common in some swamps. Melaleuca, an exotic invasive, now dominates some southern swampland. Because of the hydroperiod, understory vegetation in swamps may be sparse. Sweetspire, buttonbush and swamp dogwood are common deciduous shrubs. A number of shrubs in the Ericaceae family survive in poor acidic swamp soils by depending upon associated mycorrhiza fungi to get nutrients. Twenty-three species of vines and a large number of epiphytes (bromeliads, orchids, ferns) grow in the trees. Epiphyte diversity greatly increases in the subtropical zone. Animal diversity also depends upon the type of swamp habitat. Many animals found in swamps only spend part of their lives there, so neighboring habitat can be critical. The greatest diversity of wildlife in Florida is found around the edges of the river/floodplain swamps. The mast, fruit and other seed production, along with greater availability of nesting cavities and understory cover, attract birds and other small animals. So does the larger population of insects in the more edible canopy of river swamps. The association of these swamps with rivers greatly increases the number of fish species. Benthic invertebrates (water-bottom dwelling insects, mollusks, crustaceans) are typically the bottom of the fauna food-chain in swamps, and the high density of snails, clams and crayfish in river swamps supports many other animals. Frogs dominate the fauna of north Florida cypress swamps, because of the extremes in wet-dry cycles. In stillwater swamps with extended hydroperiods, frogs are few and ground-dwelling reptiles like turtles and snakes are rare. The sparse or nonexistent understory provides little in the way of food, nest-sites or cover. Fish diversity may be less in a stillwater swamp, but density may be great in deeper water as dry season concentrates populations--which provides important feeding sites for the water birds that concentrate in stillwater swamps. Large and endangered animals like the black bear and Florida panther with large territory requirements have been forced into swamps as more of their habitat elsewhere is destroyed. New conservation measures are now concerned with "closing the gaps" between protected tracts of land to allow pathways for these animals. Reference: Ecosystems of Florida, Ronald Myers and John Jewel, editors, Univ. Press of Florida, 1990. |
