Fire Ecology and Management of Freshwater Marshes

Alachua Lake

Freshwater marshesin the southeastern coastal plain can be found inassociation with rivers and lakes or in basins or other depressions. Tidal freshwater marshes occur along the Atlantic and Gulf coasts in association with rivers just inland of salt and brackish marshes. Freshwater marshes are associated with a variety of plant communities from salt and fresh open water systems to a variety of upland communities. The climate of marshes is variable due to the broad distribution of marshes, but rainfall patterns affecting hydrology are important factors influencing marshes. Marsh soil varies from organic soils in long hydroperiod marshes to more mineral-based soils where hydroperiods are shorter. Sedimentation also affects the character of soils in some types of marsh. Though the hydrology of marshes is variable, periods of inundation sufficient to limit encroachment of many species of woody plants, relatively shallow water, and water level fluctuation are characteristic. Most non-tidal marshes experience fluctuation of water on the scale of seasons or longer. Alternatively, tidal marshes experience daily fluctuation in water levels associated with tides. Water quality of marshes depends on the water source of the marsh and productivity varies with nutrient status.

Vegetation of marshes varies spatially, and in some cases, seasonally. Differing hydroperiods are associated with different plant associations. Long hydroperiods are associated with submerged and floating leaf herbaceous plants, whereas short hydroperiods are associated with a more diverse assemblage of plants including many grasses. Plant zonation in some tidal marshes is related to water depth. Submerged and floating leaf herbaceous plants are found in deeper water where perennial herbs and grasses are found in more shallow water. Some tidal marshes undergo a seasonal succession of vegetation ranging from mudflats in winter and spring to rank growth in summer and early fall.

Fauna of freshwater marshes include members of most major groups. Invertebrates range from conspicuous members such as flies and mosquitoes to herbivores to inconspicuous detritivores. Food chains in marshes are thought to be detritial based. Fishes are also common in marshes. Species composition varies with type of marsh and population structure will vary with recent hydropatterns. Reptiles and amphibians also use marshes. Turtles and some species of frogs and snakes are particularly common. Marshes are used extensively by a variety of birds including several endangered birds. Mammals that use marshes include common mammals of other habitats and mammals that specialize in non-forested wetlands.

Despite evidence that fire occurred in nearly all southeastern freshwater marshes, fire regimes of freshwater marshes are not well understood. Some suggest that fires were frequent in marshes occurring every1-5, 3-5 or 3-10 years. While others suggest that extensive fires were uncommon and dependent on drought. Different types of marsh likely burned at different frequencies with the wettest marshes burning least frequently and drier types burning more frequently. The natural season of fire within marshes is also not well understood. Flammability of marshes depends on vegetation type and on season. Some types of plants are better fuels than others. Fall following a hard frost is the season in which most marsh types are most flammable, which does not coincide with the season in which most uplands burned. Flammability patterns pose questions regarding season, frequency and ignition source of “natural” fires in marshes. In the absence of fire, marshes are thought to succeed toward plant assemblages dominated by woody plants. Modern alterations to marshes such as drainage and impounding water have undoubtedly altered marsh fire regimes.

Fire may affect plant species composition of marshes. Fire is thought to limit invasion of some species of woody plants, but may only prune other more fire tolerant species. The role of fire in determining herbaceous species composition is uncertain. In some cases fire is credited with determining the dominance of one herbaceous species over another, while in other cases fire is credited with increasing diversity by decreasing dominance of one species. Conversely, several studies have documented little change in species composition or diversity in marshes following fire. Fires are thought to be important in marshes for limiting peat accumulation. The availability of nutrients following marsh fires is variable and may depend on the type of fire (surface vs. peat fires).

The effects of fire on animals are less well studied. Fires can cause direct mortality of animals and can change vegetation in ways that alter animal habitat and thus animal species composition. The effect of fire on several listed species is the subject of needed research.

Prescribed fire has been used in marshes to control woody vegetation, and to manage fuel loads, to aid in the control of invasive exotic plants, for management of wildlife and for ecosystem management. Fire is used as a management tool despite a lack of understanding of natural fire frequencies or seasons. Though drought regime fire likely played an important role in the maintenance of marshes under the natural fire regime, drought regime fire is avoided by managers due to smoke management and fire control issues. Thus, the effects of prescribed fire may be different than that of natural fire. Using prescribed fire under altered hydrologic regimes, common in many marshes today, likely results in the effects of fire being different than they were prior to hydrologic alteration.