Fire Regime of Freshwater Marshes
Fire regimes of freshwater marshes are not well studied. There is evidence that fire occurred in nearly all southeastern freshwater marshes in the form of fire char on woody vegetation (Frost 1995). However, the natural frequency of fires is not well understood or agreed upon. At one extreme it has been suggested that fire frequency in marshes is low; numerous small fires occur relatively frequently in marshes but extensive fires that burn large areas occur infrequently (Lee et al. 1995, Snyder 1991). Many others feel that fire in marshes was frequent. Reported fire frequencies for marshes include 1-5 years for floodplain marshes in Florida (FNAI 1990), 1 to 6 years for strictly herbaceous marshes (Frost 1995), 1 to 3 years for strictly herbaceous basin marshes and 3-10 years for marshes with willow and button bush (FNAI 1990). A frequency of 3 – 5 years has been cited for freshwater tidal marshes adjacent to upland communities that support frequent fire, whereas areas that are isolated from uplands by channels or other barriers to fire would seldom if ever burn (Schafale and Weakley 1990 and references therein).
The frequency of fire in a marsh is related to the hydroperiod of the marsh (Kushlan 1990). Long hydroperiod marshes carry fire less frequently than short hydroperiod marshes. The fire frequency of short hydroperiod marshes, i.e., wet prairie has been cited as 1-5 and 2-4 years (Duever 1987 and FNAI 1990 respectively). See also Everglades for a discussion of fire frequencies of sawgrass marsh. Long hydroperiod marshes are thought to burn infrequently, only under drought conditions (Kushlan 1990, FNAI 1990).
The natural season of fire within marshes is also not well understood. At the southern end of marsh distribution, i.e., south Florida, there are distinct wet and dry seasons and fires are thought to have naturally occurred in the transition from dry to wet seasons (See Everglades fire regime). As one progresses northward, this pattern becomes less clear as rainfall becomes more evenly distributed throughout the year and drawdowns become less seasonally predictable. In many marshes there are patterns of vegetation dominance that vary spatially and seasonally with some vegetation types being better fuels than others. For instance broad leaved forbs such as arrowhead, alligator weed, pickerelweed, and green arrow arum are poor fuels, whereas grasses such as maidencane and switchgrass (Panicum virgatum) are better fuels, i.e., more likely to propagate fire (Wade et al. 2000). In coastal tidal marshes seasonal succession from broad leaved forbs in spring and early summer to grasses in mid-summer to fall, suggests that these marshes are least flammable in the early growing season. When forbs are dominant in marshes, propagation of fire is only successful after tissues have senesced (frost killed, drought stresses) (Wade et al. 2000).
In years of normal rainfall, basin marsh vegetation will not burn during the growing season (FLDEP 2002). Intermediate and long hydroperiod marshes are most flammable in fall and winter after hard frosts (James Weimer, personal communication). In fact, under normal conditions, This period of relative flammability may carry over, in a decreased capacity due to fuel compaction (the beginning stages of decomposition), into the onset of the growing season in marshes that do not experience tidal flushing that removes dead biomass (James Weimer, personal communication). It seems reasonable to assume that natural fire season may have been affected by seasonal changes in vegetation dominance and flammability that we observe today.
Under normal rainfall, peak flammability in marshes does not coincide with the season in which most uplands burned. This poses the question of how frequently marshes burned naturally if ignition was dependent on lightning-ignited fire spreading from fire-type uplands in the growing season when marshes are naturally less flammable. Given flammability patterns, natural lightning-ignited fires may have been extensive in marshes only under drought conditions. Another hypothesis, assuming that frequent fires maintain marshes rather than periodic severe fires, is that anthropogenic ignitions were the source of fire in marshes. Very little is understood about the Native American role in burning of marshes. However, large populations of Native Americans are known from areas adjacent to large basin marshes in Florida (FLDEP 2002).
Fire season in short hydroperiod marshes (wet prairie) may be more closely related to the fire season in surrounding uplands. In south Florida, there is a pronounced flowering response by dominant wet prairie grasses (Muhlenbergia capillaris, Paspalum monostachyum, and Schizachyrium rhizomatum) to early growing season fire (Main et al. 2002). However, in south Florida marsh fires are thought to have occurred in the early growing season (See Everglades).
Fire-driven succession in marshes is poorly documented (Wade et al. 2000), but numerous authors suggest that marshes succeed toward assemblages dominated by woody plants in the absence of fire (Frost 1995, FNAI 1990). Severe fires are thought to be important in marshes for maintenance of deeper water habitats and to prevent succession toward bog (FNAI 1990).
Fuel loads, like productivity are variable in marshes. Fuel load in a cordgrass (Spartina bakerii) marsh was approx 17 tons/acre (Wade 1991). Fuel loads of 4-18 tons / acre have been cited for coastal marshes (Wade et al. 2000).
Alterations to Natural Fire Regimes of Marshes
Many marshes have been extensively altered. Ditching and diking and a history of grazing and conversion to agriculture are common to many marshes (Kushlan 1990, Gordon et al. 1989). Ditching and diking practices have profoundly altered hydrologic regimes. Many marshes have become drier (Wade 1991) while others experience salt water intrusion or are managed as impoundments (Gordon et al. 1989, Chabreck et al. 1989). In some cases, as a result of hydrologic changes, burning by hunters and vandals has increased fire frequency (Miller et al. 1998). In addition to increased frequency, season of fires may become altered, particularly in modified marshes (Miller et al. 1998, Lee et al. 1995). Marshes in the urban interface may be particularly likely to experience changes in fire season or frequency because water is often diverted from these marshes for flood control purposes and the incidence of accidental or arson ignition is increased (Lockwood et al. 2003). Woody plant encroachment has occurred in other cases (Miller et al. 1998), and is often presumed to be due to decreased fire frequencies (See fire effects). Many marshes have experienced eutrophication as runoff from agricultural sources enters marshes. The effects of fires may be different under altered hydrologic and nutrient regimes (see below). Due to the extensive alteration of marshes, fire likely doesn’t play the same role now as prior to marsh modification (Miller et al. 1998).
Woody plants in coastal marshes affect fire propagation. Woody plants can decrease rate of fire spread, and decrease fire coverage (Wade et al. 2000). A particular problem is where invasive plants such as Chinese tallow invade marshes and stands act as firebreaks (Grace 1998) thus altering the fire regime.
Some marshes, particularly along the Atlantic coast, are intensively managed for waterfowl. A system of ditches and dikes with water control structures are used to manage water levels. Fires are often prescribed in fall after drawdown and a killing frost to promote certain plants as waterfowl foods (Gordon et al. 1989).
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