Salt Marsh

Salt and brackish marshes are found in all southeastern states with marine coasts. They are defined by periodic tidal inundation, dominance of salt-tolerant herbaceous plants and a rate of net primary production that is among the highest of all the world’s ecosystems. Coastal marshes support a rich fauna, and are important habitat for coastal birds, fur-bearing species, and as a nursery for some species of marine fishes. In recent years, there has been a special interest in conservation of seaside sparrows and other species of special concern.

Although little documentation exists on the natural fire regime in salt and brackish coastal marshes, lightning-ignited fires in other communities were likely to spread into the coastal marshes during the growing-season. There is also a long history of human ignition of fire in southeastern coastal marshes. People often burned these sites to promote wildlife resources (especially for waterfowl), increase forage for cattle and enhance ease of access for hunting and trapping. 

To date, almost all research on fire effects in coastal marsh habitats has focused on burns during the dormant season. Although effect of dormant season fire on marsh vegetation appears to be short-lived, it remains unclear how growing season burns affect marsh vegetation. Growing-season burns may alter vegetation structure and/or availability of insects compared to the results of dormant season fires. These resources may be important for nesting success of many marsh birds.

Prescribed fire is a common management tool used in coastal marsh ecosystems of the southeastern U.S. It is often used as a cost-effective way to reduce fuel and, when coupled with impoundments, is viewed as an important tool for coastal waterfowl. Although research has explored the effects of season of burn and fire frequency in adjoining upland ecosystems and, under some conditions, has determined advantages of varying fire regimes, these topics remain largely unstudied in coastal marsh areas of the Southeast. Agencies continue to use mostly dormant-season burns in coastal marshes, in part due to concerns about potential negative effects on wildlife, especially seaside sparrows. Due to the lack of research on season and frequency of burn, large gaps still exist in the scientific knowledge required to develop and implement fire management programs in coastal marshes of the southeast United States.

Salt and brackish marshes are found in all southeastern states with marine coasts.  The species composition and biology of Southeastern coastal marshes, especially in regards to plants, may vary geographically, however the basic physical conditions that define the ecosystems share similar characteristics across the region.  In general, they are defined by periodic tidal inundation, dominance of salt-tolerant herbaceous plants and a rate of net primary production that is among the highest of all the world’s ecosystems.  Recorded salinity varies widely, from 1 to over 50 ppt in Louisiana salt marshes.  Net primary production (above-ground plant material) may also be highly variable.  Although production has been estimated in only a few Florida salt marshes, the range spans from 130 to 2500 grams dry mass per square meter per year (g m ^–2 yr ^-1).

At some sites, Southeastern coastal marshes are dependent on burns to reduce woody vegetation such as saltbush, (Baccharis halimifolia) and wax myrtle (Myrica cerifera) (Wade 1991), however at other sites high salinity may be sufficient to preclude most woody plants.  On the Atlantic coast, where tides and wave action are often strong, marshes occur primarily in protected areas associated with barrier islands.  Along the Gulf of Mexico, where wave energy is generally low, marshes often occur fronting the ocean.  In Louisiana, the Mississippi Delta adds complexity to coastal marsh classification.  In that state, large areas of marsh may span gradients from salt water to brackish and intermediate marshes before finally ending in freshwater marshes.  A gradation from high salinity (>30 ppt), to brackish to freshwater is common in many areas (Figure 1 in Odum et al. 1984).  This gradation is correlated with many differences in species composition and ecological processes (Odum 1988). 

Vegetation

Depending on site location and physical characteristics (especially salinity), the dominant plant species may include cordgrass (Spartina spp.), rush (Scirpus spp.), needlerush (Juncus) and/or saltgrass (Distichlis spicta) (Gosselink 1984, Odum et al. 1984, Stout 1984, Wiegert and Freeman 1990).  Species composition is determined, in part, by geography and local elevation that, in turn, influences length of time of inundation and salinity.  Across the region, sub-dominant plant species often include saltgrass and saltwort (Batis maritime).  Coastal marshes east of Lousiana are almost exclusively dominated by black needlerush (J. roemerianus).  Along the Atlantic coast, as far north as North Carolina, there is a range of salt marsh vegetation that may be dominated by smooth cordgrass (Spartina alterniflora) or black needlerush.  In the delta coastal marshes of Louisiana, smooth cordgrass and saltmarsh cordgrass (S. patens) may dominate a saline marsh, with black needlerush and salt grass as sub-dominants.  In brackish delta marshes, saltmarsh cordgrass is dominant and the saltmarsh dominant plants listed above often represent sub-dominants.  The intermediate delta marshes are also dominated by saltmarsh cordgrass but the sub-dominants may vary. (Gosselink 1984).

Although, by definition, salt marshes are largely devoid of woody vegetation, brackish sites often include shrubs such as saltbush (Baccharis halmifolia).  Along the upper (high elevation) edges of marsh areas there may be scattered individuals of other woody species, including marsh elder (Iva frutescens).  In general, salt marsh algae are an order of magnitude less productive than salt marsh vascular plants and microalgae may be important nutritionally for many resident invertebrate (Montague and Wiegert 1990). 

Although, the vegetation structure of salt marshes is very similar, there can be distinct differences in vegetation composition.  Correct identification of dominant plants may be important for effective evaluation of fire effects.  

Mangrove swamps, the other common coastal ecosystem in the southeast, is generally limited to southern Florida.  The transition between salt marshes and mangrove swamps is at approximately 30⁰ N.  North of this point, winters are usually too cold for mangroves. 

Animals

There are numerous animal species (both aquatic and terrestrial) associated with salt and brackish marshes (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  Invertebrates include various species of crabs and mollusks and many species of terrestrial arthropods; grasshoppers and plant hoppers are common herbivores.  Although no species is usually locally abundant, invertebrates may be a species- rich group in salt marshes.  In North Carolina, a smooth cordgrass marsh supports more than 109 species of herbivorous insects (Wiegert and Freeman 1990).

Native species of amphibians are rarely found in association with salt or brackish marshes, however salt-tolerant leopard frogs have been collected in Florida (references in Montague and Wiegert 1990).  In addition, the introduced marine toad (Bufo marinus) is sometimes found in association with salt marshes of South Florida.

Several reptile species, including the American alligator (Alligator mississippiensis), use salt and brackish marshes but are not restricted to them.  In addition there are a few coastal marsh reptile species that are of conservation concern.  Subspecies of the secretive salt marsh snake (Nerodia fasciata) are endangered, as is the diamondback terrapin (Malaclemys terrapin). 

Salt marshes also provide habitat for fur-bearing species (raccoon, mink, muskrat and, the introduced nutria) and other small mammals such as rice rats (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  However, in many states, the economic value of furbearers is now of less significance than in the past, when small mammal trapping was more common.  In Atlantic coastal marshes dominated by smooth cordgrass, the rice rat is the only year round resident mammal species.  

Coastal marshes provide important habitat for many types of waterfowl and wading birds, especially species of herons and egrets (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  In addition, subspecies of seaside sparrows are of concern over much of the coastal southeast.  Habitat loss, due in part to permanent impoundment, could help to explain declining populations but marsh burning has also been a focus of concern. Unfortunately, there are scant data on seaside sparrows prior to their decline.  Other secretive marsh birds, especially species of rails and wrens, are also in need of study.

There are many sources of information for general accounts of these ecosystems, as well as species lists and summaries of many aspects of natural history.  The delta marshes of Louisiana are reviewed by Gosselink (1984). Stout (1984) profiled related ecosystems adjoining the northern Gulf of Mexico, from the Pearl River in Mississippi to Cedar Key in Florida.  Wiegert and Freeman (1990) did the same for Atlantic coastal marshes from North Carolina to Florida.  Additional information on Florida sites is contained in Montague and Wiegert (1990).  Chabreck (1972) provided a detailed description of broad marsh patterns in Louisiana and Wharton (1977) is often cited for succinct descriptions of the natural ecosystems of Georgia, including coastal marshes.

Little documentation exists on the natural fire regime (pre-European settlement) in salt and brackish coastal marshes of the southeastern United States. There is also little information on whether native peoples deliberately burned these areas. If humans did not specifically ignite coastal marshes then it is likely that any fire that occurred prior to European settlement originated in the uplands. Native peoples could have set such fire in any month of a year however lightning-ignited fires are most likely to have occurred during the growing-season when dry thunderstorms and extreme temperatures are most common. Although not confirmed with data, it is probable that lightning ignited fires from nearby upland pine forests occasionally spread into adjoining coastal marsh areas.  In addition, there is a report of spontaneous combustion creating a fire in a Louisiana coastal marsh in early August during an extreme drought (Viosca 1931). The significance of fire in different marsh types is related to dominant plant species as well as geographic position and local weather conditions.

Recent Anthropogenic Use of Fire

There is a long history of human ignition of fire in southeastern coastal marshes (reviewed by Chabreck 1981).  Trappers who supplied local fur industries often used fire as a management tool and, on a limited scale, the practice continues today.  Stout (1984) reported that trappers use frequent fires to increase ease of access and because of their belief that burns increase food for preferred pelt mammals.  Animal trails may be more noticeable after a burn (Gosselink 1984) and that at one time, burning was widely used to attract geese (Zontek 1966).  However nutria and raccoon often move from burned areas because of lack of adequate cover (Gosselink 1984).  Hoffpauir (1968) suggested that burning in Spartina patens stands encourages desirable food for ducks and muskrats.  Potential wildlife food plants such as S. olneyi and annual grasses may be excluded from the thick layer of dead vegetation that can build up in unburned sites.  Chabreck (1972) also agrees that using fire to remove heavy thatch may be useful for encouraging germination and enhancing growth of annual plant species.  However, there has been an ongoing debate over the value of marsh burning to enhance resources for waterfowl, especially if there is no ability to periodically flood the site (Chabreck et al. 1989, Nyman and Chabreck 1995).

Agency use of prescribed fire in Southeastern coastal marshes began early in the middle of the 20th century.  During the winter of 1940-1941, St. Marks National Wildlife Refuge conducted an experimental 120 acre burn in coastal marshland (Zontek 1966).  Prescribed fire has been accepted as a cost-effective way to reduce fuel as a means of minimizing wildfire occurrence and perhaps severity (Wade 1991).  Although research and observations demonstrated that growing-season fire triggered or enhanced reproduction of important plant species and, at the same time, promoted control of invasive hardwoods, broad-scale agency use of growing-season fire was not seen until the last 1-2 decades. 

Understanding of effects of fire in coastal salt marshes lags significantly behind knowledge of fire ecology of inland areas. Despite the knowledge that lightning-ignited fires occur almost exclusively during the growing season in coastal marshes (Lynch 1941) as well as the upland pine forests that surround them (Robbins and Myers 1992), virtually all prescribed burning in coastal marshes has occurred during the dormant season in recent history. Reasons for this include:

1. the fact that some dominant coastal marsh plants such as Needle rush may only burn under extreme conditions in the growing season.
2. fall and winter burning promote annual food plants used by fur-bearing species (e.g., muskrats) and waterfowl (Nyman and Chabreck 1995);
3. fears that growing-season burns will destroy bird nests; and
4. concerns that spring or summer fires will disrupt marsh processes.

To date, almost all research on fire effects in coastal marsh habitats has focused on burns during the dormant season (reviewed by Nyman and Chabreck 1995). The only published information on growing-season burns in salt marshes of the southeastern U.S. focuses on fire as a remediation tool for oil spills (reviewed by Mendelssohn et al. 1996). While dormant-season fire has been shown to improve both the quality and availability of forage habitat for resident and migratory waterfowl and fur-bearing species, effects on non-game species are poorly understood. In addition, no data are available to support the frequently stated belief that growing-season fires are almost always detrimental to salt marsh ecosystems.

See: Fire and coastal marsh birds and Fire effects on marsh vegetation

Although many areas of coastal salt marshes in the southeastern United States have declined in recent decades (reviewed in Noss et al. 1995), they continue to serve as critical habitat for many secretive marsh birds of special concern.  Breeding species of these birds include Clapper Rail (Rallus longirostris), perhaps Black Rail (Laterallus jamaicensis) and various subspecies of Seaside Sparrows (Ammodramus maritimus; Post and Greenlaw 1994).  Wintering species include Nelson’s Sharp-tailed Sparrow (Ammodramus caudacutus) , Yellow Rail (Coturnicops noveboracensis), Marsh Wren (Cistothorus palustris), and Sedge Wren (C. platensis). 

Preliminary observations for the Black Rail indicate individuals often survive fire but that adults attempt to seek shelter in moist areas rather than flee (Legare et al. 1998).  This behavior means that some individuals experience mortality in hot fires that leave few unburned patches of vegetation.  Legare et al. (1998) recommend prescribing marsh fires that create a habitat mosaic of burned and unburned patches.

The Cape Sable seaside sparrow, A. m. mirabilis, is endangered, and the dusky seaside sparrow, A. m. nigrescens, is extinct.  At times, both lightning- and human-ignited fires have been labeled as harmful for seaside sparrows (reviewed in Gabrey and Afton 2000), although the birds are obligate residents of ecosystems thought to have burned frequently prior to European settlement (Frost 1995).  In other instances, prescribed burns have been credited with producing vegetation structure that is beneficial to these same birds (reviewed in Gabrey and Afton 2000).  In earlier literature, fire was thought to have at least a short-term negative effect on two Florida subspecies, A. m. nigrescens and A. m. mirabilis (Gabrey and Afton 2000).  When a population of A.m. nigrescens experienced two burns in the same winter, singing males were reduced by half during the following breeding season (Gabrey and Afton 2000).  Taylor (1983) noted that A. m. mirabilis was generally absent in marshes that were burned the previous winter.  Two burns in one season is an extreme situation and may have been related, in part, to excess fuel build-up.  In addition, when observations were extended beyond a single year, seaside sparrow use increased in the second year following a burn (Taylor 1983; Post and Greenlaw 1994).  These positive multiple-year results are similar to those reported for A. m. fisheri in Louisiana (Gabrey and Afton 2000). 

Recent work, suggests that periodic fire may not only be tolerated by some marsh birds but may be necessary for effective management.  Gabrey and Afton (2004) considered three bird species [Seaside Sparrows (Ammodramus maritimus), Red-winged Blackbirds (Agelaius phoeniceus), and Boat-tailed Grackles (Quiscalus major)] that accounted for > 85% of birds observed on their study sites in a southern Louisiana marsh.  The first breeding season after winter burn, Blackbirds and Grackles increased and Seaside Sparrows decreased relative to previous year.  However, the patterns were reversed in second year after the burn and there was no difference in the third year.  The researchers suggest that bird abundances related to percent dead vegetation and Spartina patens (Gabrey and Afton 2004).  Tucker and Robertson (2003) reported comparable responses for Henslow’s and Bachman’s sparrows in longleaf savannas, and he demonstrated that growing-season fires enhance habitat for these savanna bird species.

Information on the effects of burning on most secretive marsh birds is generally lacking, although the recent work on seaside sparrows has clarified some issues.  However, confounding factors, such as season and time since burn, have not been considered for most species and these are areas of study that should be pursued in the future, 

Fire and Arthropod Prey

Few data document the effects of fire on arthropods in coastal marshes.  It has been suggested, but not documented, that invertebrates in coastal marshes recover rapidly following burns (reviewed in Nyman and Chabreck 1995).  Although there are no data on growing-season coastal fires, such burns in nearby inland pine forests result in decreased arthropods (abundance and perhaps diversity) in areas burned in the early summer compared to areas burned in the dormant season.  However by late summer (August), at a time when nestlings of many bird species are fledging, pine forest plots burned in the growing season supported more arthropods than did sites burned in the dormant season (Hermann et al. 1998).

Currently fire effects on marsh habitat structure and function are poorly documented.  In a community profile for salt marshes of the northeastern Gulf of Mexico, Stout (1984) suggested that repeated marsh burns would result in community succession but also stated that the details of succession and impacts on marsh functioning are not known.  Almost two decades later, the necessary research still has not been done.  A few studies have shown that, following dormant-season burns, plant species composition is rarely modified and live biomass rapidly recovers to pre-burn levels, often within a year (reviewed in Mendelssohn et al. 1996).  However, other details about the removal of marsh biomass are lacking.  Although reduction of invasive shrubs (both natives and exotics) is often listed as a reason for marsh burns, prescribed fire’s effectiveness for producing this result is rarely quantified nor are measurements usually made of fuel (biomass) reduction.  This is especially true for marshes dominated by Juncus roemerianus.

Some studies have described the relationship between fire and changes in plant production (cf. Hackney and de la Cruz, 1981).  In general, past studies have indicated that burning increases plant production, but these results were based only on dormant-season burns.  It is not known how growing-season burns affect plant production.  Vertical accretion (formation of new marsh soil) has been deemed an important process, especially in areas where rising sea level threatens to drown marshes.  This concern may be more significant in Louisiana and Texas than in north Florida.  Penland and Ramsey (1990) estimated that relative sea-level rise in Louisiana averaged 1.04 cm/yr and 0.63 cm/yr in Texas but only 0.17 cm/yr in western Florida.]                              

Prescribed burning is a common and often-preferred management tool used in coastal marsh ecosystems of the southeastern U.S. The need to reduce fuel and so to minimize severe wildfires in southeastern coastal salt marshes has been recognized for over a century (reviewed by Nyman and Chabreck 1995). Even so, few studies have evaluated the effectiveness of different seasons and frequencies of burning. This is especially puzzling given that issues of fuel reduction and ecological effects related to season of burn are being studied more and more in inland Coastal Plain pine forests. However, most research and adaptive management to date in coastal salt marshes has been directed at comparing long-unburned areas with sites recently burned in the dormant season. In these ecosystems, dormant-season burning has been the standard technique for fuel reduction for decades despite the general acceptance that, prior to European settlement, fire in both Coastal Plain inland pine forests and coastal marshes occurred primarily in the growing season. Twenty-five years ago, Hoffpauer (1968) estimated that 300,000 to 400,000 ha of Louisiana coastal marsh were burned annually.  In recent years, USFWS Region IV has burned more than 70,000 ha of salt marsh annually during the dormant season. The U.S. Park Service, numerous state agencies, and some private owners manage additional land in a similar manner. 

The ecological significance of season of burn was not recognized until the last quarter of the century. Acceptance of alternative months for prescribed burning was probably related, in part, to documentation that such fires did little or no harm to resident animal species in the short term and in the long term created desirable habitat structure. Recent studies have also demonstrated that growing-season burns may increase the availability of insects, a resource important for nesting success of many bird species.

Although reintroduction of natural processes may be impractical, use of growing-season burns in Coastal Plains pine forests provides many significant benefits when compared to dormant-season fires. Some of the positive effects include:

1. increases in flowering and/or seed production of some plant species,
2. decreases in the number of stems of encroaching hardwoods,
3. shifts in vegetation structure for many animals, and
4. increases in numbers of acceptable burn days each year, giving managers more opportunities for fuel reduction. 

It is unknown if similar benefits would be realized by applying alternative types of prescribed fires to coastal salt marshes, but continued lack of knowledge may well hamper fuel reduction and conservation efforts for these ecosystems and associated species of special concern. 

There is very little published information on specific environmental conditions that result in successful prescribed burns in coastal salt and brackish marshes. Some managers have commented that low relative humidity, light wind and continuous dead litter are required before Juncus marsh will burn cleanly.  Zontek (1966) noted that in north Florida an accumulation of 3 or more years of fuel is necessary before a coastal marsh is expected to burn cleanly. Wade (1991) provides one of the few descriptions of fire weather for a fire in a southeastern coastal marsh.  For a September burn in Spartina bakerii on a barrier island off the coast of Southwestern Florida, he reported that conditions at time of ignition (10 AM) were:

• ambient temperature = 87⁰ F
• relative humidity = 70%
• wind (at ground level) = 2-3 mph
• fuel load (1 hr fuels) = 35,000 lbs/ac

Under these conditions, a backing fire did not carry but a head fire produced 25-foot flames and a successful burn. 

Despite the high ecological significance of southeastern salt marshes, federal and state land-holding agencies currently lack sufficient information on fire effects in these ecosystems to manage fuels effectively and simultaneously conserve species of special concern.  In recent years, both land managers and scientists have called for research on the effects of shifting season of burn in coastal marshes (Gabrey and Afton 2000, Gabrey and Afton 2004) and some have suggested that past landscape alteration may need to be considered (Schmalzer 1995, Wade 1991).  Targets for evaluation should include fire behavior, plus potential changes over time in fuel load, vegetation structure and composition.  Of special interest is the relative use of different types of burned area by subspecies of seaside sparrows and other secretive marsh birds and fire effects on potential food sources (arthropods and seeds) and nest sites.