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Consequences of Fire and Freeze on a Mangrove-Marsh Ecotone

Christa L. Walker1, Thomas J. Smith III2, and Kevin Whelan3


Everglades National Park - Harney River
photo of southern florida showing study area
map showing transact one
(top) Map showing study area. Yellow box shows area of map below it. (bottom) Yellow line shows location of transact 1. Click on images for larger versions.
We are studying the episodic effects of fire and freeze on a mangrove - marsh ecotone on the Harney River in Everglades National Park. Based upon aerial photographs of this region, we know that this ecotone has migrated significantly. Sea level rise, upstream water management, drought, fire, and freeze are factors that likely influence the position of this ecotone. We established a transect across this ecotone that begins in coastal marsh, dominated by sawgrass (Cladium jamaicense) and terminates in mixed mangrove forest (Rhizophora mangle, Laguncularia racemosa, and Avicennia germinans, Conocarpus erectus). Plots were established in which all stems are being monitored to determine the changes in stem density and biomass after both fire and freeze events. After a fire occurred in the marsh in September 2000, we found that this ecotone shifted 20m towards the river. Then a freeze event in January 2001 pushed it another 20m towards the river. Both events produced a temporary expansion of the coastal marsh zone. The first freeze event in this area that we observed was January 1997 and "top killed" many of the Laguncularia. Laguncularia have the ability to resprout via reserve meristems. Based upon data from other permanent vegetation plots in this zone, we expect to have a recovery of approximately 80% of the biomass two years post freeze.


photo of a forest fire aerial photo of everglades national park photo of an aerial shot of the study area photo of transact from marsh zone to mangrove zone
Photo of forest fire.
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Aerial shot of the Everglades.
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Aerial picture of study area. [larger image] View of transact from marsh zone to mangrove zone. [larger image]


  • The objective of this study is to determine the sensitivity of vegetation to fire and freeze in this transition zone where coastal marsh adjoins mangrove forest.
  • Mangroves are almost exclusively tropical, which suggests a limitation by temperature (Hogarth 1999). Climatic factors are important determinants of species richness, stand structures, and growth dynamics in mangrove forests (Smith 1992). Freeze events seem to be a significant factor in shaping the position of the marsh-mangrove ecotone (figure 3).
  • Typically fire does not have a role in mangrove dynamics. But in Florida, mangroves are usually adjacent to coastal marshes, which readily burn. Damage from freeze has been shown to be greater in areas that have burned recently (Olmsted, Dunevitz, and Platt 1993). Factors such as hydrological regime, drought, dead wood created by hurricanes and freeze, and season of burn have a significant effect on how far into the mangrove zone these fires penetrate.
  • Along the transect, locations of the macrohabitats is based upon percent cover of each of the dominant species.
  • The ecotone is being defined where the occurrence of white mangrove (Laguncularia racemosa) and sawgrass (Cladium jamaicensis) is balanced.
  • The marsh zone is dominated by Cladium, with low densities of Fimbristylous spadicea, Blechnum serrulatum, Ipomoea saggitata, Rhabdadenia bicolor, and Toxicodendron radicans.
  • The mangrove zone is a mixture of Laguncularia, Conocarpus erectus, and scattered occurrence of Schinus terebinthifolius. Hippocratea volubilis, Randia aculeata, Acrostichum aureum, Rhabdadenia, Fimbristylous and epiphytic species such as Encyclia tampensis and Tillandsia spp. are also found in low densities.


  • Beginning in the coastal marsh zone, a 90m belt transect was established which terminates in mangrove forest. PVC poles were set every 10m, creating nine contiguous 10 X 10 plots. Center poles were established in each 10 X 10, and two 5 X 5 subplots chosen from each.
  • All stems occurring in each chosen 5 x 5 subplot were permanently tagged, measured for Diameter at Breast Height (DBH), compass bearing, and distance from center stake recorded.
  • Each stem was assessed a condition code based upon field observations: mortality from fire, mortality from freeze, damage from fire, damage from freeze, and no damage.
  • In each subplot, the % occurrence of Cladium and all species occurring within each plot was recorded.
  • Fire and weather data obtained from Everglades National Park Fire Management were incorporated into this study (Tables 1,2). Water level data obtained from USGS Global Climate Change Hydrological Monitoring Network.
Table 1. Freeze data January 2001 (temp given in °F)
Date Mean T Minimum T Wind Direction Wind Speed (MPH)
Jan 4 52 39 WNW 7
Jan 5 47 31 WNW 4
Jan 6 58 44 WSW 6

Table 2. Fire/Water depth September 2000
Date Mean T Wind Direction Wind Speed (MPH) Water Depth (ft)
Sept 13 88 ENE 4 .5

Table 3. Damage assessment (%) along transect vs. overall
Plot # 1 2 3 4 5 6 7 8 9 Overall
% mortality fire 100% 100% 100% 42% 12%
% mortality freeze 58% 68% 44% 35%
% damage freeze 24% 33% 78% 35% 32%
% no damage 9% 23% 22% 65% 21%
Total # stems 0 1 9 38 78 105 283 104 71 689


Plot Photos photo of plot 2 marsh zone photo of plot 4 fire mortality
Plot 2 marsh zone.
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Plot 4 fire mortality.
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photo of plot 5 fire and freeze mortality and plot 6 freeze recovery photo of plot 7 with top killed by Laguncularia photo of plot 9 mangrove zone
Plot 5 fire and freeze mortality and plot 6 freeze recovery.
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Plot 7 with top killed by Laguncularia.
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Plot 9 mangrove zone.
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Results and Discussion

    plot showing number of stems through plots 1 through 9
    (Above) Figure 1. [larger image]
    plot showing shift of ecotone
    (Above) Figure 2. [larger image]
    plot showing mangrove mortality for storms and freezes
    (Above) Figure 3. [larger image]
  • Stems show no recovery of above ground biomass (with one exception) in plots that sustained fire, more than one year post-burn (plots 1-5). This stands in contrast to plots that were affected by freeze only which are producing many basal sprouts (plots 6-9). This recovery after freeze damage is consistent with the recovery we have found in other vegetation plots located in this zone and with results reported by Olmsted et al. (figures 1,2, table 3).
  • The architecture of damaged or top killed Laguncularia results from its ability to retain reserve meristems, which produce basal sprouts following disturbance. This differential recovery is dependent upon locations of the reserve buds located on the trunk. Stems without overstory trees were extensively top killed (plots 6,7). This pattern of damage along ecotones has also been reported in other habitats (Olmsted et al.).
  • Along the transect, the burn was of low intensity due to the presence of surface water and inadequate wind to push the fire into unburned fuel (table 2). Cladium recovery (height, culm density, cover) seems consistent with results reported by Forthman (1973). Prior to the burn, the amount of dead leaves surpassed live, which may have implications for the proliferation of Schinus. In plot 2, a Schinus that originally established on a Cladium clump has many basal sprouts and was not permanently killed by the fire. If the fire occurred after the freeze, it may have further penetrated the mangrove zone because of a higher fuel load created by the freeze.
  • Freezing temperatures, of short duration (~2 hours), were accompanied by low wind speeds and high relative humidity in the early morning hours on January 5 (table 1). We were on site the morning of this event and signs of freeze damage were evident. In addition, this vegetation may have been previously stressed by chilling injury on January 1 when temperatures remained between 34-450 F for ~9 hours.
  • On a landscape level, wetlands are ecotones between aquatic and terrestrial systems (Risser 1995) therefore eustatic sea level rise can be expected to have an impact on the dynamics of these areas. The transition of an area from mangrove to marsh, or vise versa, will depend on the response of the local hydrological conditions to changes in sea level and climate at the synoptic scale (Smith 1997).

Future Research

  • Recently set up a mangrove seedling establishment study in burned vs. unburned sawgrass.
  • Established vegetation plots in Cladium, Eleocharis marsh communities.
  • Put in new transects on Lostman's River and Avocado Creek.
  • Initiate work on different types of mangrove-marsh ecotones, e.g., mangrove-spikerush (Eleocharis), mangrove-needlerush (Juncus).


Forthman, C., 1973. The effects of prescribed burning on sawgrass, Cladium jamaicense Crantz, in South Florida. M.S. Thesis. University of Miami, Coral Gables, Florida.

Hogarth, P. J., 1999. The biology of mangroves. Oxford University Press, New York.

Olmsted, I., Dunevitz, H., Platt, W.J., 1993. Effects of freezes on tropical trees in Everglades National Park Florida, U.S.A. Tropical Ecology, 34 (1): 17-34.

Risser, P.G., 1995. The status of the science examining ecotones. BioScience, 45 (5): 318-325.

Smith, T.J. III, 1997. Hydrologic variation and ecological processes in the mangrove forests of south Florida. Project proposal for C.E.S.I. Everglades National Park. 1992. Forest Structure. Tropical Mangrove Ecosystems (eds. A.I. Robertsons and D.M. Alongi), pp. 101-136. American Geophysical Union, Washington D.C.


Financial support for this research was received from USGS Global Climate Change Program. Additional support came from Department of Interior's "Critical Ecosystems Studies Initiative" administered by Everglades National Park. Base funds were provided by USGS Florida Caribbean Science Center. We wish to thank: Diana Rodriguez, Rene Cerezo, Karina Becerra, Phil George, Salvador Martinez, Kevin Shipp, Thijs Sanderink, and Germaine Ploos for field assistance. For technical assistance we wish to also thank: Diane Riggs, Christian DeWitt, Dafna Reiner, Rick Struhar, Gina Hernandez, Gordon Anderson, Rich Kearns, Tori Foster, and Xavier Pagan. This poster was presented at the 16th Biennial Conference of the Estuarine Research Federation in St. Petersburg Beach, Florida November 2001.

1Johnson Controls World Services, U.S. Geological Survey, Florida Caribbean Science Center, Restoration Ecology Branch at Everglades Field Station, Everglades National Park, 40001 S.R. 9336, Homestead, Fla. 33034, christa_walker@usgs.gov

2U.S. Geological Survey, Florida Caribbean Science Center, Center for Coastal and Marine Studies, 600 Fourth Street South, St. Petersburg, Fla. 33701, tom_j_smith@usgs.gov

3Johnson Controls World Services, U.S. Geological Survey, Florida Caribbean Science Center, Restoration Ecology Branch, c/o SERC FIU University Park Campus, Miami, Fla. 33199, whelank@fiu.edu

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