Short Term Dynamics of Vegetation Change Across a Mangrove – Marsh Ecotone in the South-west Coastal Everglades: Storms, Sea-level, Fire and Freeze
Christa L. Walker
Dyncorp Systems & Solutions LLC, USGS Everglades Field Station, Homestead, FL
Thomas J. Smith III
U.S. Geological Survey, Center for Water & Restoration Studies, St. Petersburg, FL
Kevin R.T. Whelan
U.S. Geological Survey, Center for Water & Restoration Studies, Miami, FL
Understanding the potential impacts of Global Climate Change, such as sea-level rise, on the newly initiated Comprehensive Everglades Restoration Plan (CERP) are crucial to its’ success. The position of the mangrove – marsh ecotone (MME) is known to have changed through time at several locations in coastal Everglades and Florida Keys. Shifts in the upstream position of the MME have been suggested as a means of monitoring sea-level impacts and to serve as an indicator of global climate change in places such as Australia and Southeast Asia.
The ecology of a MME has been examined in detail for over seven years. The site is located along the Harney River, in Everglades National Park. We established a transect across the MME which spans a distance of >350m, running from a tall mangrove forest at the river’s bank, into a sawgrass dominated plain. Five sediment porewater sampling sites are located along the transect. At each porewater site there are six porewater sippers, three at 30cm depth and three at 60cm depth. These sippers were sampled weekly for the first year (1997) and then bi-weekly thereafter. Porewater for nutrient analyses was collected at infrequent intervals. Permanent plots for measuring changes in the mangrove forest canopy and in the abundance of mangrove seedlings were established in 1998 and have been sampled yearly. An experiment was established to examine the impacts of a marsh fire on the subsequent establishment and growth of mangrove seedlings. Change in the position of this ecotone since 1927 has been determined by the analysis of aerial photographs.
The river’s edge is dominated by tall mangrove forests, particularly Rhizophora mangle and Laguncularia racemosa. Avicennia germinans is present in low numbers over the first 200m of the transect (fig.1). Forest height and average stem size decrease inland. Both Rhizophora and Avicennia disappear from the inland areas of the transect, leaving Laguncularia as the only mangrove species where the forest gives way to a marsh dominated by sawgrass (Cladium jamaicense).
Long term change in the position of the ecotone has occurred. In 1927 the MME was approximately 100m from the river bank. By 1994, the MME was 350m from the river bank, an inland shift of approximately 250m, a distance readily measurable on the photographs. Stumps of the Cabbage Palm (Sabal palmetto) can be found inside of the current mangrove forest and provides strong evidence for a change in the vegetation. Our physical data indicate that this transect, located on a large coastal island, is disconnected from upstream hydrologic signals. We thus feel that the movement of this ecotone over the past 70 years is related to a rise in sea level.
Gross sediment nutrient concentrations vary significantly from river edge to interior marsh. Total nitrogen increases from just over 1ppm (mg per gm wet sediment) in the riverine mangrove forest to more than 2ppm in the sawgrass. The trend for phosphorus is the reverse. Sediment P is highest in the riverine forest (1.2ppm) and least in the sawgrass (0.4ppm).
Tree mortality has been observed from a variety of sources over seven years including: wind (from both hurricanes and winter cold fronts), freezing temperatures, fire and lightning (fig. 2). Freezes and fires are more likely to affect stems at the ecotone. Both fires and freezes “top killed” numerous individuals of Laguncularia along the ecotone. This species stump sprouts readily. The frequent occurrence of fires and freezes may account to the large number of Laguncularia that are multi-stemmed.
In terms of fire impacts, the preliminary results from our seedling transplant experiment are interesting. We hypothesized that seedlings planted into a recently burned sawgrass marsh would have higher survival rates than ones planted under an unburned sawgrass canopy. For all three mangroves, however, the reverse was true. Individuals in the unburned marsh had greater survival. The largest effect, however, was among species. Avicennia, in both burned and unburned marshes, died rapidly, whereas 45% or more of both Laguncularia and Rhizophora survived for at least 200 days.
Results of this study indicate the mangrove – marsh ecotone is a dynamic component of landscape, capable of changing and being changed over relatively short time periods. Mangrove - marsh ecotones at more upstream locations in the Everglades ecosystem will be useful locations at which to monitor the effects of increasing freshwater inflow.
Contact: Walker, Christa L. Dyncorp Systems & Solutions LLE, c/o USGS, Beard Center, Everglades National Park, 40001 SR 9336, Homestead, FL 33034. Phone: 305-242-7800, FAX 305-242-7836, Email: POSTER, Ecology & Ecological Modeling
Figure 1. Stem density for the dominant species across the Harney River mangrove marsh ecotone transect (multiply Cladium by 100).
Figure 2. Causes of short term mortality along the Harney River transect. Storm related mortality is common near the river’s edge and in the interior forest. Freeze and fire are the major mortality factors at the mangrove – marsh ecotone.