Fire Suppression Effects in a Coastal Floating Marsh Ecosystem

Makweche, Plaxedes; Paudel, Shishir; Looft, Jerrod; Schmale, Guy; Ernat, Darcy; Battaglia, Loretta

Mississippi Delta floating marshes are characterized by emergent vegetation rooted in buoyant mats of peat that rise and fall with changing water levels. Herbaceous species, most notably the dominant grasses and sedges, build and bind the mat. These marshes are threatened by sea level rise, as well as fire suppression that favors spread of woody species. The floating marshes at our study site, Jean lafitte National Historical Park and Preserve (JlNHPP), have not been burned since establishment of the park in 1974. Many areas formerly dominated by herbaceous marsh species now have dense shrub cover. We hypothesized that mat-bUilding herbaceous species have lower aboveground biomass on mats invaded by woody species compared to those with no woody cover. We also expected that root biomass and peat would be lower in shrub thickets compared to open herbaceous marsh. In Fall 2007, we sampled aboveground and belowground biomass, as well as the peat, in herbaceous and shrub-dominated communities. Biomass of herbaceous species was lower in shrub thickets than in herbaceous marshes. Root biomass did not differ, but mineral density in the mat was greater in shrub thickets, suggesting previous submergence events and possible reunion with the underlying inorganic substrate. Figure 1. A notant marsh is shown with herbaceous ground layer and shrub canopy. Materials and Methods Figure 2. The floating peat mat is sampled using a Hargis corer. Results Aboveground_herbaceous biomass was Significantly higher in herbaceous marshes compared to that of the shrub thickets (Figure 3). Belowground biomass and amount of peat did not differ between herbaceous marshes and shrub thickets (Figures 4 and 5). There was a significantly negative relationship between mat depth and mineral density in herbaceous marsh cores (p < 0.01) but not in shrub thickets. The average mineral density was higher in thicket cores (0.0878 g cm") compared to those from herbaceous marsh (0.0720 g cm") (Figure 6); this pattern was consistent for most of the individual depth classes. Research is needed to assess the long-term effects of shrub expansion on mat sustainability and to evaluate the potential of prescribed fire to counter woody encroachment in the floating marsh ecosystems.