Mangrove carbon sequestration in the Florida Everglades
Jordan G. Barr, José D. Fuentes, and Jay C. Zieman
Department of Environmental Sciences, University of Virginia, Charlottesville, VA
Dan Childers - Department of Biological Sciences, Florida International University, Miami, FL

Mangrove forests represent one of the most geochemically and biologically active biomes (Twilley et al. 1992), yet at present no unified modeling framework exists to investigate processes governing carbon sequestration. Limited information is available to learn how mangrove forests respond to climate forcings and anthropogenic perturbations such as fresh water input dynamics. This research addresses the hypothesis that the carbon sequestration capability of the riverine mangrove ecosystem in south Florida is governed by rates of fresh water flows into the bay and abiotic forcings such as environmental irradiance, and temporal and spatial salinity gradients. The hypothesis is first evaluated through the application of coupled atmospheric-biospheric modeling systems. We are developing a coupled atmospheric-biospheric modeling system to investigate trace gas exchange between the mangrove forest and overlying atmosphere. The model consists of a plant canopy radiative transfer module to describe solar and terrestrial irradiance disposition inside the forest, a module to quantify turbulent transport through the canopy, a plant biochemistry module to estimate carbon assimilation rates, and a component to evaluate soil respiration rates. The plant biochemistry module is based on the theory developed for terrestrial ecosystems (Baldocchi and Meyers 1998, Gu et al. 1999), but it incorporates physiological characteristics determined by our research group for riverine mangroves in Shark River Slough adjacent to Long-Term Ecological Research (FCE-LTER) site SRS-6. As input to the biochemical module, local physiological characteristics are considered including Rubisco, light limited carboxylation rates, nighttime respiration rates, and stomatal conductance to water vapor diffusion for red (Rhizophora mangle), white (Languncularia racemosa), and black (Avicennia germinans) mangroves. To reflect the vertical gradients in both mangrove physiology and environmental state variables, the model represents the mangrove forest canopy as a conglomerate of layers. The net carbon exchange between the forest and overlying atmosphere is taken as the integral of the differences between photosynthetic gains and the respiratory losses.

Though the biophysical principles are essentially the same as those applied to terrestrial forest ecosystems, mangrove forests exhibit unique physiological attributes. For example, mangrove leaves exposed to direct sunlight throughout the day achieve maximum photosynthetic rates and stomatal conductance around 10:00 h local time. These optimum physiological responses occur much earlier than the time when the governing environmental variables attain their maximum daily values. As a working hypothesis, we propose that mangroves exhibit unique physiological responses to the local environment due to two coupled processes. First, as the atmospheric evaporative demand increases (as reflected in the high vapor pressure deficits) mangroves need to reduce the stomatal conductance to conserve water. Mangroves must maintain low salt concentrations by either excluding salt in their xylem, as is the case with red mangroves, or ridding themselves of salt, as is the case with black mangroves. Second, mangrove leaves need to cope with exceedingly high radiational loadings. Because of the unique mangrove-water relations, evaporative cooling may not be the most effective mechanism to protect foliage from excessive radiational heating.

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