Carbon Exchange Between the Atmosphere and Subtropical Forested Cypress and Pine Wetlands

Carbon dioxide exchange between the atmosphere and forested subtropical wetlands is largely unknown. Here we report a first step in characterizing this atmospheric-ecosystem carbon (C) exchange, for cypress strands and pine forests in the Greater Everglades of Florida as measured with eddy covariance methods at three locations (Cypress Swamp, Dwarf Cypress and Pine Upland) for two years. Links between water and C cycles also are examined at these three sites, and methane emission measured only at the Dwarf Cypress site. Each forested wetland showed net C uptake from the atmosphere both monthly and annually, as indicated by net ecosystem exchange (NEE) of carbon dioxide (CO2). For this study, NEE is the difference between photosynthesis and respiration, with negative values representing uptake from the atmosphere that is retained in the ecosystem or transported laterally via overland flow (unmeasured for this study). NEE was greatest at the Cypress Swamp (-900 to -1000 g C per m2 year), moderate at the Pine Upland (-650 to -700 g C per m2 year), and least at the Dwarf Cypress (-400 to -450 g C per m2 year). Changes in NEE were clearly a function of seasonality in solar insolation, air temperature and flooding which suppressed heterotrophic soil respiration. We also note that changes in the satellite-derived enhanced-vegetation index (EVI) served as a useful surrogate for changes in -NEE at these forested wetland sites.