Carbon exchange between the atmosphere and subtropical forested cypress and pine wetlands

W B Shoemaker,F Anderson,S L Graham,D B Botkin,J G Barr

doi:10.5194/bg-12-2285-2015

Abstract

Abstract. 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 2 years. Links between water and C cycles are also examined at these three sites, as are 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 the 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). Atmospheric C uptake (NEE) was greatest at the Cypress Swampp (−900 to −1000 g C m2 yr−1), moderate at the Pine Upland (−650 to −700 g C m2 yr−1) and least at the Dwarf Cypress (−400 to −450 g C m2 yr−1). 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.

Highlights

On global scales, wetlands are generally considered sinks for atmospheric carbon dioxide (Troxler et al, 2013; Bridgham et al, 2006) and natural sources for methane emission (Whalen, 2005; Sjogersten et al, 2014)
In addition to the insight provided on the role of subtropical forested wetlands in the global carbon cycle, this research is expected to be useful for determining consequences of land-use changes in the Everglades region
A variety of subtropical forested and non-forested wetland ecosystems are present in Big Cypress National Preserve (BCNP), including Pine Upland, Wet Prairie, Marsh, Hardwood Hammocks, Cypress Swamps, Dwarf Cypress and Mangrove Forests as formally characterized by McPherson (1973) and Duever et al (1986, 2002)

Summary

Introduction

Wetlands are generally considered sinks for atmospheric carbon dioxide (Troxler et al, 2013; Bridgham et al, 2006) and natural sources for methane emission (Whalen, 2005; Sjogersten et al, 2014). Wetlands in southern Florida’s Greater Everglades (http://sofia.usgs.gov/) are expansive subtropical ecosystems that are carbon (C) accumulating over geologic timescales (Jones et al, 2014). We report a first step in characterizing modern rates of atmospheric–ecosystem carbon (C) exchange, for cypress strands and pine forests in the Greater Everglades of Florida. In addition to the insight provided on the role of subtropical forested wetlands in the global carbon cycle, this research is expected to be useful for determining consequences of land-use changes in the Everglades region. Canal building and drainage projects in south Florida have reduced the original extent of the Everglades (Parker et al, 1955), decreased peat accretion rates and total carbon stocks, and reduced ecosystem services. State and federal governments are planning and executing complex projects to restore the Everglade’s wetlands (http://www.evergladesrestoration.gov/) while concurrently avoiding flooding in urbanized areas and maintaining water supply

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