Environmental controls over net exchanges of carbon dioxide from contrasting Florida ecosystems

Net CO2 exchange estimated using eddy covariance and relaxed eddy accumulation indicated that evergreen pine upland and deciduous cypress wetland ecosystems in north-central Florida had similar apparent light compensation points during the growing season (125 vs. 150 mu mol PPFD.m(-2).s(-1)), but that maximum rates at 1800 mu mol PPFD.m(-1).s(-1) at the cypress ecosystem were only 59% of those at the pine ecosystem (8.9 vs. 15.2 mu mol CO2.m(-2).s(-1)). During both the summer and winter months at the pine ecosystem, net CO2 exchange in the daytime was a curvilinear function of PPFD, with no significant seasonal differences in slope or intercept. In contrast, net CO2 exchange at the cypress ecosystem was minimal during the daytime in the winter. Net CO2 exchange during the righttime was an exponential function of air temperature at both sites, with Q(10) values of 2.0 and 1.9 for the pine and cypress ecosystems, respectively. Lower nighttime fluxes of CO2 occurred at the cypress ecosystem across the entire temperature range. Both of these relatively sparse canopies stored CO2 during stable atmospheric conditions. Mean maximum net CO2 exchange during the daytime and mean nighttime net CO2 exchange for these ecosystems were highly contrasting, and together resulted in a relatively low rate of annual carbon accumulation in the wetland when compared to the aggrading pine ecosystem. However, values reported here are within the ranges of values for other boreal, temperate, and tropical forest ecosystems.