Plant photosynthesis is a key biochemical process for carbon (C) cycling in wetland ecosystems. Mangrove wetlands are large C reservoirs along tropical and subtropical coastlines that play a critical role in the global C cycle. Existing on the terrestrial-marine ecotone, variation in mangrove forest structure is controlled by resources gradients (e.g., nutrients, light), regulators (e.g., salinity), and hydroperiod. In the Florida Coastal Everglades (FCE), these environmental gradients and natural disturbances (i.e., hurricanes) create a matrix of distinct riverine and scrub mangrove forests across the landscape. We questioned how variation in mangrove island microtopography (i.e., habitat) modulates environmental effects on photosynthesis in scrub mangroves of the FCE. During 2019, we measured monthly rates of leaf gas exchange of scrub Rhizophora mangle trees in the southeastern FCE. We hypothesized that stomatal conductance (gsw) and rates of CO2 assimilation (A) would decline with increases in water level and salinity, with higher differences at the edge of mangrove islands relative to the center, where inundation and salt stress are greatest. We modeled the effects of seasonal water level and soil porewater salinity on A and gsw using linear mixed-effects models that accounted for seasonality and mangrove island habitat (i.e. center vs. edge).
Results/Conclusions
Water levels varied between 0 and 60 cm and were highest during the wet season (May-October) compared to the dry season (November-April). Soil porewater salinity ranged between 15 and 30 ppt, with higher values at the edge compared to the center of mangrove islands. Rates of A maximized at 15.1 µmols CO2 m-2 sec-1 and rates of gsw were typically less than 0.2 mols H2O m-2 sec-1, both of which varied over time. Rates of gsw and A were greater in the dry season than the wet season and greater in the mangrove island center than the edge. After accounting for season and mangrove habitat, porewater salinity had a slightly positive effect on A, but a negligible effect on gsw, with effects being consistent across seasons. Similarly, there was a positive marginal effect of water level on A in both the wet and dry seasons, but the effect of water level on gsw was not statistically significant. Our findings suggest that the interaction between permanent flooding and salinity varies with season, that mangrove microtopographic habitat modulates the effects of inundation and salt stress, and that water levels and water salinity constrain leaf A more than they affect gsw of scrub Rhizophora mangroves.