Wetland shrub encroachment alters landscape land-atmosphere carbon and water exchange in subtropical Florida

Background/Question/Methods

Encroachment of woody shrubs into graminoid-dominated wetlands can impact ecosystem carbon and water cycling due to differences in species physiology. In subtropical Florida, shortened hydroperiods primarily resulting from water management activities have led to the expansion of Carolina willow (Salix caroliniana) in sawgrass (Cladium jamaicense) marsh communities. Broadleaf deciduous woody shrubs like willow are morphologically and ecophysiologically different from the herbaceous sawgrass sedge and other emergent vegetation that typically dominates these subtropical fens, potentially compromising ecosystem function. In this study, we assessed differences in leaf gas exchange between willow and sawgrass in Blue Cypress Marsh Conservation Area (BCMCA).  Stomatal conductance (g_s) and photosynthetic CO₂ exchange (A_net) were measured across a range of photosynthetically active radiation (PAR; 0-2000 μmol m^-2 s^-1).  Leaf area index (LAI; m² leaf m^-2 ground) was determined for each species and used in conjunction with land cover estimates to extrapolate leaf measurements to the plant canopy and assess the consequences of shrub encroachment on landscape atmospheric carbon and water exchange.   

Results/Conclusions

Willow had higher average rates of leaf g_s (0.261 mol H₂O m^-2 sec^-1) and A_net (8.052 μmol CO₂ m^-2 sec^-1) than sawgrass (0.153 mol H₂O m^-2 sec^-1; 6.436 μmol CO₂ m^-2 sec^-1).  However, willow had lower water use efficiency (WUE, 46.67 μmol CO₂ mol H₂O^-1) than sawgrass (55.94 μmol CO₂ mol H₂O^-1). While sawgrass land cover was more than double that of willow, a greater LAI and lower WUE resulted in a disproportionate effect of willow on the landscape, with diminishing projected landscape water availability despite marginally increased C assimilation. Therefore, shrub encroachment can compromise wetland water availability, with potential positive feedbacks from autogenic drying causing further species invasion or increased risk of wetland loss. Climate drying or water management could accelerate shrub invasion, further compromising wetland health and services.