Shrub Encroachment Increases Transpiration Water Loss from a Subtropical Wetland

Abstract

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 have led to the expansion of Carolina willow (Salix caroliniana) in sawgrass (Cladium jamaicense) marsh communities, potentially compromising ecosystem health. In this study, we assessed differences in leaf gas exchange between willow and sawgrass in Blue Cypress Marsh Conservation Area (BCMCA). Stomatal conductance (gs) and photosynthetic CO2 exchange (Anet) 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. Willow had higher average rates of leaf gs and Anet than sawgrass. However, willow had lower water use efficiency (WUE) and greater LAI, resulting in greater loss of water through transpiration by willow populations and diminishing projected landscape water availability despite marginally increased C assimilation. Climate drying or potential positive feedbacks of shrubs to autogenic drying may accelerate shrub encroachment and increase risk of wetland loss.