Evapotranspiration—Flooding Feedback in Forested Wetlands

Abstract:

Evapotranspiration (ET) is a principle efflux from flooded swamps. However, for many reasons (e.g., difficulties in measuring across small gradients), controls over ET in wetlands, and swamps particularly, remain unclear. We investigate how the interaction of vegetation and hydrology controls ET and energy partitioning in flooded forests in three different hydrologic landscapes of the southeastern USA: mesohaline coastal shrub forest, floodplain hardwood forest, and backswamp baldcypress-tupelo forest. Energy components were measured by eddy covariance, sapflow, Bowen ratio energy balance, and evaporation pan to partition canopy transpiration, understory transpiration, and evaporation from floodwaters. Together, we found the energy balances of forested wetlands often differed from that of terrestrial systems, because of the potentially substantial role of evaporation from the free water surface, and flood / salinity stress inhibiting transpiration. Highest ET rates were observed in the coastal shrub forest in flooded conditions, when saline water levels dropped below the root zone (on hummocks) with concurrent lateral advection of dry air. The other two sites behaved similar to terrestrial systems, with increasing transpiration during periosd of greater water availability, although with radiation transmitted to the understory mostly partitioned to latent heat which partially compensated for the variability in canopy transpiration. Identifying these differential controls over ET is critical to understanding long-term trajectory of these systems. Many wetlands of the southeastern US have been impounded and disconnected, necessitating some degree of balance between ET and precipitation for sustainability. The positive feedbacks between ET and water levels in the coastal system suggest lower ecosystem resilience than the other two systems that showed negative feedbacks between ET and water levels.