Emergent Ecohydrologic Thresholds as a Cause of Loss of Forested Wetlands in Hydrologically Altered Environments

Forested wetlands in the southeastern United States are critical not only for timber products and wildlife habitat, but also because they benefit society by moderating hydrological, biogeochemical, and geomorphological processes. However, extensive modification of the landscape in the lower Mississippi River deltaic plain has resulted in a radically altered hydrologic environment. In this new hydrologic environment, many forested wetlands remain flooded for the majority of the year, causing shifts in dominant tree species, increased tree mortality, and widespread loss of forested wetlands. Evapotranspiration is a key component of the water budget in swamp-forests with evapotranspiration rates that often exceed open-water evaporation rates. In disconnected and impounded swamps, the transpiration flux may be crucial for lowering the water table and creating conditions that are necessary for the reproduction and survival of trees. However, even the most flood tolerant species are inhibited by extended, deep flooding; one effect is stomatal closure and decreased transpiration. Therefore when flooding is greatest, the evapotranspiration flux, which reduces flooding, is lowest. This potential positive feedback cycle between flooding and reduced transpiration invites the question of whether there are threshold conditions that result in mortality and loss of swamp forests. In this proposed project, we will investigate whether the human-altered hydrology of the Lower Mississippi River region has introduced a new vulnerability to this system due to this ecohydrologic feedback. An understanding of ecohydrologic feedback cycles and thresholds are necessary to improve the current inadequate management practices for these vital ecosystems.