Prolonged flooding alters forested wetland function

Abstract

Standing dead trees, or snags, and other vegetative structures contribute to the delivery of forested wetland ecosystem functions (i.e., things that wetland do). This study evaluated > 150-day flood induced changes in vegetation community structure and wetland functional capacity shifts in a > 30 year-old restored Mississippi Alluvial Valley wetland. Flooding significantly altered Hydrogeomorphic (HGM) wetland functional assessment variables (e.g., snag and tree density; ground vegetation cover; woody debris and log biomass), shifting forested wetland functional capacity indexes (FCIs). For example, few snags were present prior to the floods, and flooding increased mean snag density from 2.0 snags/ha to 105 snags/ha. Only 8 % of study locations exhibited snag densities observed in fully functional forests before the floods, which increased to 42 % post-flood. The remaining 58 % locations surpassed fully functional wetlands snag densities, and now contain excessive snags. Overall, changes in vegetation structures within the restored wetland augmented the delivery of ecological functions. The wetland FCIs for Nutrient Cycling, Carbon Export, Maintain Plant Communities, and Fish and Wildlife Habitat cumulatively increased by a mean value of 0.44 (range = −0.30–1.30; median = 0.44), representing a mean increase of 19 %. Results would differ in mature forests with natural snag distributions, where extensive flood-induced snags would likely decrease functional capacity. The restored wetlands functional trajectory will continue evolving in response to changes in log and woody debris distribution as snags decay, and forest succession occurs. Future floods extending into the growing season, when tree mortality risk increases, will further influence the delivery of wetland functions.