Effects of altered flows in the Congaree River on the floodplain forests of the Congaree National Park, SC

The Congaree National Park (CNP) protects the largest remaining tract of old-growth bottomland forest in the United States.  Changes in the hydrology of the Congaree River and flooding patterns of the CNP have occurred since Saluda Dam was constructed in 1930, with unknown effects on the floodplain forests.  We analyzed the size and age structure of floodplain forests to test for evidence of long-term changes in species composition.  Existing diameter (DBH) increment and mortality data from nine 1-ha plots established in 1989 were analyzed to derive age-DBH models for canopy species.  The models were then applied to data from these plots and an additional 16 1-ha plots surveyed in 2004, to estimate the age of each tree and allocate them to year-of-establishment classes.  Non-metric multidimensional scaling (NMDS) ordination and Trajectory Analysis identified 13 plots with a community trend from the oldest age class (trees established prior to damming) through to younger age classes that paralleled the trend from slough forests to bottomland hardwood (BLH) forests.  Since 1930, cohorts of trees on these sites have been increasingly dominated by species typical of less-flooded conditions.  Though the observed patterns are consistent with long-term shifts towards drier forest types, alternative interpretations exist.  The BLH species that dominate younger age classes in sloughs are mostly established on elevated microsites and may not survive to reach the canopy.  Some BLH sites may be undergoing a succession towards less flood-tolerant species that is primarily driven by sedimentation, rather than changes in the hydrological regime.