Differential recovery of a deepwater swamp forest across a gradient of disturbance intensity
On the Savannah River Site, South Carolina, USA, large areas of floodplain swamp forest of baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica) were destroyed by the cumulative impacts of cooling-water discharges over a 35-year period of nuclear reactor operations. In one floodplain area, four years after thermal discharges ended, we analyzed the pattern of forest recovery across a disturbance gradient spanning from a site of chronic thermal impact and extensive sediment deposition to sites of intermittent thermal impact and little substrate change. Across this spatial gradient, we measured density and size structure of cypress and tupelo and assessed regeneration success in relation to density of surviving canopy trees and to substrate changes. Compared with undisturbed forest, canopy tree density was lower in all disturbed sites and decreased progressively with greater site disturbance. Density of tree regeneration decreased in parallel with declining canopy tree density; however, regeneration was particularly low in the site of chronic impact, where very few canopy trees had survived and where substrates had been modified by sedimentation. Size structures suggested that tree recruitment had occurred synchronously during a 5-year period of regional drought and minimal river flooding. Thus, cypress-tupelo recovery was influenced both by availability of seed sources and by site conditions, but floodplain hydrology also affected regeneration. The pattern of differential recovery across the disturbance gradient has allowed the use of natural regeneration potential in efforts to restore the pre-disturbance forest, and it also illustrates several key factors in wetlands design.