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Forest composition and growth in a freshwater forested wetland community across a salinity gradient in South Carolina, USA

Liu, Xijun, Conner, William H., Song, Bo, Jayakaran, Anand D.
Forest ecology and management 2017 v.389 pp. 211-219
Morella cerifera, Taxodium distichum, basal area, forest communities, forest health, freshwater, lowland forests, overstory, plant litter, primary productivity, saltwater intrusion, sea level, shrubs, stemwood, strawberries, swamps, tree and stand measurements, trees, understory, water salinity, South Carolina
Tidal freshwater forested wetlands (TFFW) of the southeastern United States are experiencing increased saltwater intrusion mainly due to sea-level rise. Inter-annual and intra-annual variability in forest productivity along a salinity gradient was studied on established sites. Aboveground net primary productivity (ANPP) of trees was monitored from 2013 to 2015 on three sites within a baldcypress (Taxodium distichum) swamp forest ecosystem in Strawberry Swamp on Hobcaw Barony, Georgetown County, South Carolina. Paired plots (20×25-m) were established along a water salinity gradient (0.8, 2.6, 4.6PSU). Salinity was continuously monitored, litterfall was measured monthly, and growth of overstory trees ⩾10cm diameter at breast height (DBH) was monitored on an annual basis. Annual litterfall and stem wood growth were summed to estimate ANPP. The DBH of live and dead individuals of understory shrubs were measured to calculate density, basal area (BA), and important values (IV). Freshwater forest communities clearly differed in composition, structure, tree size, BA, and productivity across the salinity gradient. The higher salinity plots had decreased numbers of tree species, density, and BA. Higher salinity reduced average ANPP. The dominant tree species and their relative densities did not change along the salinity gradient, but the dominance of the primary tree species differed with increasing salinity. Baldcypress was the predominant tree species with highest density, DBH, BA, IV, and contribution to total ANPP on all sites. Mean growth rate of baldcypress trees decreased with increasing salinity, but exhibited the greatest growth among all tree species. While the overall number of shrub species decreased with increasing salinity, wax myrtle (Morella cerifera) density, DBH, BA, and IV increased with salinity. With rising sea level and increasing salinity levels, low regeneration of baldcypress, and the invasion of wax myrtle, typical successional patterns in TFFW and forest health are likely to change in the future.