Long-term tree productivity of a South Carolina coastal plain forest across a hydrology gradient

In recent years, there has been an increased interest in examining changes in forest systems in response to drought, flooding, hurricanes and climate change. In the southern United States, forested wetlands are of special interest because of the extent of these forests. Coastal plain forested wetlands are among the most vulnerable to these climatic impacts. One of the problems in developing management practices for these coastal areas is the difficulty in adequately describing productivity relations and predicting how the structure and function of these communities might be affected by natural or anthropogenic disturbances. Community response to environmental change often occurs over a period of years, and the majority of reported studies are for 1-3 years in duration. This study documents long-term changes (10 years) in structure, composition and growth along a catena of high water table forested sites of an ancient beach ridge landscape in coastal South Carolina. Aboveground net primary production (ANPP) of trees was monitored from 2000 to 2009 on three sites within a longleaf pine-swamp blackgum forest system on the southern end of the Waccamaw Neck area of Georgetown County, SC. Permanent study plots (20 x 25 m) were established across a moisture gradient (Dry, Intermediate, and Wet). Water levels were continuously monitored, litterfall was measured monthly and growth of trees >= 10 cm diameter at breast height was monitored on an annual basis. Annual litterfall and tree production values were summed to provide estimates of ANPP. The study site was under severe drought conditions July 2001 through late summer 2002 and again in 2007. Diameter growth was affected in all three sites, but with different patterns. It seems that diameter growth in the Wet site was more sensitive to drought conditions in 2001-02 and 2007 than either Dry or Intermediate sites. While droughts did not seem to have a significant impact on litterfall in the Wet site, litterfall in the Intermediate site was more sensitive to the drought than either Dry or Wet sites. ANPP was significantly lower in both Intermediate and Wet sites in 2001 at < 602 g/m(2). Highest ANPP (> 1 000 g/m(2)) occurred in the Intermediate and Wet sites in 2003 following a return to more normal water levels at the end of the drought. Maximum tree production occurred on the Wet site in 2003 (657 g/m(2)), which exceeded total ANPP of any site in 2001. In the Dry site, ANPP remained relatively consistent throughout the study when compared to Wet and Intermediate sites. While litterfall estimates are well defined with 3-5 years of data, data collection is continuing to assess impact of drought on stem growth across the gradient, which is still not clear with 10 years of data.