ORGANIC MATTER DYNAMICS IN FOUR SEASONALLY FLOODED FOREST COMMUNITIES OF THE DISMAL SWAMP 1

Budgets of organic matter dynamics for plant communities of the Great Dismal Swamp were developed to summarize an extensive data base, determine patterns of biomass allocation, transfer and accumulation, and make comparisons with other forested wetlands. Aboveground net primary production on the flooded sites (1,050-1,176 g m-Z y r l ) was significantly greater than on a rarely flooded site (83 1 g m-2 y r l ) . Estimates of belowground net primary production were comparable to aboveground production on flooded sites (824-1,221 gm-2 y r l ) . However, productivity was nearly three times greater belowground than aboveground on the rarely flooded site (2,256 g m-2 y r l ) . Aboveground productivity in Dismal Swamp forests is relatively high compared to other forested wetlands. This is attributed to the timing and periodic nature of flood events. Fine root turnover is shown to be an important source of soil organic matter. Estimates indicate that roots contribute about 60% of the annual increment to soil organic matter. Leaf litter contributes 6-28O/o and wood debris contributes 5-15%. Comparisons with other forested wetlands suggest that detritus accounts for greater than half of the total organic matter (living + dead) in many wetland systems. THE BIOGEOCHEMICAL CYCLES of freshwater forested wetlands are unique among forested ecosystems. Flooding leads to predictable and dramatic changes in the solubility and chemical reactivity of important soil nutrients (Gambrel1 and Patrick, 1978) and influences rates of elemental import, export, storage, and processing (Kuenzler et al., 1980; Brinson, Lugo, and Brown, 1981). Secondary production in wetlands and associated aquatic systems is intimately related to organic matter and nutrient processing rates (Benke, 1984). Certain aspects of organic matter dynamics, such as litterfall and decay rates, have been the subject of many ecosystem level studies at a variety of freshwater forested wetland sites. However, there have been comparatively few I Received for publication 29 May 1987; revision accepted 28 December 1987. We thank Elizabeth Hahn and Millie Symbula for field assistance. Revision of the manuscript was aided by helpful comments from Christopher Dunn, Mike Scott, William Mitsch, and an anonymous reviewer. This work is based on a thesis submitted by the first author to Old Dominion University in partial fulfillment of the M.S. degree requirements. The research was partially supported by Old Dominion University. Much of the data that was synthesized in this paper came from studies supported by National Science Foundation grants DEB-7708609, DEB7708609-A0 1 and BSR-8405222. Manuscript preparation was aided by contract DE-AC09-76SR00-8 19 between the United States Department of Energy and the University of Georgia's Savannah River Ecology Laboratory. efforts at an ecosystem level synthesis of all the major storage and transfer rates on a given site. Such studies provide valuable insight into patterns of productivity, storage, and mass balance which are often overlooked in less complete investigations (Vogt, Grier, and Vogt, 1986). We present a synthesis of several studies on organic matter dynamics in the Great Dismal Swamp, Virginia. These data permit between-site comparisons of net primary production, aboveand belowground detrital transfer rates, and the annual contribution of various litter sources to soil organic matter from various litter sources. In addition, patterns of organic matter storage in Dismal Swamp forests are compared with other freshwater forested wetlands. STUDY SITES-The Great Dismal Swamp occurs on 85,000 ha of the Atlantic coastal plain in southeastern Virginia and northeastern North Carolina. The Suffolk escarpment is the artesian head of an aquifer that underlies the swamp's eastward sloping surface (Lichtler and Walker, 1979). Water movement is thought to be quite slow and in a southeasterly direction, excgpt through the numerous ditches that dissect the swamp's surface. Hydroperiod is highly dependent on annual patterns of precipitation; flooding generally occurs during the winter and spring. During periods of low rainfall the water table can drop > 1 m below the September 1 9 8 81 MEGONIGAL AND DAY FLOODED FOREST COMMUNITIES 1335 TABLE 1 . Selected features of four long-term research sites in the Great Dismal Swamp