SEASONAL AND INTER-ANNUAL VARIATIONS IN HYDROLOGY AND PLANT COMMUNITY STRUCTURE IN MID-ATLANTIC WETLANDS

Wetlands are transitional between upland and deep-water habitats; a boundary that may change due to seasonal and inter-annual variations in hydrological conditions. In some ecosystems, seasonal changes mark the difference between the growing season and the dormant season. The humid temperate Mid-Atlantic region of the United States, the concept of a dormant season is not valid in all years. The purpose of this paper is to examine seasonal and inter-annual variations in hydrological conditions and plant community structure in freshwater marsh and forested wetlands in the Maryland Coastal Plain. We examine the relationships among climate, wetland hydrology, and plant communities. We evaluate the length of time required to develop accurate measures of plant community diversity, and whether the sequencing of wet and dry years influences plant diversity and wetland indicator status. The wetland is maintained by streamflow generated during storms and as winter baseflow, augmented by groundwater discharge. A network of 70 nested piezometers and wells were installed in the wetland and adjacent uplands and monitored continuously for 5 years, including both wet and drought years. A subset of wells in the wetland and adjacent upland were chosen for monitoring of hydrology, groundwater chemistry, and plant communities for 2-3 year periods. Sites were chosen primarily along two transects in the marsh and one transect in the forested wetland. Plant community structure was evaluated by plots that were re-sampled every three months. A species-area analysis was used to determine appropriate plot size (7m2). An evaluation of the pant species at each plot indicates that ~ 2years of sampling were required to obtain consistent species diversity. Most sites contained 20-30 plant species, usually with 5 dominant species that covered 80% of the area. The dominant species varied seasonally, with associated changes in the wetland indicator status. The seasonal pattern of plant community variations, however, were similar from year to year, even though hydrological conditions showed significant inter-annual variations.