HIGH MARSH-FOREST TRANSITIONS IN A BRACKISH MARSH: THE EFFECTS OF SLOPE


Joseph I. Hmieleski (Under the direction of Mark M. Brinson, Ph.D.) Department of Biology, May 1994.


Abiotic and biotic variables were assessed along brackish marsh-upland continua at the Virginia Coast Reserve/LTER, to determine the effect of slope on the position of vegetation zones through its effect on physicochemical variables. Four transects through the high marsh-upland transition were established to represent extremes of slope and proximity to tidal creek (i.e., Flat Far, Flat Near, Steep Far, Steep Near). Vegetation zones were delineated as high marsh, transition, and forest based on differences in percent cover of grass, shrub, and tree species. Vegetation cover, elevation, and soil characteristics were sampled along all transects; hydrologic and pore water variables were measured along two of the four transects, Steep Near and Flat Near, from November 1991 to August 1993.

The transition and forest zones of both flat transects occupied significantly lower elevations and were more hummocky as compared with the steep transects, while the high marsh occupied similar elevations. The flat transition zone had less drainage and a higher mean annual water table, higher and more variable mean groundwater and pore water salinities, and similar redox potentials for the 10 cm depths in the transition and forest zones. However, the steep slope showed significantly lower redox potential at 20 cm depth for all zones which is hypothesized to be related to the discharge of anaerobic groundwater. Hydrogen sulfide was very low throughout, but was significantly higher for the flat transect high marsh. Results of univariate analysis were supported by multivariate factor analysis which identified three factors accounting for 69.4% of the variation in the data: brackish water, redox potential, and sulfide variables. Factor scores for brackish water and redox potential for the flat transition and forest zones were significantly higher than for corresponding steep zones, but similar in the high marsh zone. All of these trends suggest that the dominant source of water is atmospheric for the flat transition and drainage and groundwater discharge for the steep transition.

Degree of slope affects transition zone size and position, such that the steep transition zone is narrower and changes more abruptly than the flat transition. Hydroperiod, salinity, and redox potential are believed to directly influence the distribution of plant species and their relative dominance. I could not determine whether hydroperiod or salinity controlled the position of the transition zone. However, the position of the transition zone roughly corresponds to the distance at which mean salinity decreases most rapidly. This occurs at the base of the forest area for the steep slope due to drainage or groundwater discharge, but at a greater distance from the tidal creek and at lower elevation for the flat slope. Marsh vegetation may restrict overland flow over long distances from the tidal creek during flood events, thus allowing the glycophytic vegetation to inhabit lower elevations, further away from the creek. Hummocks appear to allow glycophytic vegetation to colonize closer to the tidal creek thus increasing the width of transition zones.