Transition between High Marsh and Forest in Mainland Marshes:

Observations in the Vicinity of Phillips Creek

Joseph I. Hmieleski and Mark M. Brinson, East Carolina University

The literature on rising sea level and its influence on migrating marshes suggests that the rate of lateral movement will be inversely proportional to slope of the landward surface (Titus 1987). This logic holds until slopes become so diminished that they are virtually flat. In such cases, landward migration should occur instantaneously once sea level reaches a threshold appropriate to cause flooding on the flat surface. This is unlikely to take place, however, because distance itself becomes a dominant factor in modulating marsh migration over relatively flat slopes. Resistance to overland flow from tidal sources will limit the transport saline water landward. Stated differently, slope must be taken into consideration when predicting the rate of marsh migration and the elevation occupied by the transition zone.

Approach: Elevation and distance from a source of salt water are both variables of slope that should be examined independently for their influence on overland migration of marshes. The forest-high marsh transition is the leading front of migration. To examine how the forest-high marsh transition varies with slope and elevation, we installed four transects across transition zones shown in Fig. 2 as combinations of steep slope, flat slope, near to a tidal creek, and far from a tidal creek. Details of sampling points and representative distances are shown in Fig. 3. Linear regressions were used to determine overall slope for each of the transects (Fig. 4). For the flat transects, microtopographic relief determined over 2 m intervals normally exceeded differences in macrotopography across entire transects. Transition zones were identified from changes in species composition of vegetation . In contrast to high marsh which is dominated by Spartina patens and Distichlis spicata, the transition zone contains shrubs, small trees, and graminoids (Juniperus virginiana, Baccharis hamilifolia, Ilex opaca, Iva frutescens, and Panicum virgatum). The forest landward of the transition zone is dominated by Pinus taeda individuals that are considerably larger than the few stunted individuals occurring in the transition zone.

Mean elevations of the high marsh, transition, and forest portions of the transects show that elevations in the transition zone and forest zone are significantly higher on steep transects than they are on flat transects (Fig. 5). Mean elevations of the high marsh did not differ among transects. Depths to water tables were also greater for steep slopes (Fig. 6). Mean pore water salinities at 10 and 20 m depth were significantly higher for the flat than the steep transect. Even the pattern of salinity showed greater variation in the steep than the flat slope (Fig. 7).

Conclusions: Previous studies have concluded that slope is an important variable in the rate of landward transgression of salt marshes. However, distance becomes important in determining the elevation at which the transition of vegetation will occur. Presumably it also influences the rate of movement of the leading front. Flat slopes are occupied by transitional vegetation at lower elevations than with steeper slopes. Consequently, the elevation of the transition zone of vegetation varies depending on whether the process occurs on a steep or relatively flat slope.

An understanding of the transition from forest to high marsh is critical because it occurs on and modifies the same geomorphic surface that will be occupied by subsequent changes in state including intertidal low marsh and subtidal lagoon. The transition has socioeconomic implications for land use. The forest-high marsh transition is the first that renders land effectively useless for most silviculture, agriculture, and urbanization.

References:

Brinson, M.M., R.R. Christian, and L.K. Blum. In review. Multiple states in the sea-level induced transition from terrestrial forest to estuary. Estuaries

Hmieleski, J. I. 1994. High marsh-forest transitions in a brackish marsh: The effects of slope. Master's thesis, East Carolina University, Greenville, North Carolina. 129pp.

Titus, J. G. 1987. The greenhouse effect, rising sea level and coastal Wetlands. EPA-230-05-86-013. Office of Policy, Planning and Evaluation, Environmental Protection Agency, Washington, D.C. 152 pp.