The relationship of below-ground hydrology to canopy composition in five tidal freshwater swamps

Abstract

The relationship between ground-water fluctuations and the canopy compositions of five tidal freshwater swamps were examined. Tide gauge data were compared with measured ground-water fluctuations to assess the role of tides in driving the hydrologic regimes of tidal swamps. Flooding in tidal swamps was found to be closely associated with high tides. As the tides dropped through the hummock zone (elevated area, about 15 cm high, to which the trees are restricted), drainage of the hollows (low areas between hummocks) occurred at the same rate as the falling tides. After the tides dropped below the elevation of the hollows, the lowering of the water table became much slower than that of the falling tides, presumably because peat in the root zone inhibited drainage. When the subsequent rising tides reached the elevation of the water table, the water table rose vertically with the tides. Flooding recurred in the hollows as the tides rose above the surface of the ground. The composition of the canopy was found to be related to the mean water-table depth (WTD) (i.e., the depth in relation to the hummock elevation at which the soil was flooded 50% of the time) and not to the duration, depth, or frequency of flooding. Tidal swamps dominated byFraxinus spp. andNyssa sylvatica varbiflora occurred in the wetter sites (mean WTD=17 cm), whereas swamps dominated byAcer rubrum andLiquidambar styracifiua occurred in less wet sites (mean WTD=21 to 30 cm). Tidal freshwater swamps are precariously positioned at the upper portion of the tidal range where they seem to keep pace with a rising sea level by accumulating biomass (logs and roots). Potential threats to the distribution and areal extent of tidal freshwater swamps include any abrupt changes in the rate of sea-level rise, periodic logging, or blockage of their upriver migration due to incompatible land use.