A Multivariate Analysis of Vegetation Patterns in Tidal Freshwater Swamps of Lower Chesapeake Bay, U.S.A.

Abstract

RHEINHARDT, R. D. (School of Marine Science, College of William and Mary, Gloucester Point, VA 23062). A multivariate analysis of vegetation patterns in tidal freshwater swamps of lower Chesapeake Bay, U.S.A. Bull. Torrey Bot. Club 119: 192-207. 1992.-The woody and herbaceous vegetation of 23 tidal freshwater swamps was sampled along the Pamunkey River, a tributary of the York River (a subestuary of Chesapeake Bay). Tidally driven water level fluctuations were monitored and recorded in selected swamps. Four vegetative life-forms were examined and ordinated with Detrended Correspondence Analysis: trees (canopy and sapling sized), woody subcanopy (shrubs and understory trees), vines, and herbs. Species distribution patterns were compared in relation to edaphic factors, a flooding index, and duration of flooding in the root zone. On the basis of the canopy composition, two tidal swamp communities were found, both subjected to a tidally forced hydroperiod regime within the upper 15 cm of their root zones, the approximate height of the hummocks. Nyssa biflora-Fraxinus spp. dominated swamps are best developed toward the more downriver reaches in the wetter sites, which contain more hollows, a higher organic matter content, and higher calcium levels. In contrast, Acer rubrumLiquidambar styraciflua-Nyssa biflora dominated swamps are more common throughout the mid- to upriver reaches at less wet sites with lower organic matter and calcium levels. Taxodium distichum was found to co-dominate in two swamps that may represent relic conditions for the wetter sites. Although low in canopy diversity, the tidal swamps are floristically rich in herbaceous and subcanopy species, ranking them among the most speciose in temperate North America. The microtopographic complexity (the hummocks vs. hollows pattern) appeared to be strongly related to species distribution patterns in the canopy, vine, and herbaceous strata. Although the ordinations segregated canopy and herbaceous strata similarly, the woody subcanopy did not segregate into the same two communities established by the canopy and herb strata, suggesting that the canopy may be partitioning different resources than the woody subcanopy.