Title

Vegetation patch dynamics in freshwater tidal wetlands

Date of Completion

January 1994

Keywords

Biology, Botany|Biology, Ecology|Biology, Limnology

Degree

Ph.D.

Abstract

This study investigates causes of the mosaic pattern of vegetation occurring in freshwater tidal wetlands of the Connecticut River using the concept of patch dynamics. I examined: (1) the extent of the vegetation mosaic accounted for by floristic composition and geographic site differences; (2) differential site availability for seasonal colonization along complex hydrologic gradients of flood stress and flood disturbance, and suitability of experimental gap conditions to seasonal regeneration; (3) differential species availability for seasonal regeneration from the seed and bud bank; and (4) differential species performance of colonists from the seed and bud bank as alternate modes of regeneration with respect to complex hydrological gradients.^ The vegetation mosaic attributed to plant community structure and geographic sites accounted for 51% and 19%, respectively, of the total floristic variation. Accordingly, the representation of vegetation types was not the same among geographic sites. Differential site availability for colonization into seasonal gaps or bare space varied directly with increasing flood stress and flood disturbance. Differential species availability, based on an inventory of 68 species present in the vegetation of the preceding year, consisted of 72 species emerging from the seed bank and 30 species emerging from the bud bank, of which 21 species were shared. Differential species performance resulted from a trade-off between alternate modes of regeneration. Seasonal regeneration from both buds and seeds diminished with increasing levels of flood stress, yet unlike the bud bank, the seed bank responded positively to flood disturbance. Confirmed experimentally, the trade-off between seeds and buds is most pronounced under more severe gap conditions. However, upon disturbance in sites ordinarily less disturbed, regeneration from buds exceeded seeds.^ In general, hydrologic gradients accounted for the vegetation mosaic by determining which species, mode of regeneration, and range of response are the most appropriate for local persistence. However, under benign conditions of minimal flood stress and flood disturbance, the actual vegetation mosaic consisted of multiple, alternate, asuccessional vegetation types. The implications for conservation suggest re-examining traditional object-oriented practices in favor of preserving the ecological processes of the Connecticut River tidelands ecosystem. ^