INFLUENCES OF HOLOCENE CLIMATE AND WATER LEVELS ON VEGETATION DYNAMICS OF A LAKESIDE WETLAND

Sediment lithology, pollen, and plant-macrofossil data from the Paynte r Site, southern Ontario, revealed three wetland developmental stages during the past 11000 years: (i) a Carex, Eupatorium, and Eleocharis d ominated marsh with some Larix, Abies, and Picea (ca. 11000-8300 cal y ears BP); (ii) a Verbena hastata and Mentha arvensis marsh (ca. 8300-7 460 cal years BP); and (iii) a white cedar (Thuja occidentalis) swamp (7460 cal years BP to present). There were no significant successional changes in the initial diverse marsh (stage 1) for about 2700 years; its high taxon richness was maintained by fluctuating water levels. Th e succession from marsh to swamp at 7460 cal years BP was caused by mi d-Holocene warm and dry climate, which corresponded with cedar expansi on elsewhere in Ontario. The swamp peat record was interrupted at ca. 6400 cal years BP by declining water levels, culminating in a dry peri od, as indicated by a sandy layer, rare macrofossils, and a low sedime nt-accumulation rate (0.012 cm/year). The reappearance of cedar swamp macrofossils since 3200 cal years BP corresponded with the recovery of water levels owing to a more humid late Holocene climate and flooding from isostatic tilt. Human disturbance such as damming and logging ca used the development of historical cedar and thicket swamps with abund ant Alnus rugosa and weedy taxa. This sequence of wetland development did not match the present vegetational gradients from Carex-Typha herb marsh, through Myrica-Decodon shrub marsh and Alnus-Fraxinus thicket, to Thuja swamp. At this site, the wetland development was mainly infl uenced by allogenic factors such as water levels and climate rather th an autogenic factors.