Influences of Holocene climate and water levels on vegetation dynamics of a lakeside wetland

Abstract

Sediment lithology, pollen, and plant-macrofossil data from the Paynter Site, southern Ontario, revealed three wetland developmental stages during the past 11 000 years: (i) a Carex, Eupatorium, and Eleocharis dominated marsh with some Larix, Abies, and Picea (ca. 11 000–8300 cal years BP); (ii) a Verbena hastata and Mentha arvensis marsh (ca. 8300–7460 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. The succession from marsh to swamp at 7460 cal years BP was caused by mid-Holocene warm and dry climate, which corresponded with cedar expansion elsewhere in Ontario. The swamp peat record was interrupted at ca. 6400 cal years BP by declining water levels, culminating in a dry period, as indicated by a sandy layer, rare macrofossils, and a low sediment-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 caused the development of historical cedar and thicket swamps with abundant 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 influenced by allogenic factors such as water levels and climate rather than autogenic factors. Keywords: wetland succession, climate change, hydrological change, Holocene, paleoecology, disturbance, space-for-time substitution, plant macrofossils, Thuja expansion, Ontario.