HYDROSERAL DEVELOPMENT IN SOUTHERN ONTARIO - PATTERNS AND CONTROLS

Palaeoecological data from a range of wetland types were synthesized i n order to study the development of wetland ecosystems in southern Ont ario. The present-day nature of the wetland system occurring in a topo graphically defined basin was shown to be controlled in part by basin morphology. Stratigraphic and vegetational (pollen and macrofossil ana lysis) data were used to compare the sequence of communities occupying basins during the postglacial. In systems where terrestrial peat over lies lake sediment, the broad stages of development were relatively un iform (from a lake to a shallow open water wetland to a herbaceous mar sh or fen to a woody plant-dominated community), although the exact na ture of each stage varied widely between basins. The accumulation rate of sediment within such basins varied by up to two orders of magnitud e during the postglacial. Allogenic and autogenic factors have been ac ting on these systems throughout the postglacial, and the sensitivity of individual sites to changes in these environmental conditions will also have varied. Even the widespread land clearance associated with E uropean settlement in the area in recent centuries has not affected so me wetland systems detectably? while others have either been destroyed or profoundly changed. The patterns of hydroseral development, both t iming and direction, were compared with known regional environmental c hanges.