<p> An open Quercus-dominated vegetation association, known locally as the oak plains, was found at a number of locations in southern Ontario until disturbance by European settlers in the late 18th and early 19th centuries. Two contrasting theories have been suggested in the literature regarding the origin of the oak plains. One suggests they developed as the result of burning by pre-European natives, while the other considers them to be relics of a warmer, drier mid-Holocene climate. In this paper, the factors which led to the development of the oak plains are examined. The hypothesis that the oak plains resulted from native burning of the natural vegetation was tested by pollen analysis of a 5 m sediment core from Decoy Lake, a small kettle basin near Paris, Ontario located in an area mapped by early surveyors as oak plains. The Decoy Lake record was then compared to those of two nearby lakes supporting mesic forests. This palaeoecological analysis was supported by an investigation of physical factors controlling the historical distribution of the oak plains in a study area between Cambridge and Long Point on Lake Erie. </p> <p> The distribution of the oak plains and other vegetation associations in pre-settlement times, reconstructed from early survey records, correlated fairly well with the texture of soils and underlying Quaternary parent materials. Within the defined study area, the oak plains were restricted almost exclusively to well-drained soils overlying coarse-textured till and sandy outwash and deltaic deposits. Climatic factors and topography varied within the study area, but showed little correlation with the distribution of vegetation associations. <p> <p> The fossil pollen record at Decoy Lake indicates that a QuercusPinus- herb pollen assemblage, unique to southern Ontario, was found from 4000 yr BP until pre-settlement times. This suggests that the oak plains have existed in the area for at least 4000 years. The oak plains replaced an assemblage dominated by Pinus strobus. The warm, dry Hypsithermal appears to have allowed Pinus strobus to remain dominant on the well drained soils around Decoy Lake until after 5000 yr BP, 2000 to 3000 years longer than at other southern Ontario sites. The Picea zone (11,800 yr BP to 10,100 yr B P), Pin us banksiana/resinosa zone ( 10,100 yr BP to c. 9000 yr BP), and the replacement of Pinus banksiana/resinosa by Pinus strobus (c. 9000 yr BP) occurred contemporaneously with other records from southern Ontario. </p> <p> The hypothesis that anthropogenic factors resulted in the development of the oak plains was rejected since this association developed 2500 years before the onset of agricultural activity by natives in southern Ontario. Instead, it appears post-Hypsithermal increases in moisture, perhaps coupled with an amelioration of winter temperatures, led to the replacement of Pinus strobus by the oak plains in some areas of well-drained soils between 6300 yr BP and 4000 yr BP. The pollen record from Decoy Lake provides the first evidence from southern Ontario for substantial vegetation response to mid to late Holocene climatic change. </p> / Thesis / Master of Science (MSc)