Postglacial vegetation history of oak savanna in southern Ontario

Sizable areas of oak savanna were present in southern Ontario when the region was first settled by Europeans. The objective of this study was to reconstruct the vegetation history of oak savanna at a site in Ontario and to test the hypothesis that the savanna was created by Indian burning. The fossil pollen, plant macrofossil, and fossil charcoal records of the sediments of a small lake were used to reconstruct postglacial vegetation development in an area formerly occupied by oak savanna. The fossil record from the site extends from approximately 11 800 BP to the present. The initial vegetation around the lake was an open Picea woodland similar to that reported from other late glacial sites in southern Ontario. Pinus banksiana forest dominated the vegetation of the site between about 10 000 and 9000 BP. Pinus strobus replaced Pinus banksiana and remained the dominant tree species in the area until approximately 6000 BP. The persistence of Pinus strobus as the dominant tree species until 6000 BP is unique in southern Ontario. Between 6000 and 4000 BP the Pinus strobus forest was replaced by oak savanna. Oak savanna occupied the area until land clearance by Europeans at approximately A.D. 1850. The early date at which oak savanna developed makes it unlikely that Indian burning was the cause of savanna formation. Determining the cause of the late persistence of Pinus strobus-dominated vegetation and its replacement by oak savanna is difficult. It is possible that the late persistence of Pinus strobus in the study area is related to dry climatic conditions during the mid-Holocene (~8000 to 6000 BP). The establishment of oak savanna may have been caused by the transition to moister climatic conditions in the later half of the Holocene. The particularly dry and well-drained substrates associated with oak savanna may have restricted occupation of these regions by more mesic tree species and maintained herb and graminoid dominated openings by promoting natural fires. Key words: Palaeoecology, fossil pollen, Holocene, oak savanna, southern Ontario.