BISON EXTIRPATION MAY HAVE CAUSED ASPEN EXPANSION IN WESTERN CANADA

The aspen Populus tremuloides parkland that forms the northern margin of the North American grassland or prairie has been variously attributed either individually or in different combinations to the ecological effects of prairie fire, fire suppression, frequent drought, and grazing. Plains bison Bison bison bison, formerly abundant on the western North American plains, inhibited growth of aspen by browsing, wallowing, trampling and toppling. Historical references show that aspen populations have expanded over the past century, and fossil pollen evidence suggests that the expansion occurred mainly after the near extinction of bison but before European homesteading and subsequent fire suppression in the late 1800s. We suggest that a prehistoric cycle of grassland, fire, aspen suckering, bison activity, and return to grassland was interrupted by the removal of bison, allowing aspen expansion.