PALEOVEGETATION AND PALEOCLIMATIC CHANGES IN THE YUKON AT 6 KA BP

The most recent paleoenvironmental change to affect the Yukon centres around 6.0 ka. In the forested southern Yukon, black spruce (Picea mar iana) and green alder (Alnus crispa) expanded their populations at mos t sites between 6.5 and 6.0 ka. Even in the semi-arid region of SW Yuk on these species increased their populations, although slightly later at 5.5 ka. These vegetation changes in the south imply cooler and wett er growing seasons, i.e. more mesic conditions. In the region of the u pper Blackstone River of central Yukon, the modern vegetation consists of shrub tundra with scattered groves of white spruce (Picea glauca) and even fewer black spruce. Open forests of predominantly white spruc e occupied the region as early as 9.5 ka, but between 6.5 and 6.0 ka w hite spruce declined as black spruce became the dominant tree, coincid entally with an increase in green alder. By 5.0 ka the vegetation had acquired its modern composition. As in the south, these changes imply cooling. Less evidence is available on the expansion of alder and blac k spruce in the northern Yukon. Both species increased in forested are as (forest-tundra) at 6.0 ka. These changes again imply cooling. Becau se both black spruce and green alder were present in Yukon well before 6 ka, these vegetation changes cannot be ascribed to migration lags.