Late Quaternary vegetation history of Kettlehole Pond, southwestern Yukon

A pollen diagram with a detailed chronology reveals Late-Pleistocene and Holocene vegetation changes which, in combination with previously published data, provide information on regional vegetation changes in the western boreal forest of the southwestern Yukon. A Populus woodland with an understory of Shepherdiacanadensis and extensive open areas dominated by Artemisia occurred from 11 030 to 9250 BP. Juniperus populations expanded at 9700 BP and then more-mesic forest communities developed when Piceaglauca populations increased at 9250 BP and Populus declined. At 6100 BP there is a remarkable shift from Piceaglauca woodland with Juniperus to a mixed spruce forest in which P. mariana was the dominant species. At 4100 BP conditions altered to favour P. glauca which expanded at the expense of P. mariana, and Juniperus again became important in the vegetation. By 1900 BP Pinuscontorta had become a dominant tree as P. mariana declined, the latter eventually disappearing from the local vegetation. There is no evidence that grasslands were more extensive in the early Holocene, as has been hypothesized for the southwestern Yukon. The vegetation sequence implies an initial period of aridity from 11 030 to 9250 BP, when summer warmth was probably greater than that of the modern climate, a period of increased effective moisture between 9250 and 6100 BP, when Piceaglauca was abundant, even greater effective moisture between 6100 and 4100 BP, when Piceamariana was the dominant forest tree, and then a prolonged period of increasing aridity beginning at 4100 BP and culminating in the development of the modern semi-arid climate.