REVERSION OF FOREST TO TUNDRA IN THE CENTRAL YUKON

Pollen and plant macrofossil analyses of sediments from three sites in the central Yukon that are presently in shrub tundra provide a record of former forest establishment. Shrub tundra with groves and gallery forests of balsam poplar occupied the region between 10 000 and 8000 BP. At 9400 BP white spruce (Picea glauca) populations expanded, and open white spruce woodlands persisted until 6500 BP when black spruce (Picea mariana) and green alder (Alnus viridis) populations increased, resulting in open spruce woodlands with a distribution of species probably similar to that commonly found today in the northern boreal forest: white spruce on drier south-facing slopes and on alluvial sites with balsam poplar, and black spruce on colder, wetter sites on north-facing slopes and valley bottoms. At 5000 BP forest began to revert to shrub tundra, abruptly at first but then more gradually. The modern groves of spruce in the region, which are mostly white spruce, therefore are probably relict populations surviving in favorable microsites. These results support the conclusion from other recent studies that Alaska and northwest Canada experienced warmer summers than today from as early as 10 000 BP to almost-equal-to 6000 BP in response to increased summer insolation arising from changes in the earth's orbital parameters as predicted by the Astronomic Theory of climate change. The decline of forest after 5000 BP is not associated with any significant changes in the abundance of shrub or herb pollen types, suggesting that the ground vegetation has behaved independently of the tree populations.