Reversion of forest to tundra in the central Yukon

Pollen and plant macrofossil analyses of sediments from 3 sites that are presently in shrub tundra provide a record of former forest establishment. Shrub tundra with groves and gallery forests of balsam poplar Populus balsamea occupied the region between 10 000-8000 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 S-facing slopes and on alluvial sites with balsam poplar, and black spruce on colder, wetter sites on N-facing slopes and valley bottoms. At 5000 BP forest began to revert to shrub tundra, abruptly then more gradually. The modern groves of (mostly white) spruce are probably relict populations surving in favorable microsites. The ground vegetation apparently behaved independently of the tree populations. -from Author