Recent seedling establishment at timberline in Jasper National Park, Alta.

Dendrochronological investigations of the age structure of timberline seedlings at three sites in Jasper National Park suggest that the timberline in these areas experienced two major periods of seedling establishment during the past 40 years: an early episode dating from the middle 1940's to the early 1950's and a later episode dating from the late 1960's to the early 1970's. Analysis of possible factors responsible for triggering these seedling invasions indicates a strong association with higher-than-average mean minimum summer temperatures and little association with mean summer precipitation.