Monday, August 2, 2010 - 4:00 PM

COS 2-8: Yukon tree rings and winter warming: Effect of early freeze-thaw events on white spruce and lodgepole pine growth

Elizabeth A. Nelson and Sean C. Thomas. University of Toronto


Dramatically increasing temperatures across Canada’s north have resulted in much earlier thaw events, leaving northern forests vulnerable to freeze-thaw damage. Our research examines whether increased frequency of freeze-thaw events is linked to growth reductions in lodgepole pine (Pinus contorta) and white spruce (Picea glauca) in the Yukon Territory, Canada. Tree core samples were collected from 11 sites across the Yukon, covering a range of ecoregions, climate zones, and fire history, sampling all major forest communities accessible by road and located near the network of long-term weather stations. Over 50 tree cores from each site were sampled, analysed for ring-width, cross-dated and averaged to generate yearly ring growth at each site for each species. Ring growth was then compared against the frequency of freeze-thaw events, defined as days with above-freezing maximum temperatures and below-freezing minimum temperatures in the first seven months of the year. To determine the intensity of the freeze-thaw events, we calculated the daily temperature range  for each event and summed these values over the first seven months of the year. We also separated the freeze-thaw events into early thaw events and late frost events, and compared the frequency and intensity of these events against ring growth.


Our results indicate a positive relationship between ring growth and freeze-thaw events at most sites, with late frost events having the most significantly positive effect on forest growth. White spruce populations were more responsive to freeze-thaw parameters than co-occurring lodgepole pine populations, and were more likely to benefit from increased freeze-thaw frequency and intensity. These results suggest that critical temperature fluctuations during the winter and spring seasons may actually benefit hardy conifer species more than previously assumed. In conclusion, our results indicate that northern forests will suffer more from decreased late frost event frequency than they will benefit from increased early thaw frequency, therefore growth declines across the Yukon are likely to continue in response to warming conditions.