Climate Change in the Canadian Boreal Forest: The Effect of Warming, Frost Events, Cloud Cover and CO2 Fertilization on Conifer Tree Rings

Anthropogenic climate change is expected to dramatically affect boreal forests, not only through warming effects, but through changes in seasonal and diurnal temperature patterns, precipitation, cloud-cover, and direct effects of rising CO2. My doctoral research examines the impact of these changes on dominant boreal forest conifer species, using dendrochronological methods. Through my analysis of white spruce (Picea glauca) and lodgepole pine (Pinus contorta) tree rings across five Yukon Territory sites, I found that white spruce growth is showing growth declines in response to all three measured climate changes, with negative correlations between tree ring increment and spring and summer temperature, spring frost events, and growing season cloud cover. Lodgepole pine populations exhibited growth enhancement with increasing spring maximum daily temperatures, but generally neutral responses to warming summers, higher frost event frequency and increased cloud cover. To evaluate the effect of rising CO2 on boreal forest growth, I examined three representative managed forest stands across Canada, first building a model of climate effects, and examining temporal trends in the residual growth patterns. I found evidence for CO2 fertilization in Ontario black spruce (Picea mariana) and Manitoba white spruce populations, particularly at younger ages, but no growth enhancement in Yukon lodgepole pine. These results taken together suggest that Yukon white spruce may suffer pronounced growth declines under continued climate change, but more eastern spruce populations may be better able to benefit from increased carbon availability. Yukon lodgepole pine populations are less vulnerable to the impacts of climate change, but are also unlikely to exhibit significant growth increases in response to increasing temperature, frost events, cloud cover or rising CO2. The results from this thesis have important implications for future management of the Canadian boreal forest under progressive climate change.Ph