Forest Tree Population Response to Climate Variability and Climate Change: Implications for Reforestation

This dissertation aims to quantify the sensitivity and adaptive capacity of forest tree species to climate change in Canada and North America, with applications of guiding sustainable forest management through case studies focusing on Alberta. The general idea is that management interventions should focus on ecosystems, species or populations that are most likely to experience stress or mortality, or alternatively to focus on new forest management opportunities associated with warming climate conditions at high latitudes. The research addresses several specific problems at different spatial scales. The research starts at the broadest scale, for North America, with a remote-sensing based vulnerability assessment of forest ecosystems to historical droughts. One of the most striking findings was a very high spatial diversity of vegetation response to historical climate variability. Broad continental patterns of vegetation response are readily apparent and conform to expectations, with southern interior ecosystems being limited by water availability and boreal populations limited by short growing seasons. However, within these broad geographic trends in growth response, finer scale (and often contradictory) response patterns emerge. For example, across the western boreal forest local patches show populations restricted in growth by warm summer temperatures and drought. Next, a species-specific analysis investigates the adaptive capacity of an important boreal forest tree species, white spruce, using dendrochronological analysis. Results showed evidence for population differentiation in resistance and recovery parameters, but provenances conformed to approximately the same growth rates under drought conditions and had similar resilience metrics. The lack of populations with better growth rates under drought conditions is contrary to expectations for a wide-ranging species with distinct regional climate and we provide a counter example for drought tolerance traits, where assisted migration prescriptions may be ineffective to mitigate climate change impacts. An analysis of population response in the context of climatic conditions across Canada supports the view that northeastern Canada will provide a refugium for white spruce under climate change, while the species is sensitive to growth reductions under climate change in the western boreal. In a case study for Alberta, a comprehensive series of genetic test plantations was analyzed to determine the optimal climate niche of selected planting stock. The results suggest that seed transfers can improve growth in some cases. However, the climate change vector does not always align with geographic gradients, which makes finding well adapted seed sources difficult or impossible. This issue may partially be addressed by relying on additional silvicultural adaptation options to address climate change. The case study provides a methodological template of how jurisdictions can determine feasibility as well as magnitude and direction of assisted migration prescriptions to adapt their reforestation programs to new planting environments. When assisted migration is used to address climate change, tree seedlings may have to be moved to substantially colder environments in anticipation of climate warming over their life span. To assess the risk associated with moving planting stock north or to higher elevation, a climatic assessment of frost risk associated with assisted migration was performed. The results indicate that late spring and early fall frost risks do not change significantly for transfers toward the north. In contrast, moving planting stock toward higher elevation generally leads to a substantial increase in exposure to unseasonal frosts. In conclusion, transfers toward the north are preferable to transfers up in elevation in reforestation for the most important commercial tree species in western Canada that were evaluated in this study. Lastly, the research explores how results may be translated to management prescriptions through an on-line seed selection tool for forest managers to identify the overall best planting stock for a reforestation site, synthesizing multiple criteria including vulnerability, adaptive capacity and growth of species and genotypes.

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