Print PageThe Intergovernmental Panel on Climate Change predicts global air temperature to increase 3-6° C in the 21st Century, most notably within polar regions. In response to this warming, it is widely predicted that the northern range limit of boreal forest tree species will shift northward. Increasing temperature alone, however, may not facilitate a biome-wide northward migration. For instance, local adaptation to environmental variables, such as photoperiod, that are not expected to change with climate warming, may act to inhibit northward migration at ecological timescales. A clear tradeoff between growing season length (determined by photoperiodic cues) and cold tolerance has been established in this species. The aim was to identify the extent to which local adaptation benefits or limits the colonization success of genotypes of balsam poplar (Populus balsamifera) into northern environments under present and future temperature scenarios. 150 seedlings, representing 75 genotype pairs (clones), of balsam poplar were planted in a common garden in Alaska at 64.8º N. Source populations were chosen from every 5º latitude between 55 -75º N. For each genotype pair, one seedling was artificially warmed in an open-top chamber. Vertical and diameter growth, the change in the number of lateral buds and leaves, as well as three growth components (growing season length, photosynthetic rate, and insect herbivore damage for all seedlings) were measured. Linear mixed effect models were used to test for effects of latitude and warming on the dependent growth variables.
Results/Conclusions
There was a significant effect of source latitude on seedling growth (and all growth components except herbivory) indicating that southern genotypes perform better than northern genotypes in northern environments. Warmed seedlings had a significantly longer growing season length than control seedlings, caused primarily by delayed bud set in the fall. The warming treatment also increased overall height growth, number of leaves, and photosynthetic rates for both northern and southern seedlings, suggesting that balsam poplar would grow better at northern latitudes in a warmer climate. This is in contrast to other boreal-forest trees, such as white and black spruce, which are believed to be near their upper-temperature thresholds. A path analysis to discriminate between direct and indirect effects of source latitude and warming treatment is underway. This study shows that adaptation to longer growing season lengths does not inhibit the establishment of southern populations of balsam poplar in northern environments, and balsam poplar could likely migrate northward of current treeline in a warmer climate.
