Assessing intraspecific variation in frost tolerance of northern tree species: implications for assisted migration and climate change

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Ironically, assessing whether trees are frost tolerant is critical for helping managed forests adapt to global warming, because one of the most promising adaptation strategies, within-range assisted migration, relies on planting native species using seeds or seedlings that have been imported from warmer climates. This strategy not only assumes that southern transplants will be better adapted to rising temperatures than local trees, but also that they are tolerant of current conditions, particularly growing-season frost. While transplant experiments show that this is true of some species, it is not true of all the species that have been studied, and too few species have been studied to fully assess both the promise and risks of within-range assisted migration. Assessing the frost tolerance of naturally established trees is also critical because global warming will advance the onset of spring growth while also amplifying temperature swings, making trees more vulnerable to frost damage, regardless of whether they are transplants or not. Trees may be particularly vulnerable if they initiate growth at the very beginning of the growing season, as a necessary, but risky adaptation to short growing seasons near the northern limit of a species' range. However, it remains uncertain whether peripheral northern populations are generally more vulnerable to frost than lower-latitude populations, highlighting the need to study naturally established trees as well as transplants. Clearly, predicting and mitigating the impact of climate change requires a better understanding of how frost influences the growth and survival of trees. Toward this end, the proposed research will assess the frost tolerance of transplants and naturally established trees. The three main objectives of the research are: 1) to assess the survival, growth and frost tolerance of transplanted hardwoods, using new transplant experiments; 2) to assess the impact of frost on the long-term growth of a transplanted conifer, using an existing transplant experiment; 3) to assess the impact of frost on naturally established trees, using a national tree-ring database. The proposed research will be of broad interest to scientists studying climate change adaptation because it will test fundamental assumptions about assisted migration. The results will also be valuable to society at large, including natural resource managers and policy makers who need to know how climate change and assisted migration will impact forests. Finally, the results of the new transplant experiments will be of economic interest to the forest sector, whose silvicultural strategies must adjust to the impacts of global warming.