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

具有讽刺意味的是，评估树木是否耐寒对于帮助管理的森林适应全球变暖至关重要，因为最有希望的适应策略之一，即范围内辅助迁移，依赖于使用从温暖气候进口的种子或幼苗种植本地物种。这一策略不仅假设南方移植的树木比当地树木更能适应不断上升的温度，而且还假设它们能耐受当前的条件，尤其是生长季节的霜冻。虽然移植实验表明，这对某些物种是正确的，但并非对所有被研究的物种都是正确的，而且被研究的物种太少，无法充分评估范围内辅助迁移的前景和风险。评估自然生长的树木的抗冻性也很重要，因为全球变暖将提前春季生长的开始，同时也会放大温度波动，使树木更容易受到霜冻损害，无论它们是否是移植的。如果树木在生长季节的一开始就开始生长，那么它们可能特别脆弱，这是一种必要的、但有风险的适应，以适应一个物种范围的北部极限附近的短生长季节。然而，目前还不确定的是，北方外围的种群是否比低纬度的种群更容易受到霜冻的影响，这凸显了研究自然生长的树木以及移植的必要性。显然，预测和减轻气候变化的影响需要更好地了解霜冻如何影响树木的生长和生存。为此，拟议的研究将评估移植物和自然建立的树木的抗冻性。本研究的三个主要目的是：1)利用新的移植试验，评估移植硬木的存活、生长和抗冻性；2)利用已有的移植试验，评价霜冻对移植针叶树长期生长的影响；3)利用国家树木年轮数据库评估霜冻对自然生长树木的影响。这项拟议中的研究将引起研究气候变化适应的科学家的广泛兴趣，因为它将检验有关辅助移民的基本假设。研究结果对整个社会也很有价值，包括自然资源管理者和决策者，他们需要了解气候变化和辅助移民将如何影响森林。最后，新的移植试验的结果将对林业部门产生经济利益，林业部门的造林战略必须适应全球变暖的影响。

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.