U. S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 970 S. Lusk Street, Boise, ID, 83706, USA.
USDA Forest Service, Rocky Mountain Research Station, 1221 S. Main St., Moscow, ID, 83843, USA.
Am J Bot. 2019 Jul;106(7):922-934. doi: 10.1002/ajb2.1320. Epub 2019 Jul 11.
Physiological responses to temperature extremes are considered strong drivers of species' demographic responses to climate variability. Plants are typically classified as either avoiders or tolerators in their freezing-resistance mechanism, but a gradient of physiological-threshold freezing responses may exist among individuals of a species. Moreover, adaptive significance of physiological freezing responses is poorly characterized, particularly under warming conditions that relax selection on cold hardiness.
Freezing responses were measured in winter and again for new foliage in spring for 14 populations of Artemisia tridentata collected throughout its range and planted in a warm common garden. The relationships of the freezing responses to survival were evaluated in the warm garden and in two colder gardens.
Winter and spring freezing resistance were not correlated and appeared to be under differing selection regimes, as evident in correlations with different population climate of origin variables. All populations resisted considerably lower temperatures in winter than in spring, with populations from more continental climates showing narrower freezing safety margins (difference in temperatures at which ice-nucleation occurs and 50% reduction in chlorophyll fluorescence occurs) in spring. Populations with greater winter freezing resistance had lower survivorship in the warmest garden, while populations with greater spring freezing resistance had lower survivorship in a colder garden.
These survivorship patterns relative to physiological thresholds suggest excess freezing resistance may incur a survival cost that likely relates to a trade-off between carbon gain and freezing resistance during critical periods of moisture availability. This cost has implications for seed moved from cooler to warmer environments and for plants growing in warming environments.
生理对极端温度的响应被认为是物种对气候变异性的种群响应的主要驱动因素。植物在其抗冻机制中通常被分为避免者或耐受者，但在一个物种的个体中可能存在生理阈值冻结响应的梯度。此外，生理冻结响应的适应意义还描述不足，特别是在变暖条件下，对冷硬度的选择放松。
对在其整个范围内收集的 14 个三齿蒿种群的冬季和春季新叶进行了冻结响应测量，并在温暖的普通花园中进行了种植。在温暖的花园和两个较冷的花园中评估了冻结响应与存活率的关系。
冬季和春季的抗冻性没有相关性，而且似乎处于不同的选择机制下，这从与不同种群气候起源变量的相关性中可以明显看出。所有种群在冬季的抗冻能力都明显低于春季，来自更大陆性气候的种群在春季的冻结安全裕度更窄（冰核形成温度与叶绿素荧光降低 50%之间的差异）。冬季抗冻能力越强的种群在最温暖的花园中的存活率越低，而春季抗冻能力越强的种群在较冷的花园中的存活率越低。
相对于生理阈值的这些存活模式表明，过度的抗冻能力可能会产生生存成本，这可能与在关键水分可利用期内碳增益和抗冻能力之间的权衡有关。这种成本对从较凉爽环境转移到较温暖环境的种子以及在变暖环境中生长的植物都有影响。
