Winter cold sensitivity of European temperate broadleaf forest trees in the face of climate warming: Spatial patterns, mechanisms and effects of genotype

气候变化正越来越多地影响全球森林的活力。然而，目前的研究大多集中在夏季干旱日益严重和频繁导致的森林生长衰退和树木死亡，而关于冬季气候变化对森林健康影响的研究则少之又少。尽管冬季变暖，但在北部温带地区，土壤霜冻事件的规模和频率预计将增加，这是因为土壤对积雪减少造成的冷空气的隔热程度降低。在温带阔叶林中，土壤霜冻程度的增加会增加细根的死亡率，改变碳和养分循环以及树木内部的碳分配，并阻碍树干的生长。然而，冬季寒冷敏感性对森林生长动态的大范围影响及其背后的生理机制还没有得到充分的了解。我推测，除了夏季干旱和炎热之外，由于积雪减少而导致的仲冬土壤降温可能是温暖世界中温带树木的另一个压力源，这种压力源主要是通过细小的根系对冬季土壤温度较低的敏感性而起作用的。该项目将土壤降温实验与树轮研究相结合，对三种常见的、分布广泛的欧洲阔叶树欧洲山毛榉、白桦树和无柄栎树进行了验证。将探索和扩展现有的树木年轮数据库，以调查冬季天气特征(特别是低温极端天气、寒冷总量、积雪持续时间)如何影响树木生长，以及这种影响在三个物种之间、欧洲地区之间以及上个世纪较暖和较冷几十年之间的差异。为了了解树木生长对冬季寒冷敏感的机制，并检测当地对冬季寒冷条件的适应，将在室外和气候室设施中进行冬季不同土壤温度状况下的土壤温度操纵实验，树苗来自三个树种的冷缘种群和中心源种群。幼树对冬季土壤温度处理的地上和地下响应(生长和死亡速率、根N吸收能力、根呼吸)将在接下来的生长季进行调查，并与源种群的遗传结构有关。该项目将更好地了解树木生长对冬季寒冷的敏感性及其潜在机制，并评估对冬季寒冷的适应潜力。此外，欧洲各地的风险区将被划定，预计由于冬季气候变化，森林生长动态将发生变化。

Climate change is increasingly affecting the vitality of forests worldwide. However, current research mostly focuses on forest growth decline and tree mortality due to increasing severity and frequency of summer droughts, while the influence of winter climate change on forest health has been studied much less. Despite warmer winters, the magnitude and frequency of soil frost events are projected to increase in northern temperate regions due to reduced insulation of the soil against cold air caused by declining snow cover. In temperate broadleaf forests, increased soil frost severity can increase the mortality of fine roots, alter carbon and nutrient cycling as well as carbon partitioning within trees, and hamper tree stem growth. Yet, both the large-scale consequences for forest growth dynamics of winter cold sensitivity and the physiological mechanisms behind it are not sufficiently understood. I hypothesize that soil cooling in the middle of winter due to declining snow cover may represent an additional stressor to temperate trees in a warmer world besides summer drought and heat and that this stressor mainly acts through the sensitivity of the fine root system to low winter soil temperatures. This project tests these hypotheses by combining a soil cooling experiment with tree-ring studies on the three common and widely distributed European broadleaf trees European beech (Fagus sylvatica), silver birch (Betula pendula), and sessile oak (Quercus petraea). Existing tree-ring databases will be explored and extended, in order to investigate how winter weather characteristics (in particular low temperature extremes, cold sums, snow cover duration) influence tree growth and how this influence differs between the three species, between regions in Europe, and between warmer and colder decades of the last century. To understand the mechanisms explaining winter cold sensitivity of tree growth and to detect local adaptation to winter cold conditions, a soil temperature manipulation experiment with different soil temperature regimes during winter will be conducted outdoors and in a climate chamber facility with saplings from cold-marginal and central source populations of the three tree species. The above- and belowground response (growth and mortality rates, root N uptake capacity, root respiration) of the saplings to the winter soil temperature treatment will be investigated during the following growing season and related to the genetic constitution of the source populations. The project will deliver a better understanding of winter cold sensitivity of tree growth and its underlying mechanisms and assess the adaptation potential to winter cold. Further, risk areas across Europe will be delimitated, in which changing forest growth dynamics are to be expected due to winter climate change.