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

气候变化正日益影响全球森林的活力。然而，目前的研究主要集中在夏季干旱日益严重和频繁导致的森林生长下降和树木死亡，而冬季气候变化对森林健康的影响研究甚少。尽管冬季变暖，但预计北温带地区土壤霜冻事件的规模和频率将增加，这是由于积雪减少导致土壤对冷空气的绝缘性降低。在温带阔叶林中，土壤霜冻严重程度的增加会增加细根的死亡率，改变树木内部的碳和养分循环以及碳分配，阻碍树木的生长。然而，冬季寒冷敏感性对森林生长动态的大规模影响及其背后的生理机制都没有得到充分的了解。我推测，除了夏季干旱和炎热之外，冬季中期由于积雪减少而导致的土壤冷却可能是温带树木在温暖世界中的另一个压力源，并且这种压力源主要通过细根系统对冬季低土壤温度的敏感性起作用。该项目通过将土壤冷却实验与三种常见且分布广泛的欧洲阔叶树（欧洲山毛榉（Fagus sylvatica））、白桦（Betula pendula）和无梗橡树（Quercus petaea）的树木年轮研究相结合来验证这些假设。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（特别是极端低温、寒冷和积雪持续时间）如何影响树木生长，以及这种影响在三个物种之间、在欧洲不同地区之间以及在上个世纪温暖和寒冷的几十年之间有何不同。为了了解树木生长对冬季寒冷敏感性的机制，并检测当地对冬季寒冷条件的适应，我们将在室外和气候室设施中对三种树种的冷边缘和中心源种群的树苗进行冬季不同土壤温度制度的土壤温度操纵实验。在接下来的生长季节，将调查幼树对冬季土壤温度处理的地上和地下反应（生长和死亡率、根系氮吸收能力、根系呼吸），并与源种群的遗传构成有关。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，并评估树木对冬季寒冷的适应潜力。此外，整个欧洲的风险区域将被划定，其中由于冬季气候变化，预计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.