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

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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.