Bark lesions and sensitivity to frost in beech and Norway spruce

Strong temperature fluctuations and pathogen attacks can cause bark lesions, visible signs of wounds in the phloem and cambium. Pathogenic insects and fungi are able to invade lesions that have not healed, further increasing the damage. The ability of the tree to withstand climatic and biotic stress can be lowered by air pollution, especially N deposition, with subsequent alterations in the soil chemistry and in the nutrient status of the tree. It was concluded that air pollution, as indicated by epiphytic lichens on the tree stem, and excess nitrogen affected the frequency of beech bark lesions. The frost sensitivity of both beech and spruce bark was related to the concentrations of nutrients in the trees. A low C/N ratio and low concentrations of Cu, K and Zn in the mineral soil were related to high frost sensitivity in beech bark. Countermeasures, such as liming and vitality fertilization, had some positive effects on the bark chemistry. A tendency towards fewer bark lesions on beech treated with lime was found, and at one of three Norway spruce sites treated with a vitality fertilizer the treated trees were less sensitive to frost than the control trees. When measuring the sensitivity of bark to frost, influences of soil treatment and site specific conditions can be hidden by a stronger influence from the freezing treatment, depending on the hardiness status of the bark. The smaller the variation in concentration of an element, the more difficult it is to detect any influence upon the frost sensitivity.