Deep winter frost, causing severe bark necroses, and insect defoliation are two of the causal factors for the present oak damages in northern Germany. In earlier investigations, a majority of oak stands had shown high leaf nitrogen concentrations. Therefore, the effect of nitrogen status and of insect defoliation on the frost hardiness of the bark of adult oaks was tested. At several dates during winter, samples from the living inner bark tissue were taken from adult sessile (Quercus petraea [Matt] Liebl) and pedunculate oaks (Q robur L) i) with normal or elevated leaf nitrogen concentrations, and ii) defoliated or nondefoliated in the preceding spring. Frost hardiness of bark was determined by electrolyte leakage after artificial freezing in the laboratory. During frost periods in January and February, oaks with lowered CIN ratios in bark or leaves as well as defoliated trees tended to reduced frost hardiness. Although the differences were insignificant for some temperature treatments, it is concluded that the effect of winter frost on oak damage is enhanced by a supply of excess nitrogen and by preceding insect defoliation.
