THE EFFECT OF EXCESS NITROGEN AND OF INSECT DEFOLIATION ON THE FROST HARDINESS OF BARK TISSUE OF ADULT OAKS

Deep winter frost, causing severe bark necroses, and insect defoliatio n are two of the causal factors for the present oak damages in norther n Germany. In earlier investigations, a majority of oak stands had sho wn high leaf nitrogen concentrations. Therefore, the effect of nitroge n 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 p etraea [Matt] Liebl) and pedunculate oaks (Q robur L) i) with normal o r elevated leaf nitrogen concentrations, and ii) defoliated or nondefo liated in the preceding spring. Frost hardiness of bark was determined by electrolyte leakage after artificial freezing in the laboratory. D uring frost periods in January and February, oaks with lowered CIN rat ios in bark or leaves as well as defoliated trees tended to reduced fr ost hardiness. Although the differences were insignificant for some te mperature treatments, it is concluded that the effect of winter frost on oak damage is enhanced by a supply of excess nitrogen and by preced ing insect defoliation.