The nitrogen status of oak stands in northern Germany and its role in oak decline

Abstract

The supply of excess nitrogen is considered to be a predisposing factor to the present decline of oak in northern Germany. Therefore, an investigation was started on the nitrogen status of various oak stands affected by decline. In the summer of 1992, samples of healthy leaves from both healthy and damaged oaks were taken from 18 stands of adult pedunculate and sessile oaks. In the coastal plain, where the bulk N deposition is rather high (> 15 kg ha^-1 a^-1), soil solutions from two adjacent stands of pedunculate oak differing in the degree of crown damage were sampled. In the winter 1992/93, the frost hardiness of bark tissue sampled from sessile oaks differing in leaf N content was determined.

In most stands, the N concentrations of the leaves from healthy oaks were elevated as compared to literature data (> 27 mg g^-1 D. M.). In a considerable number of the stands, also the ratios N/P, N/Mg and N/K were increased. Only in few stands, significant differences between leaves from healthy and from damaged trees were found. The NO₃ ^- concentrations of the soil solutions were high and, in the declining stand, did not decrease distinctly with soil depth. In January and February, the frost hardiness of bark tissue from oaks with high leaf N content tended to be lower, but the differences were significant in only one case. The results of the analyses of leaves and soil solutions indicate that most of the forest ecosystems investigated are approaching the stage of N saturation. The lack of distinct differences between healthy and damaged oaks, however, leads to the hypothesis that the actual outbreak of oak decline is due to other factors like frost, drought or insect attacks.