CORRELATIONS BETWEEN NUTRIENT STATUS AND DEGREE OF FOLIATION IN MATURE OAK STANDS OF NORTHWESTERN GERMANY

In the summer of 1992, foliar nutrient concentrations and the surplus of inorganic cations compared to inorganic anions (as a measure of car boxylate concentrations) were determined in trees with various degrees of foliation in 18 mature stands of sessile (Quercus petraea [MATT.] LIEBL.) or pedunculate oak (Q. robur L.) in northwestern Germany where increased leaf loss connected with damage to oaks had occurred in the eighties. In sessile oaks, a weak but significantly positive correlat ion was found between the degree of foliation and the leaf-N concentra tions; whereas the degree of foliation did not correlate with the foli ar concentrations of other nutrients. The vast majority of the sessile oaks showed, at least in tendency, an inadequate supply of P. In pedu nculate oaks, high leaf-N concentrations were detected even in several poorly foliated trees. In this species, the foliar Mg concentrations, which were within the range of Mg deficiency in about one quarter of all trees, correlated weakly but significantly with the degrees of fol iation. P deficiency was not as widespread as in sessile oak. In neith er species was K deficiency observed. The foliar concentrations of sol uble inorganic N compounds (NH4+ and NO3-) were negligible and amounte d, at maximum, to approx. 1% of the total N concentration in both case s. The mean foliar N concentrations of all stands investigated were si gnificantly correlated with the average bulk N deposition rates of the preceding years (from 1985 to 1990) measured in the vicinity of the s tands. The results indicate that in the stands investigated, the forma tion of an increased leaf area is promoted by higher leaf-N concentrat ions in the sessile oak; whereas a low Mg availability may be a limiti ng factor the production of leaf biomass in about one quarter of the p edunculate oaks. Comparisons of foliar carboxylate concentrations with bulk deposition rates and literature data provided no indications of an exhaustion of the neutralization capacity for protons resulting fro m assimilation of NH4+ taken up by the leaves from the atmosphere.