Do admixed broadleaves improve foliar nutrient status of conifer tree crops?

We investigated three Douglas fir-red alder stands in Oregon, USA, and five Norway spruce-European beech stands in Bavaria, Germany, to test the hypothesis that admixtures of broadleaved trees improve nutritional status of conifers. At each site needle samples from 20 to 30 conifer trees with varying proportions of broadleaves adjacent to the sample trees were taken and analysed for needle mass and the macronutrients N, P, K, Ca and Mg. The neighbourhood of each tree was described by the proportion of deciduous basal area within an 8 m circle. Ordination methods (principal component analysis; redundancy analysis) were used to test the dependency of the multivariate nutritional pattern in conifers on deciduous neighbours and rank correlation and regression were used to analyse bivariate relationships between tree species composition and descriptors of needle status. The statistical analyses yielded no evidence that deciduous admixtures improve foliar nutrition of conifers. Alongside with other empirical studies this shows that beneficial effects of broadleave admixtures on conifer nutrition are less common than postulated. Future research investigating the whole causal path relevant for tree nutrition and growth is necessary to improve our knowledge in the complex field of nutrition of mixed species forests. (C) 2002 Elsevier Science B.V. All rights reserved.