Impact of the admixture of European beech (Fagus sylvatica L.) on plant species diver- sity and naturalness of conifer stands in Lower Saxony Auswirkungen der Einbringung von Buche (Fagus sylvatica L.) auf die Artendiversität und Naturnähe von Nadelholzbeständen in Niedersachsen

The promotion and extension of continuous cover mixed stands with a simultaneous reduction of conifer-monocultures play a major role in current silvicultural practices in Central Europe. It is assumed that the admixture of the natural dominant beech (Fagus sylvatica) in pure non site-specific conifer stands automatically indicates better conditions in terms of nature conservation and forest management. To test this hypothesis three different conifer-beech-comparisons of pure and mixed stands in Lower Saxony are studied, analysing plant species diversity and naturalness of understorey vegetation as one important indicator for the ecological status of forests. Each comparison includes pure coniferous stands (Picea abies, Pinus sylvestris, Pseudotsuga menziesii), mixed coniferous-beech-stands, and pure beech stands on similar acidic mineral soils where the potential natural vegetation will be an oligotrophic beech forest ( LuzuloFagetum ). The age of stands varies between 50 and 150 years. To specify tree species influence on site conditions and vegetation, the study also includes light climate and soil data of the stands. It is observed that, with regard to all comparisons, the admixture of beech reduces plant species diversity but increases naturalness of the stands. The intensity of beech admixture effects differs. While in Scots pine stands the impact of admixed beech is very noticeable, with the mixed stands being nearly identical with pure beech stands, the species change in Douglas-fir and Norway spruce stands proceeds more slowly. Assuming that the status in nature conservation and forest management is improving with increasing plant species diversity and increasing naturalness, the results of this study show a contrary development on a stand scale, as the potential natural vegetation of the L u z u l o - F a g e t u m is in its self very species poor on vascular plants.