Oligotrophie in einer eutrophen Umwelt: Zur synsystematischen Umgrenzung und standörtlichen Gefährdung sand-oligotropher Quellfluren im Teutoburger Wald (NW-Deutschland)

Abstract

POLLMANN, W.; LETHMATE, J.: Oligotrophy in an eutrophic environment: On the synsystematical definition and nature conservation of sand-oligotrophic spring vegetation in the Teutoburger Wald, NW Germany. – Hercynia N.F. 35 (2002): 157-179.The phytosociology and habitat ecology of spring vegetation were investigated in the Riesenbecker Osning (NW Germany). Concerning nutrient support and chemical water characteristics, most spring vegetation belongs to oligotrophic and weak productive communities. In the territory four communities were distinguished; Caricetum fuscae, Juncus acutiflorus-community, Molinia caerulea-Betula pubescens-community and Carici elongatae-Alnetum, and their characteristics were described. In the weakly buffered and atmospheric polluted Riesenbecker Osning, this study fills the gap in the contemporary treatment of defining vegetation types, indicating atmospheric nitrogen eutrophication and endangering of amphibic zones in relation to critical loads. The sand-oligotrophic Caricetum fuscae contains several differential species together with the Molinia caerulea-Betula pubescens-community, and, for the first time, can be differentiated as the subassociation of Polytrichum commune in Northwest Germany. The Juncus acutiflorus-unit describes a dominant community in the Caricion alliance that lacks several Sphagnum species. Molinia-Betula swamp forests rich in bog mosses are established when draining has created sites for tree growth. In contrast to this the establishment of alder (Alnus glutinosa) forests is dependent on soils that are characterized by a higher nutrient content of the groundwater and a higher pH-value. Generally in mesotrophic habitats, alder forests are found in a Sphagnum-rich community with Carex acutiformis. Chemical analyses of spring waters evaluated low pH-values in combination with high loads of nitrogen, sulfate, light and heavy metals (e.g., 29.3 mg NO3 - l-1, 38.1 mg SO4 2- l-1, 5.6 mg Al3+ l-1, 110.0 μg Pb2+ l-1). A decline in the oligotrophic vegetation and an increase in more nitrogen-dependent species have been observed. Accordingly, a loss in species diversity could be shown, and Sphagnum species have been replaced by more nitrophilous species. Depression of the groundwater level and melioration, and eutrophication of the habitats seem to be the main causes that endanger the Caricion-communities. The Caricetum fuscae polytrichetosum rich in Sphagnum species that is growing on flooded, acid and peaty sites is especially endangered by raised atmospheric nitrogen deposition. Losses in diversity, documented for spring vegetation in South Germany after acidification and atmospheric pollution, could also be observed in the Riesenbecker Osning towards uniform dominant communities. In the Riesenbecker Osning, this study of sand-oligotrophic habitats forms the basis for a monitoring system for a spatial evaluation of environmental impacts in relation to critical loads. Comparative studies in exactly localised sites may document future changes in water chemistry by changes in species composition.