IS THE PRESENCE OF MESOTROPHIC PLANT-COMMUNITIES IN THE PEAT-BOGS OF HAUTES-FAGNES (BELGIUM) CONNECTED WITH TONALITE INTRUSIONS IN THE REVINIAN LAYERS

Within the Hautes-Fagnes Nature Reserve (Belgium) acidic and oligotrophic biotopes are developing on very acidified stony silt raised bogs. However, they are locally dominated by mesotrophic species. Based on the fact that there are several outcrops of tonalite in the region and particularly one in the Nature Reserve, a hypothesis to explain these singularities of the vegetation implies discontinuities of the bedrock provoked by intrusions of this magmatic rock in the Revinian layer covering the region. Seventeen plots containing Phragmites australis were examined and vegetation was described. Silt and soil water were sampled and analyzed for mineral contents in 15 sites. The differences of mineral composition between the P. australis plots and their surrounding environment confirmed the more mesotrophic characteristics of these plots (i.e. higher pH, higher Ca and Mg contents, lower Al content). However, the lower Zn, Pb and Fe contents would lead to reject the hypothesis of intrusions of tonalite since these elements are more abundant in the tonalite and its mineralization than in the Revinian rock. Nevertheless, the overall ecological conditions of the examined plots and the differences of plant species composition suggested that the observed chemical environment could be evoked for explaining the distribution of the mesotrophic species in the oligotrophic environment of the Nature Reserve. Consequently, an alternative hypothesis is proposed.