Study area - The Meuse district
The Meuse district (Mosan) has an altitude fluctuating between 200 and 300 m and enjoys a better air quality than the previous districts. It shelters an exceptional lichen flora owing to the numerous natural rock outcrops that are to be found in the main valleys, especially along the Meuse, Viroin, Lesse and Ourthe. Those calcareous rocks are usually very compact and date back to the Devonian or Carboniferous. The lichen flora associated with the Xerobromion communities show strong submediterranean affinities, and many species are at the northern limit of their distribution area. Other outcrops, covering a smaller surface, are markedly siliceous and also of high interest. The lichen forest flora is rather trivial, with a few exceptions linked to the best preserved fragments with old trees. Trees along roads are locally very interesting, although air pollution and especially the removal of Ulmus have strongly altered the lichen flora. Semi-naturals habitats like the Mesobromion communities and heathlands are now very reduced but nevertheless still shelter an interesting lichen flora.