quantitative study of the epiphytic vegetation on pollarded trunks of Fraxinus excelsior at Havra, Osteroy, western Norway

The epiphytic vegetation on 19 pollarded trees of Fraxinus excelsior at the farm Havrå, Osterøy, western Norway was investigated. Each trunk was divided into a basal zone, a middle zone and a top zone. In each zone the four different aspects were analysed by squares (12 sampling units from each trunk). Within a total of 225 sampling units, 173 taxa were recorded (84 lichens, 72 bryophytes, 17 vascular plants). The epiphytic vegetation was classified into seven TWINSPAN groups, and it is shown by detrended correspondence analysis (DCA) that these groups are mainly separated by the first two DCA axes. In an attempt to explain the floristic composition, several environmental variables were measured, and the floristic and environmental data were analysed by canonical correspondence analysis (CCA). A pollarded tree, depending on its age, represents a continuum of perhaps several hundred years. Some of the recorded epiphytic lichens are very rare in Norway, and the pollarded trees thus contribute greatly to the biodiversity of the old cultural landscape. Several epiphytic species take advantage of favourable light conditions on trunks in the open fields. At Havrå, woodland has developed by tree colonization of old meadows and open fields during the last 40 years after the cessation of earlier managements. Some floristic differences between the epiphytic vegetation on trunks in the open fields and in the woodland are found. This suggests a change in the epiphytic vegetation because of the creation of a more shady habitat in parts of the area during a few decades.