A 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 a t the farm Havra, Osteroy, 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 ep iphytic vegetation was classified into seven TWINSPAN groups, and it i s shown by detrended correspondence analysis (DCA) that these groups a re mainly separated by the first two DCA axes. In an attempt to explai n the floristic composition, several environmental variables were meas ured, and the floristic and environmental data were analysed by canoni cal 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 pollar ded trees thus contribute greatly to the biodiversity of the old cultu ral landscape. Several epiphytic species take advantage of favourable light conditions on trunks in the open fields. At Havra, woodland has developed by tree colonization of old meadows and open fields during t he last 40 years after the cessation of earlier managements. Some flor istic differences between the epiphytic vegetation on trunks in the op en 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.