1. The pattern of epiphyte communities on birch was examined at five sites in north-west Scotland. 2. The age structure of the birchwood at the main study site (Bettyhill) is described and growth rates of birch at all five sites are compared. The historical background of these woodlands is also discussed (Chapter 2). 3. Methods of assessing epiphyte cover are evaluated and a point sampling technique is proposed as being the most suitable for comparing communities on all ages of bark surface (Chapter 4). 4. Epiphyte communities were classified according to James et al (1977) and the ground vegetation related to the classification of McVean & Ratcliffe (1962). 5. Variation in epiphyte communities within a site is primarily determined by age of bark surface, and consequently can be related to age of the whole tree. It is also affected by the age of stand, exposure and density of the trees. 6. The age-determined pattern is divergent, in so far as the same pioneer species occur at all sites but there are marked differences at the later stages. The largest differences between sites are in the communities found towards the base of old trees. 7. The effect of height in the canopy on epiphyte communities is most marked at an exposed site with high rainfall (Skye), where age is of relatively lesser importance than at other sites. The effect of height is least at a sheltered site with high rainfall, and age here accounts for a high proportion of the variation in epiphytic vegetation. 8. The upwards extension of basal bryophyte communities on the trunk places the sites in the same order as their total annual rainfall. 9. Foliose Parmeliaceae, especially Hypogymnia physodes, are generally infrequent and restricted to sheltered trees at the west coast sites (Loch a'Mhuilinn and Rhegreanoch). This is possibly due to the effects of strong winds causing rapid dehydration in the upper trunk and crown, and heavy, often wind-driven rain, impairing establishment from soredia. 10. Variation in the field layer, like that of epiphytes, is affected by maturation of the trees. It also demonstrates an age-related sequence, which is not however divergent since middle-aged stands (30 to 50 years) show greater uniformity than young stands. 11. The pattern of variation in microclimate at the bark surface was examined at Bettyhill. Light intensity, vapour pressure deficit and rate of evaporation all tend to increase with height in the canopy (Chapter 8). 12. Variation in bark characters was investigated (Chapter 7). pH decreases with age of bark surface, and independently can be shown to increase with height above the ground. It is lower on trees in exposed situations. Bark moisture-holding capacity was not related to age or height above the ground of bark surface. 13. The assimilation response of several lichens (including Hypogymnia physodes and Parmelia sulcata) to different light and moisture levels was examined by means of infra-red gas analysis. The results of this technique revealed that light compensation points were related, to the light intensities in which they grow in the field. Seasonal variation in light response may be related to the presence of a leafy canopy, and the moisture optima for assimilation may vary in relation to monthly rainfall totals. 14 The course of epiphyte succession is primarily determined allogenically by the growth of the host trees and their effect on microclimate at the bark surface. This is borne out by the fact that macroclimatic differences between sites cause variation in epiphyte communities. Inherent change in the properties of the bark may also play a part. 15. An autogenic effect of the epiphytes on their community dynamics was investigated in terms of competition (by photography of permanent quadrats) - 16, their effects on properties of the bark - 17, and by artificially removing epiphytes from the trunk - 18. 16. Larger species with their margins farther away from the bark surface can grow over adpressed foliose lichens, which in turn grow over crustose species. Under favourably moist conditions the faster growing bryophytes often take over. 17. Certain species may modify the texture or pH of the bark surface making establishment of others easier than on bare bark. 18. Species characteristic of an earlier stage occasionally invade rapidly on removal of the epiphytes, but the usual trend is for the removed community to become re-established.