The respective ecological amplitudes of the two British tree species of Betula are known only imperfectly. B. pendula is better represented on drier, lighter soils and is more of a southern, eastern and lowland species than B. pubescens, which belongs rather to the wetter, colder conditions of north, west and upland Britain. The contribution of the two species to different types of woodland community is briefly reviewed in this paper, and consideration is given to the status of ‘pure’ birchwoods. In some cases, the latter may have been derived from other woodland types; in others, by invasion of heath or grassland. However, some relict birchwoods above the altitudinal limits of dominance by other trees, and in the extreme north and west of Scotland, may be ‘natural’ or ‘climax’ woods.
Birches show a number of characteristics of ‘r-selected’ plants and hence tend to be trees of secondary or seral woodlands. Factors determining the capacity of birch species to colonize bare ground or vegetation such as heath and grassland, are reviewed, with special reference to a study of invasion of Calluna heath at Dinnet, Aberdeenshire. The importance of the development of gaps in the Calluna canopy, or alternatively of fire, in providing suitable sites for establishment, is emphasized.
Changes in the ground vegetation following the establishment of birch stands are outlined, as well as the subsequent development of seral birchwoods. Attention is drawn to the fact that many upland birchwoods are becoming moribund, and to the effects of grazing and other influences in preventing regeneration. The possibility of reversing this trend is discussed.