Vegetation succession under developing birch woods
Birch readily colonizes heather moorland in the absence of regular burning, and successional changes in the dominance of various plant species take place beneath the developing woodland. Birch (unlike Calluna vulgaris and many conifers) has long been thought to have an ameliorating effect on soils. Changes such as this in association with other environmental changes accompanying the development of birch woodland can have complex and often interactive effects on the species involved in a succession of this sort. It is not known to what degree the changes in species dominance beneath developing birch can be attributed to soil changes or to other factors such as changing light levels, leaf litter, or grazing animals, for example. This thesis examines some of the species changes taking place beneath birch stands of different ages which have colonized heather moorland in north-east Scotland and attempts to elucidate some of the mechanisms controlling these species changes. (1) Examination of seed inputs revealed that this factor restricts the colonization of some, but not all, later successional species. (2) Germination of most species was unaffected by age of birch stand, but early growth of all species improved with increasing age of birch stand. (3) Results of experiments altering light intensity, nutrient availability and simulated grazing gave information on the relative importance of these factors in controlling the species changes beneath the birch stands studied. (4) The predictive ability of a Markov model in the simulation of succession under birch was shown to be limited; the use of this model did generate some interesting hypotheses about the nature of some of the processes of change operating in this successional system.