Effects of Experimental Interference with Stand Structure on Establishment of Seedlings in Callunetum

Most Callunetum in Britain is plagioclimax vegetation that has developed from the former forest over many centuries (Dimbleby 1962; Gimingham 1964; Anderson 1967). Current burning and grazing practices in the uplands tend to prevent re-invasion by trees. However, even when not burnt or grazed, most upland Callunetum reverts to woodland very slowly. Previous studies (Miles 1973b, 1974) suggested that lack of available seed of potential invaders and soil infertility both contributed to this stability. Other studies (Miles 1972, 1973a) also showed that seedling establishment within undisturbed Callunetum was very poor compared with that on ground, suggesting that features of the stand structure further influence stability. The occurrence of very low seedling densities in vegetation has often been noted (Tamm 1956; Sagar & Harper 1960; Cavers & Harper 1967; Putwain, Machin & Harper 1968; Miles 1973a). Milthorpe (1961) stated that 'It is reasonably certain that the establishment of plants from seed in vegetation occurs only in bare areas arising from the death of previous occupants or from incomplete coverage'. The aim of this study was therefore to examine the effects on establishment from seed of two simple kinds of experimental interference with the structure of Callunetum. The first involved the successive removal of different layers of the vegetation, and the second the creation of different sized patches of soil.