Control of relative abundance of perennials in chalk grassland in southern England. I. Constancy of rank order and results of pot- and field-experiments on the role of interference

(1) A study of the relative abundance of perennial species in chalk grassland in southern England revealed great constancy $(P < 0.001)$ in rank order: (a) across any one site (twenty stands of 0.48 m2in an area of 3480 m2), (b) from year to year (1980-82) and (c) between two sites 150 km apart. Comparison of these records with transformed data from phytosociological accounts in the literature provides evidence of high constancy of rank order over much larger areas and longer periods. Ancient chalk grassland is thus an ideal community for investigation of the mechanisms determining the relative abundance of species. (2) Two experiments were carried out to determine the extent to which the order of relative abundance may be explained by short-term (1-2 years) ability to interfere with growth of plants of other species in the vegetative phase. (3) In an outdoor pot experiment, fourteen dicotyledons were grown in monocultures and in mixtures with one or two standard species (a grass and a dicotyledon). After 18 weeks there was a significant positive correlation between interference ability in the experiment and mean relative abundance in the field. Essentially, the same results were found after 70 weeks. (4) In a field experiment, six dicotyledons were grown in monocultures, and in all possible mixtures of two species, in plots on an area of chalk grassland cleared of turf. After 62 weeks, the order in interference effects paralleled the order of relative abundance in the grassland. There was also a positive correlation between interference ability and the mean height of rosette leaves of each species. This suggests the importance of above-ground interference. (5) After 115 weeks, and for 3 years afterwards, additional elements of the perennial life-history became important in determining performance in the plots, especially longevity of adult individuals, degree of lateral spread and the frequency of establishment of seedlings. (6) We suggest that the control of relative abundance in chalk grassland involves steady-state relationships: (i) between species with taller (and more widely-spreading) and shorter individuals and (ii) between species with longer- and shorter-lived individuals. The balance between species is largely determined by the frequency and timing of grazing and by the frequency of regeneration from seed.