(1) On release in 1936 from intensive grazing by rabbits the biotic potential of Grassland A emerges over 38 years in a sequence of physiognomic dominants-Festuca ovina, Hieracium pilosella, Thymus drucei-and the entry of seventeen species additional to the original eleven in the permanent plot. The relationships of the dominants to each other, to the subordinate species and of both to weather factors is discussed and used in the interpretation of the phenomena of change. (2) Quantitative data on floristic composition were obtained from a permanent plot (10 x 160 cm) inside an enclosure (6 x 6 m) by the use of a grid of 1024 squares each with side of 1.27 cm, charted annually near the beginning of July. (3) Changes in the abundance of species are compared with meteorological data for ground frost (December to March), frost-heaving and rainfall (September, October, November), and with data for germination (May and June) and for survival in spring-all factors found to be important during charting of the plot at all times of the year from 1961 to 1969, and now applied to the interpretation of the graphs for the whole period 1936-73. Of similar significance is the competitive power of the dominants, in which emphasis is placed on population structure and behaviour. (4) On the basis of the relative importance of the weather factors and period of physiognomic dominance a sequence of four regimes is recognized and analysed. The Festuca regime (1936-49) had an all-aged mature population of Festuca ovina, distributed over the plot but covering less than half of it. Both the dominant and subordinate species were influenced by the wide range of weather factors, particularly by the combination of winter frost and spring drought. In the Hieracium regime (1950-58) H. pilosella formed a rapidly developing almost even-aged population distributed over the whole plot. It showed a smooth curve of rise and fall without response to a wide range in weather factors. At the high competitive power of its building and mature phases, other species diminished rapidly, remained few or were absent, but the same set of species later increased as degeneration was followed by surface soil erosion. The Interregnum (1959-62) was marked by spring drought, disruption of H. pilosella and much erosion. The dominants, H. pilosella and Festuca ovina were redistributed but the total cover remained low. Species sensitive to drought were few or absent, but larger perennials increased. During the Interregnum Thymus drucei established and spread locally. Later, the Thymus regime (1963-73) began at the onset of higher rainfall as T. drucei spread and developed quickly and unevenly, forming an uneven-aged population. While cover was still low the other species increased: on further spread of T. drucel the other species diminished. (5) The leached residues and debris from frost-heaving form a mobile surface-soil layer whose distribution, thickness and stability are closely linked with the phasic distribution of the dominant.
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A. S. Watt (1981) studied this question.