A quantitative study of gap recolonization in two contrasted limestone grasslands
Gaps differing in size, form and season of origin (September 1978 and March 1979) were created artificially in adjacent north- and south-facing grasslands in North Derbyshire. Detailed measurements revealed marked differences between the two sites in patterns of vegetative regrowth and regeneration from seed. Specific effects of gap type and time of origin were detected on both slopes. Creation of gaps exerted pronounced effects upon the relative abundance of established components of the north-facing turf. Accumulations of bryophytes were considerably less after three years than in undisturbed turf and Avenula pratensis tended to be reduced in comparison with potentially faster-growing species such as Plantago lanceolata and Holcus lanatus. Effects of comparable magnitude were not induced on the southfacing slope and it is suggested that this may be related to the high level of disturbance normally experienced by the vegetation on this slope. On the north-facing slope where seedlings are comparatively scarce in undisturbed turf, gap creation promoted spring germination in species which maintain relatively persistent seed reserves at or near the soil surface. In marked contrast on the south-facing slope, large numbers of seedlings appeared in the autumn in both gaps and control areas. Such patterns are consistent with previous laboratory studies which revealed differences in germination requirements between the species concerned. On the south-facing slope seedling survival was often highest in areas where resprouting vegetative shoots were relatively dense. However, the benefit of this apparent 'shelter effect' rarely extended to seedling size which tended to decline in close proximity to vigorous vegetative shoots. In the concluding section of the thesis, the main results of the investigation are reviewed in relation to the influence of aspect upon vegetation structure, current theories on the determinants of 'species richness' and opportunities to control species composition by more informed grassland management.