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.