An investigation of the influence of drought and other soil factors on the structure of a calcareous grassland.

Previous experimental investigations (Grime & Curtis 1976; Hillier
1984), and long term climatic monitoring (Rorison et al 1986a,b) have
revealed the frequent occurrence of drought during the summer period in
calcareous grassland on the south-facing slopes of limestone dales in
the Peak District of North Derbyshire, Northern England. The main
objective of the studies in this thesis was to examine the role of
drought as a determinant of the characteristics of one selected
calcareous grassland system at Tongue End in Millersdale. Particular
attention has been given to the interaction of drought with soil
heterogeneity and mineral nutrient stress. Efforts have been made to
differentiate between mechanisms of drought tolerance and drought
avoidance, and this has allowed some assessment of the contribution of
diversity in morphology, life history and physiology to the maintenance
of species-richness in the vegetation at the site.
Three complementary methods of investigation were adopted; (1) analysis
of spatial patterns by grid sampling of the field site. (2) comparative
study of 17 component species in standardized environments. (3)
synthesis of plant communities in turf microcosms simulating certain
aspects of the Tongue End habitats. In the final chapter (Chapter 8) an
attempt is made to integrate results from the observations and
experiments described in previous chapters.
Evidence from the investigation suggests that on the south-facing slope
at Tongue End, floristic diversity is maintained by naturally
occurring phenomena, in which drought and low nutrient status interact
with soil depth and rock exposure. These factors are responsible for
high seedling mortality, gap creation and spatial heterogeneity. The
hypothesis is formulated that (1) the species -rich community
established on the south-facing slope at Tongue End contains an
assemblage of plants most of which are attuned to both regular
predictable disturbance and chronic nutrient-deficiency. (2) drought
interacts with the mosaic in soil depth and rock exposure within the
studied site; species with the potential to develop deep root-systems
are locally prominent in circumstances where there is access to
continuously moist subsoil.