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.