A comparison between primary grasslands and abandoned quarries of the magnesian limestone, county Durham, with special reference to scrub invasion

Abstract

Seven grassland Sites of Special Scientific Interest on the Magnesian Limestone of County Durham were botanically surveyed using the methodology of the National Vegetation Classification. Both primary grasslands and grassland developed in abandoned quarries were included, but the sampling was limited to open calcicolous grassland and scrub-invaded grassland, with areas of impenetrable scrub and woodland being avoided. Characteristic of these grasslands is blue moor grass, Sesleria albicans, which reaches its southern lowland limit in County Durham. The results were analysed using multivariate techniques to reveal patterns in community composition and possible environmental factors causing this variation. Six main communities are described, comprising both grassland and scrub vegetation. A successional sequence linking these communities and their sub-communities is postulated. It is suggested that abandoned quarries are floristically different from primary grasslands, but may achieve similarity in time if there is a nearby seed source of S. albicans. Scrub invasion, mainly by hawthorn, Crataegus monogyna, is ubiquitous in both the primary and secondary grasslands and presents the major threat to the integrity of the limestone flora. The botanical survey reveals a significant change to the flora under scrub, with some of the low-growing calcicoles being partially replaced by tall nutrient-demanding grasses and herbaceous species. However, the invasion of willow (Salix spp.) at one of the abandoned quarries has had negligible impact on the ground flora. The two most common scrub species, hawthorn and dog rose (Rosa canina agg.) appear to have more influence on the ground flora than any other species. It is suggested that the age of these species is more important than density in altering the ground flora. The results of this investigation have been used to make tentative suggestions for management of the sites.