The Response of Grasslands on Sugar-Limestone in Teesdale to Application of Phosphorus and Nitrogen

Abstract

For over a century the hills at the head of Teesdale in the northern Pennines have been noted for their unusual vegetation and, in particular, for the remarkable concentration of rare plant species. Many of these, such as Carex capillarist and C. ericetorum, Dryas octopetala, Gentiana verna, Helianthemum canum, Kobresia simpliciuscula, Minuartia stricta, Polygala amara, Tofieldia pusilla and Viola rupestris, occur at about 500 m above sea-level on outcrops of metamorphosed rock known as 'sugar-limestone'. They are localized in their distribution on these outcrops, some being confined to Cronkley Fell in Yorkshire and some to Widdybank Fell in Durham, only a few being found on both. A distinctive feature of the outcrops of sugar-limestone is the patchiness of their vegetation, there being numerous bare areas of various sizes (Pigott 1956). This is clearly a result of erosion by wind over the drier parts and, more locally, of the erosional widening of the beds of streamlets and the cutting back of springs at their heads. The soils of the outcrops are in consequence very shallow rendzinas, either well drained and dry, or flushed and permanently moist; and they often contain high concentrations of lead, barium and fluoride associated with metamorphism of the limestone. The grassy cdver of the outcrops is grazed by sheep and was formerly grazed by rabbits also; the sward is very often short as well as being rather sparse and broken by many bare patches. The opportunity to conduct experiments on the vegetation of the sugar-limestone arose in 1967 when construction of the dam above Cauldron Snout began. This, after completion in 1971, now impounds the headwaters of the Tees and submerges the lowest part of the outcrops on the west slope of Widdybank Fell. The aim of the experiments was to discover whether the shortness and patchiness of the turf can be explained wholly by the combination of heavy sheep-grazing with erosion by wind and water, or whether chemical features of the soil play an important contributory part. It was hoped further to see whether at least some of the rare species might depend for their survival on this shortness and patchiness of the vegetation.