An investigation into the impact of environmental change upon the vegetation of Widdybank Fell, Upper Teesdale
Widdybank Fell is located in the Upper Teesdale National Nature Reserve in the Northern Pennines. It is internationally renowned for its unique late-glacial relict assemblage of plant species, particularly those on the "sugar limestone” outcrops. Cow Green Reservoir was constructed in the early 1970s and flooded the lower western slopes of Widdybank Fell. At the time there was concern amongst ecologists that the reservoir might alter the local climate and have adverse effects on the remaining vegetation. A comprehensive vegetation survey of Widdybank Fell was carried out by Jones (1973). Aspects of this survey were repeated as part of the present study. There have been significant changes in the composition of some of the plant communities since the 1970s. These changes include a considerable loss of bryophyte diversity and lichen abundance. Overall there has been a decline in "stress tolerant" species, and a loss of calcicole species on the calcareous grasslands. Few of the nationally rare plant species have changed in abundance. Using data from the meteorological stations at Widdybank Fell and from nearby Moor House, it has been demonstrated that the presence of Cow Green Reservoir has resulted in significant changes in the local climate. These changes are consistent with those expected by a classic "lake effect" and include the all year round moderation of minima (e.g. resulting in a reduction in the number of ground frosts). The reservoir has also produced cooler mean air temperatures in spring and warmer mean temperatures in autumn. Despite the observed local climate impact of Cow Green Reservoir it seems most likely that other factors have been responsible for the observed vegetation changes. Acid deposition has probably caused the loss of calcicoles on the calcareous grasslands, and atmospheric nitrogen deposition could explain the loss of bryophyte diversity and lichen abundance.