The influence of various ecological factors on the distribution of gortigolous lichens, in Horsley Hope ravine, near Consett, Co. Durham

Abstract

Rose, Hawksworth and Coppins 77recorded a rich, corticolous lichen flora from Horsley Hope Ravine, despite its position just S.W. of Consett - a major source of S02-pollution. This study attempts to explain the apparently anomalous presence of a luxuriant lichen vegetation in the ravine. Three possible explanations were considered:1. Prevailing winds transport S02 N.E., so that the entire area S.W. of Consett, including the ravine, is relatively unpolluted. 2. The ravine is protected from S02-pollution by its physiographical shape and/or woodland cover. 3. The variations of certain abiotic factors within the ravine account for its diverse lichen flora. Records of corticolous lichen frequency in the area indicate that: (a) the lichen vegetation is impoverished S.W. of Consett, and (b) the E. sides of trees (exposed to S02-laden winds) support a poorer lichen flora than the W. sides. It is thus concluded that the distribution of lichens S.W. of Consett is affected by S02-impaction, so that explanation 1. may be discounted. Measurements of S02, and other doiotic factors, showed that S02-concentrations, light intensities and substrate moisture-content varied significantly within Horsley Hope Ravine. Transects down the sides of the ravine revealed two distinct patterns of lichen distribution: (i) the variation in frequencies of species downslope, (ii) the consistent, preference of species for the upper or lower sides of trees. Pattern (i) was correlated with downslope changes in S02-levels and light; pattern (ii) was explained by variations of S02, light, and substrate moisture-content. Species similarly distributed according to these two patterns were grouped into four lichen units. A model was developed which showed that the luxuriant lichen flora, and distribution of these four units, in Horsley Hope Ravine may be explained by (in order of importance): reduced levels of S02, variations in light intensity, and substrate moisture-content. The decline in S02-levels in the ravine was ascribed to topographical shelter from S02-bearing winds.