A comparison of plant responses to the extreme drought of 1995 in northern England

1 In an investigation of the impact of the extreme drought of 1995, measurements of the relative water content (RWC) of leaves were conducted on a wide range of herbaceous species, distributed over a vegetation mosaic coinciding with local gradients in soil depth in a limestone dale at Burton in northern England. Variation in leaf RWC was analysed in relation to soil depth, soil water potential and species composition of the vegetation at the sampled sites. 2 Despite the unusual severity of the drought, low RWC was confined to the vegetation of shallow daleside soils and rock outcrops where turgor remained conspicuously higher in tap-rooted forbs such as Sanguisorba minor and Pimpinella saxifraga. Over the sampled area as a whole, it was apparent that desiccation-avoidance through continued access to subsoil moisture was more important as a controller of leaf RWC than restriction of transpiration. 3 Under the extreme conditions of 1995 differences in water relations became apparent between several pairs of congeners and morphologically similar species co-existing in the same communities. 4 In species with distributions concentrated on the deeper soils of the plateau and valley bottom, a consistent correlation was established between declining frequency of occurrence and falling values in RWC. This relationship was due to the failure of outlying populations on shallow soil to maintain turgor and suggested that extreme moisture stress was acting as a resetting mechanism restricting the excursion of drought-sensitive species on to the areas of shallow soil. 5 With the exception of a small number of tap-rooted plants, species associated with the shallow soils and rock outcrops showed a negative relationship between abundance in communities and leaf RWC; this supports the hypothesis that areas of shallow soil are suboptimal for such species and merely provide a refuge from the competitive effects of the more robust species occupying the deeper soils.