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, me asurements of the relative water content (RWC) of leaves were conducte d on a wide range of herbaceous species, distributed over a vegetation mosaic coinciding with local gradients in soil depth in a limestone d ale at Burton in northern England. Variation in leaf RWC was analysed in relation to soil depth, soil water potential and species compositio n of the vegetation at the sampled sites. 2 Despite the unusual severi ty of the drought, low RWC was confined to the vegetation of shallow d aleside soils and rock outcrops where turgor remained conspicuously hi gher in tap-rooted forbs such as Sanguisorba minor and Pimpinella saxi fraga. Over the sampled area as a whole, it was apparent that desiccat ion-avoidance through continued access to subsoil moisture was more im portant 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 morphologicall y similar species co-existing in the same communities. 4 In species wi th distributions concentrated on the deeper soils of the plateau and v alley bottom, a consistent correlation was established between declini ng frequency of occurrence and falling values in RWC. This relationshi p was due to the failure of outlying populations on shallow soil to ma intain 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 sma ll number of tap-rooted plants, species associated with the shallow so ils 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 r efuge from the competitive effects of the more robust species occupyin g the deeper soils.