A Chalk Flora on the Lower Greensand: Its Use in Interpreting the Calcicole Habit

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Abstract
A small area, referred to as the calcareous sand, was found on the Lower Greensand in Surrey, within 1 km. of the chalk, bearing a herbaceous flora closely resembling that of chalk grassland. Only 8 of the 45 spp. of constancy 3 or over, recorded by Tansley & Adamson for chalk grassland, were absent from this flora. The soil contained chalk intimately mixed with the sand; it was probably scattered on the land for agricultural purposes more than 50 years ago. An adjacent area on typical acid greensand soil, bearing a vegetation of the same general type as the calcareous sand, and subject to more or less similar physiographic and biotic conditions, was studied in order to use it as a rough "control experiment." The soils of the calcareous sand and acid sand areas hardly differ significantly in mechanical analysis fractions; they are both quite distinct from chalk grassland soil in this respect, but the calcareous sand is similar to it in pH and the possession of free chalk. Therefore comparison of the flora of the calcareous sand with that of chalk grassland on the one hand, and of the acid sand (supplemented by other greensand records) on the other, affords a means of studying the respective importance of physical and chemical soil factors in determining the calcicole and calcifuge habit in a few of the species of the chalk and of the greensand respectively. For the vast majority of chalk grassland species the factors determining the calcicole habit are evidently chemical. This is in agreement with the view commonly held. It has been possible to group some of the species into the following classes: (1) Calcicoles determined by chemical soil factors such as Avena pubescens, Bromus erectus, Helianthemum vulgare; (2) Calcifuges determined by chemical soil factors: e.g., Calluna vulgaris, Deschampsia flexuosa; and (3) Calcifuges determined by physical soil factors: e.g., Plantago coronopus, Saxifraga tridactylites. The available data do not make it possible to consider the very hypothetical class of "physically determined calcicoles." Avena pratensis is the most important chalk grassland species absent from the calcareous sand. A simple wilting exp. indicated that for grassland vegetation drought is a more serious problem on the Folkestone beds on the Lower Greensand than it is in similar situations on the chalk.

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