The Composition of Wet-Heath Vegetation in Relation to the Water-Table

Abstract

Listen

Translate article

Introduction. The vegetation which is the subject of this paper is found on extensive areas of wet heath-land in south-eastern England. What will be called the central associes is a community dominated by Molinia caerulea and Calluna vulgaris, and in which Erica tetralix is usually abundant. The bases of dead Molinia shoots decay rather slowly in the wet habitat and thus there are built up tussocks of varying height, which are an important rooting medium both for the Molinia itself and for the Ericaceae. Under varying conditions any one of these plants may increase in importance at the expense of the other two. As would be expected, increase in Calluna is the usual accompaniment of transition to drier sites, and the change from wet-heath to Callunetum is commonly marked by the entry of Erica cinerea, Ulex minor and Deschampsia flexuosa. Relatively greater abundance of Erica tetralix is found in transitional stages between the central associes and valley-bog communities dominated by Sphagnum spp. In other wet situations Molinia assumes complete dominance. It has then a markedly tussocky habit, the tussocks reaching 30 cm. and sometimes much more in height; and Myrica gale is often associated with it. The wet Molinietum is frequently seen in the bottoms of sloping valleys cut into the sides of plateaux, and in other situations where the ground-water may be supposed to have appreciable flow. The association of Molinia with moving ground-water is well known; it was described on the Yorkshire moors by Jefferies (1915) and on Scottish moors by Fraser (1933). Transitions of the central associes to a rather drier type of Molinietum are also found. In this, tussock building is less pronounced, but there is no ciearly marked distinction between the drier and wetter types of Molinietum; they grade imperceptibly into one another. But whereas at the wet extreme the growth of pine and birch invaders is extremely stunted, and the development of woodland seems impossible, the drier Molinietum is readily colonized by vigorously growing Betula spp., especially B. pubescens. Molinia may also dominate considerable areas of dry heath. But this quite distinct community, in which there is no accumulation of dead shoot bases, and which includes