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

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Abstract
An area is described in which occur most of the communities characteristic of wet-heath vegetation in s.e. England. The "central" association is dominated by Molinia caerulea and Calluna vulgaris, with Erica tetralix very abundant. The dead roots and shoot bases of Molinia accumulate and form tussocks which are an important part of the rooting medium for all the plants. There are transitions to Callunetum, Sphagnum dominated valley bog, and Molinietum. Vegetation profiles and root systems are illustrated, and typical soil profiles described. On 150 plots, a quantitative description of the vegetation was made, the mean tussock height measured, and the water table depth determined at 2-week intervals for a number of years. The relation between these quantities was investigated statistically. The amount of Molinia was greatest in situations characterized by a fluctuating water table. Where the water-table fluctuated but little, the proportion of Molinia was low and that of Erica and Sphagnum spp. was high. Calluna was inversely related to tussock height; tussock height in its turn was positively related to water table fluctuation and negatively related to the mean summer depth of water table. Consequently a great part of the variation in composition of the vegetation could be related to water table depth and fluctuation. Water-table fluctuation is considered to be a reflection of soil permeability and ground-water movement, and hence possibly of oxidation-reduction conditions below the water-table, a zone of the soil freely permeated by roots of Molinia and to some extent also by Erica. There was evidence that the water table effects on any one species were partly direct and partly achieved through alteration of the vigor of its competitors.

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