The vegetation of the South Burn Valley, Waldridge Fell, County Durham, was examined using an objective, quantitative phytosociological method. The vegetation units found to be present in the valley using this technique were then described and classified. Certain parameters relating to the chemical and physical environment of each vegetation unit were measured and variations in these factors used to account for the distribution of the vegetation units within the valley. The valley floor is broad and flat and situated below a series of spring lines. The majority of the area has a substrate composed of peat and the importance of water table height, ground water chemistry and tne degree of decomposition of this peat are emphasised as being the major environmental factors controlling the distribution of the main vegetation units of the valley floor. In a small area of the valley floor, which has willow car growing upon it, the substrate is composed of mineral soil. Here the chemistry of the water supplying the area and the nature of the substrate are important environmental factors. The valley sides are steep and composed of a sandy substrate and the dry nature and base-poor conditions of this habitat, seem to, in all probability account for the type of vegetation found growing on these sites. In a study of the population dynamics of the major tree species of the vegetation units, evidence was obtained showing that a change in tree species composition is taking place. Alder and Birch are the main dominants of the valley floor at present, but the evidence presented here suggests that these species are not regenerating and that ash is moving in to become the dominant tree of the valley floor. However, the small area of willow carr presently found in the valley will probably remain with such a species composition, but with a change in the predominant willow species. Further a small area of birch wood with a moss carpet ground layer appears to be losing its tree component and will probably change to a sphagnum bog. The tree composition of the valley sides also appears to be changing. On the fell side birch is being replaced by oak while, on the opposite side of the valley, sycamore is moving in to replace the birch. Reasons for these observed changes are given
