UNSATURATED ZONE PORE-WATER CHEMISTRY AND THE EDGE EFFECT IN A BEECH FOREST IN SOUTHERN ENGLAND

Twenty one pore water chemistry profiles were obtained for a range of inorganic solutes from the Chalk unsaturated zone in or adjacent to Bl ack Wood, a 2.4 km(2) mature beech wood in southern England. The depth sampled was normally 10 m, but some boreholes were shallower and one was deeper (30 m). Towards the centre of the wood, average pore water concentrations were: Cl (17-25 mg 1(-1)), SO4 (20-40 mg 1(-1)) and NO3 -N (5-10 mg 1). In small clearings within the wood, concentrations of Cl (12-20 mg 1(-1)) and SO4 (27-36 mg 1(-1)) were somewhat lower but t he average concentration of nitrate-N was higher (16 mg 1(-1)). Pore w ater chloride and nitrate concentrations under a small area of ash wer e lower than under the beech. There was a significant increase in the concentration of a number of solutes, especially Cl, Na, Mg and SO4, c lose to the exposed western edge of Black Wood. This 'edge effect' dec reased exponentially with a half distance of about 9 m. The effect was less consistent at the more sheltered eastern edge. Average pore wate r concentrations of up to 310 mg 1(-1) Cl and 312 mg 1(-1) SO4 were fo und at the western edge. Paradoxically, close to the western edge pore water nitrate concentrations were often very low, frequently less tha n 1 mg 1(-1) NO3-N. Using the parameters derived from a simple exponen tial model of the Black Wood data, calculations suggested that the edg e effect would lead to significantly enhanced Cl and SO4 pore water co ncentrations in forests of a few hectares, a size typical of many of t hose currently being planted. The consistently lower moisture content of the Chalk close to the forest edges suggested that groundwater rech arge may have been lower there.