Effects of environmental change, including drought, on water use by competing Calluna vulgaris (heather) and Pteridium aquilinum (bracken)

1. Competition for water between Calluna vulgaris (heather) and Pteridium a quilinum (bracken) was studied in conditions of increased temperature, drou ght and increased nitrogen supply. All these factors increased the intensit y of competition for water, with the combination of drought and increased n itrogen having the greatest effect on water use. 2, Both species increased water-use efficiency in response to increased nit rogen and drought. The effects of temperature were however, equivocal. Call una had a greater water demand than Pteridium but acclimated to water stres s more readily. 3, Calluna was the superior competitor for water; its water-use efficiency was reduced as a consequence of its roots depleting water from the Pteridiu m rooting zone. Pteridium, the poorer competitor, increased water-use effic iency to cope with reduced water availability owing to competition. 4. There was a strong relationship between carbon isotope discrimination (D elta) and instantaneous water-use efficiency for both species, but discrimi nation provided a more sensitive measure of seasonal water-use efficiency. Reconstruction of the plant's history of wafer-use efficiency by retrospect ive measurement of Delta proved a useful technique for Calluna leaves but w as inappropriate for Pteridium rhizome.