The effect of increased deposition of atmospheric nitrogen on Calluna vulgaris in upland Britain

Regular (monthly) additions of NH4NO3 (4-12 g N m(-2) yr(-1)) were made ove r a period of 8 vr (1989-98) to areas of moorland in North Wales dominated by the ericaceous shrub Calluna vulgaris. Results from the early stages of the experiment (1990-94) have shown marked and dose-related increases in sh oot extension and canopy height in response to the nitrogen treatments, wit h significantly higher shoot nitrogen contents. The nitrogen-related stimul ation in the growth of the C. vulgaris canopy over this period has resulted in large accumulations of litter on the high-nitrogen-treated plots (6.0 k g m(-2) in plots treated with 12 g N m(-2) yr(-1), compared with 3.8 kg m(- 2) for the water controls). Litter nitrogen concentrations were also signif icantly increased at the higher rates of nitrogen addition, leading to a do ubling of the total return of nitrogen to the litter layer over the experim ental period. These changes in vegetation structure were associated with la rge reductions in the abundance of the bryophyte and lichen species normall y present under the untreated canopy. Results since 1994, however, show lit tle increase in shoot extension in response to the nitrogen treatments, wit h no clear dose response to increasing levels of addition. These findings a re associated with a dose-related increase in the susceptibility of the nit rogen-treated areas of the C. vulgaris canopy to late winter injury, charac terized as browning of the shoot tips in early to late spring. These result s indicate that deleterious effects are non accumulating as a result of the long-term addition of nitrogen to these moorland plots.