Effects of simulated N deposition on understorey vegetation of a boreal coniferous forest

1. We report the short-term effects of simulated nitrogen (N) deposition on the understorey of a boreal forest in northern Sweden. Doses of (15)N double-labelled NH(4)NO(3) (0.5, 12.5, 25.0 and 50.0 kg N hd(-1)) were applied to 1 m(2) plots in the summer of 1995 and plants were harvested the following autumn. 2, No significant treatment effects were found in either above- or below-ground biomass which was distributed as follows: the ericaceous shrub Vaccinium myrtillus contributed 76%, the grass Deschampsia flexuosa 4%, and the bryophytes Dicranum majus and Pleurozium schreberi 20%. 3. The recovery of applied N in these species was 24, 27, 27 and 32% of the 0.5, 12.5, 25.0 and 50.0 kg N ha(-1) applications, respectively, and thus the recovery increased with the N dose. 4, In the 0.5 kg N hd(-1) treated plots, the highest concentrations of fertilizer-derived (15)N were found in the bryophytes, while in plots given the three higher N applications, leaves of D. flexuosa had the highest concentrations. 5. N application resulted in elevated concentrations of free amino acid N, which indicate increased storage of N in plant tissues. Furthermore, the N application resulted in increased damage to V. myrtillus by natural enemies. The severity of disease caused by two foliar parasitic fungi showed a clear numerical response to N treatment, as did the amount of herbivory as a result of Lepidoptera larvae.