EFFECTS OF SIMULATED N DEPOSITION ON UNDERSTOREY VEGETATION OF A BOREAL CONIFEROUS FOREST

1. We report the short-term effects of simulated nitrogen (N) depositi on on the understorey of a boreal forest in northern Sweden. Doses of N-15 double-labelled NH4NO3 (0.5, 12.5, 25.0 and 50.0 kg N hd(-1)) wer e applied to 1 m(2) plots in the summer of 1995 and plants were harves ted the following autumn. 2, No significant treatment effects were fou nd in either above- or below-ground biomass which was distributed as f ollows: the ericaceous shrub Vaccinium myrtillus contributed 76%, the grass Deschampsia flexuosa 4%, and the bryophytes Dicranum majus and P leurozium 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) ap plications, respectively, and thus the recovery increased with the N d ose. 4, In the 0.5 kg N hd(-1) treated plots, the highest concentratio ns of fertilizer-derived N-15 were found in the bryophytes, while in p lots given the three higher N applications, leaves of D. flexuosa had the highest concentrations. 5. N application resulted in elevated conc entrations of free amino acid N, which indicate increased storage of N in plant tissues. Furthermore, the N application resulted in increase d damage to V. myrtillus by natural enemies. The severity of disease c aused by two foliar parasitic fungi showed a clear numerical response to N treatment, as did the amount of herbivory as a result of Lepidopt era larvae.