Slow recovery of boreal forest ecosystem following decreased nitrogen input

1. Ecosystem recovery after decreased input of nitrogen was examined in two different fertilization experiments where the fertilization had been termi nated for 9 and 47 years, respectively. 2. The species composition of the understorey vegetation showed no signs of recovery 9 years after the fertilization was terminated. Increased sporoca rp production of mycorrhizal fungi was seen on formerly fertilized plots co mpared with plots still receiving N, but the species composition showed lar ge differences compared to control plots. 3. In the second experiment, examined 47 years after termination of fertili zation, N favoured bryophytes such as Brachythecium reflexum (Starke) Schim p., Plagiothecium denticulatum (Hedw.) Schimp. and the leaf-parasitic fungu s Valdensia heterodoxa Peyronel (attacking Vaccinium myrtillus L.) was more abundant in the formerly N-treated plots than in controls. The abundance o f Hylocomium splendens (Hedw.) Schimp., the most common bryophyte under nor mal N conditions, showed a contrasting pattern, with less abundance in the formerly N-treated plots than in controls. Sporocarp production of N-sensit ive mycorrhizal fungi was lower in the formerly N-treated plots. No differe nce in plant species composition was noticed for vascular plants. 4. These results contrast with other studies that have interpreted reduced N leakage and nutrient levels in trees after decreased N input as a rapid e cosystem recovery. The present study suggests that the time needed for reco very of the ecosystem biota may be substantial in originally N-limited ecos ystems.