Long-term responses of understory vegetation in boreal Scots pine stands after nitrogen fertilization

Nitrogen fertilization can increase above- and belowground forest growth and carbon storage in low nitrogen (N) environments. However, it may also induce changes in other parts of the ecosystem, such as altered composition and diversity of the ground vegetation. These changes may occur, for example, because of increased availability of nitrogen and light depletion due to a denser tree canopy. We studied vegetation changes at 11 experimental sites in Pinus sylvestris stands with low N-deposition, along a south-north gradient in Sweden. We estimated the relative cover of individual species and the data were analysed with a linear model, using total amount of fertilizer-N added, years since last fertilization, site and site index as independent variables. The relative cover of the dominant dwarf shrubs (Vaccinium myrtillus and Vaccinium vitis-idaea) increased following fertilization. In the bottom layer, N significantly increased the total cover of two of the three dominant species (Pleurozium schreberi and Dicranum spp.). For the third species (Hylocomium splendens) no detectable effect was found. For lichens as a group, the cover decreased following N fertilization. No effect of N fertilization on species diversity was detected at any of the sites, and the forest vegetation types remained unchanged.