Commercial forest fertilization causes long-term residual effects in ground vegetation of boreal forests

Due to an increasing demand of forestry products, nitrogen (N) fertilization of coniferous forests may in the future become a standard silvicultural practice in parts of the boreal region that is not too remotely situated. Beside positive effects on productivity, forest fertilization may induce changes in species composition of the ground vegetation. More information on the magnitude of such side effects is needed to evaluate whether commercial forest fertilization is in accordance with principles of sustainable forestry and preservation of biodiversity.We examined the effects of forest fertilization on ground vegetation in boreal forest by comparing species composition in unfertilized stands and stands fertilized twice (>20 years since the last fertilization) with 150 kg N ha−1 per event. By using stands that had been clear-felled and replanted 8–11 years ago, we addressed whether effects of fertilization persists over more than one forest generation. The ground vegetation in fertilized stands was denser, showed decreased species evenness, lower biodiversity, and higher Ellenberg N values. The abundance of dwarf shrubs was reduced by more than 40%, while grasses and some nitrophilous herbs increased by >100%. For bottom layer species, fertilization favoured litter-dwelling bryophytes (e.g. Brachythecium spp.), while Hylocomium splendens and ground living lichens were disfavoured.For the dwarf shrub Vaccinium myrtillus, fertilization reduced the density of current annual shoots by >60% and berry production by >70%. Fertilization had, however, no effect on berries produced per annual shoot. Leaf tissue N concentrations were higher in V. myrtillus, and Deschampsia flexuosa growing in previously fertilized stands, indicating N enrichment although more than 20 years had past since the last fertilization event.Because residual effects of commercial forest fertilization on forest ground vegetation appear to be substantial and extend over more than one forest generation, we question whether forest fertilization, as performed today in the Nordic countries, can be considered a long-term sustainable silvicultural practice that is in accordance with principles of preservation of biodiversity.