Cryptogams and dwarf evergreen shrubs are vulnerable to nitrogen addition in a boreal permafrost peatland of Northeast China

Aim Boreal peatlands are very sensitive to nitrogen (N) enrichment due to the low soil N availability. Our aim was to reveal how increased N availability altered plant composition in boreal permafrost peatlands with dense vascular plants (>50%). Location Greater Khingan Mountains, Northeast China. Methods A nitrogen addition experiment with four N addition levels (0, 3, 6, and 12 g N m(-2) year(-1)) was conducted in a poor fen in the continuous permafrost zone with vascular plant coverage above 60%. Plant diversity, community composition, and above-ground biomass were investigated after four, six, and eight years of N addition. Results Nitrogen addition decreased species richness and diversity but enhanced total above-ground biomass. Plant height determined the plants' responses to N addition, and the magnitudes increased with N addition level and experimental duration. The relative coverage of high-stature (>50 cm) species increased, but the relative coverage of low-stature (<50 cm) species decreased with elevating N addition levels. When N addition proceeded, the relative coverage and above-ground biomass of tall deciduous shrubs and nitrophilous grasses increased, whereas the relative coverage and above-ground biomass of dwarf evergreen shrubs, mosses, and lichens declined. Regardless of N addition levels, cryptogams (i.e., mosses and lichens) and a dwarf evergreen shrub (Chamaedaphne calyculata) disappeared after eight-year N addition. Conclusions Cryptogams and dwarf evergreen shrubs are highly vulnerable to increased N availability, and N enrichment-induced biodiversity loss, especially the disappearance of cryptogams, will deteriorate ecosystem structure and function in boreal permafrost peatlands.