How Species Dominance and Water Table Depth Modulate the Responses of Bog Vegetation to Simulated Nitrogen Deposition?

English Abstract:

Increased atmospheric nitrogen (N) deposition results in changes in the competitive ability of plant species, most importantly between Sphagnum mosses and vascular plants, in nutrient-poor peatlands. Our long-term experiments (10-15 years) examine the responses of the Mer Bleue bog to simulated N deposition with and without phosphorus (P) and potassium (K) addition. We analyzed patterns of vegetation change in response to N deposition focusing on interactions of species composition, water table level and cumulative nutrient doses. We asked how the species composition and water table depth affected the vegetation response to N addition and whether there are temporal and directional differences in responses between the N only and NPK treatments. The data were collected in 2000−2014 in a Sphagnum capillifolium and dwarf shrub dominated site. The applied N doses were 0, 1.6, 3.2, and 6.4 g N m^-2 yr^-1, with 5 and 6.3 g P and K m^-2 yr^-1, respectively, in the NPK treatments.  We have monitored water table depths relative to the moss surface in wells and the species abundance by counting point intercept hits in 60 × 60 cm sub-plots within each triplicate 3 × 3 m treatment plot. Based on a multivariate analysis of the 2014 species data, the main environmental gradients underlying the current species distribution were water table depth and cumulative N addition. The abundance of mosses decreased clearly with increasing cumulative N dose, while the overall vascular plant abundance did not differ markedly between control and fertilization treatments. The abundance of the most common vascular species, Chamaedaphe calyculata, increased with increasing water table depth and N dose. We will present more detailed analyzes of the species distribution relative to temporal, water table and nutrient addition gradients, because the long-term data showed that species abundances and the magnitude of treatment effects varied over the study periods.