Responses of epiphytic lichens to an experimental whole-tree nitrogen-deposition gradient

P>Here, we examined the responses of the epiphytic lichens Alectoria sarmentosa and Platismatia glauca to increased atmospheric nitrogen (N) deposition in an old-growth boreal spruce forest, to assess the sensitivity of these species to N and define their critical N load. Nitrogen deposition was simulated by irrigating 15 trees over a 3 yr period with water and isotopically labeled NH4NO3, providing N loads ranging from ambient to 50 kg N ha-1 yr-1. Thallus N concentration increased in both species with increasing N load, and uptake rates of both NH4+ and NO3- were similar. Photobiont concentration increased linearly with increased N in both species, saturating in A. sarmentosa in the third year at the highest N loads (25 and 50 kg ha-1 yr-1). The simulated N deposition decreased the phosphorus (P) concentration in A. sarmentosa, and increased the N : P ratio in both species. Significant responses in lichen chemistry were detected to inputs of 12.5 kg N ha-1 yr-1 or higher, suggesting that resources other than N limit lichens at higher N loads. However, the data also suggest that N saturation may be cumulative over time, even at low N.