Responses of epiphytic lichens to an experimental whole-tree nitrogen-deposition gradient
New Phytologist: 2010 188:1075-1084
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.
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