An Ecophysiological Approach to Quantifying Nitrogen Fixation by Lobaria oregana

Lobaria oregana is an epiphytic macrolichen associated with old-growth Douglas-fir forests in the Pacific Northwest. Nitrogen fixation by this often-abundant cyanolichen provides an ecologically significant input of new N to the forest ecosystem. This study estimates annual N2 fixation by L. oregana using a model based on physiological field measurements and laboratory experiments. Meteorological data from the Wind River Canopy Crane site and the H. J. Andrews Experimental Forest are used to calculate annual N2 fixation rates, assuming that hydration and temperature are the two parameters controlling nitrogenase activity. At the crane site, estimated annual N2 fixation is 1.5 kg ha−1. In the H. J. Andrews Experimental Forest, L. oregana may fix 2.6–16.5 kg N2 ha−1 yr−1 depending on its stand-level canopy biomass. The model's predictions are checked by using published growth rates and standing L. oregana biomass estimates to calculate independent values for annual N2 fixation at each site.