Canopy epiphytes, including bryophytes and lichen, harbor cyanobacteria that function as an important nitrogen (N) source in N-limited Pacific Northwest (PNW) forests supporting forest productivity by supplying stand-level N in mid to late succession forests. In environments that experience little or no N deposition, cyanobacteria associated with canopy epiphytes fix substantial quantities of N2. However, cyanobacteria cease to fix N2 in the presence of increasing rates of N deposition. Increasing atmospheric pollution associated with the growing transportation sector in the PNW threatens to uncouple the intricate community relationships in the canopy and the role they play in N cycling. This study evaluated how urbanization affects seasonal N2 fixation rates and cyanobacteria presence in Isothecium myosurorides (Bird) in Acer macrophyllum stands in Western WA. Seasonal sampling was conduced in A. macrophyllum trees in the Hoh Rainforest, WA and in Seattle, WA. Single rope climbing techniques were used to access the entire crown and replicated moss core samples were collected at four heights to assess differences in N2 fixation levels throughout the canopy due to different rates of N and metal throughfall deposition, temperature, and moisture levels.
Seasonal N2 fixation rates associated with I. myosurorides collected from the Hoh Rainforest and Seattle were estimated by measuring acetylene reduction rates in laboratory incubations. We observed a significant seasonal effect across both collection locations on N2 fixation rates, with the highest levels occurring in spring 2016. Spring fixation levels ranged from 48-3,606 μmol m-2 day-1 in the Hoh Rainforest and 4-504 μmol m-2 day-1 in Seattle. There was also a significant difference in N2 fixation rates across the collection locations, with higher N2 fixation associated with I. myosurorides collected from the Hoh Rainforest. Canopy collection height, on the other hand, did not reveal any significant effect on rates of N2 fixation. Despite their importance, and the iconic nature of PNW moss-covered forests, remarkably little is known about how these arboreal communities impact the biogeochemistry and functional biology of PNW forest ecosystems particularly with respect to such issues as resilience to anthropogenic change. This research enhances our understanding of the potential inimical role that transportation related N deposition plays in altering epiphytes community relationships in Western Washington.