Red alder (Alnus rubra), a nitrogen(N)-fixing deciduous broadleaf tree, can strongly influence N
concentrations in western Oregon and Washington. We compiled a database of stream N and GIS-derived landscape
characteristics in order to examine geographic variation in N across the Oregon Coast Range. Basal area
of alder, expressed as a percent of watershed area, accounted for 37% and 38% of the variation in summer
nitrate and total N (TN) concentrations, respectively. Relationships between alder and nitrate were strongest in
winter when streamflow and landscape connections are highest. Distance to the coast and latitude, potential
surrogates for sea salt inputs, and watershed area were also related to nitrate concentrations in an all-subsets
regression analysis, which accounted for 46% of the variation in summer nitrate concentrations. The model with
the lowest Akaike’s Information Criterion did not include developed or agricultural land cover, probably because
few watersheds in our database had substantial levels of these land cover classes. Our results provide evidence,
at a regional scale, that background sources and processes cause many Coast Range streams to exceed proposed
nutrient criteria, and that the prevalence of a single tree species (N-fixing red alder) exerts a dominant control
over stream N concentrations across this region.Keywords: Forests, Rivers/streams, Environmental regulations, Biogeochemistry, Pacific Northwest, Nutrient