Compton, J.E., M.R. Church, S.T. Larned and W.E. Hogsett. 2003 . Nitrogen saturation in forested watersheds of the Oregon Coast Range: The landscape role of N2-fixing red alder. Ecosystems 6:773-785. WED-02-154
Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species influence is the role of N2-fixing red alder (Alnus rubra) in the biogeochemistry of Pacific Northwest forests. To understand the connection between the distribution of red alder and watershed-scale nutrient losses, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder), particularly when near-coastal sites were excluded (r2 = 0.65 and 0.68, for nitrate-N and DON). Broadleaf cover within the entire watershed, including uplands, was more strongly related to nitrate and DON concentrations than riparian broadleaf cover alone. Nitrate dominated over DON in hydrologic losses (92% of total dissolved N), and nitrate concentrations were much more spatially variable than DON. Annual N losses were highly variable among watersheds (2.4 to 30.8 kg N ha-1 yr-1), but were described by a multiple linear regression combining broadleaf and mixed broadleaf-conifer cover (r2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single species, where N inputs from pure and mixed red alder stands are the major control on N losses from these coastal watersheds.