Salmon and alder as nitrogen sources to riparian forests in a boreal Alaskan watershed

Anadromous Pacific salmon (Oncorhynchus spp.) transport marine-derived nitrogen (MDN) to the rivers in which they spawn. Isotopic analyses of riparian vegetation in a boreal Alaskan watershed indicate that trees and shrubs near spawning streams derive ~24-26% of their foliar nitrogen (N) from salmon. Basal area growth of white spruce (Picea glauca) is enhanced at sites receiving MDN inputs. This marine N subsidy appears to be less important to riparian ecosystems where symbiotic N fixation by alder (Alnus crispa) is prevalent, although salmon carcasses may nonetheless be an important source of other marine-derived nutrients affecting productivity in these forests. These findings illustrate the complexity of interactions surrounding riparian forests and the interconnectedness of river and riparian ecosystems.