Salmon in the trees: an assessment of a dendrochemical technique for detecting marine-derived nitrogen in riparian tree rings

Quantifying the relationship between salmon escapement and riparian tree-ring δ15N could contribute greatly to understanding trends in historic salmon abundance. Such an understanding could have far-reaching consequences for understanding historic carrying capacities of river systems and help guide future restoration efforts. This study investigates the reliability of using naturally occurring isotopic variations in annual tree rings to produce quantifiable estimates of historic salmon runs. Three study areas with temporal and spatial changes in salmon spawning abundance were examined. I found that currently available techniques for removing mobile nitrogen are not sufficient to overcome problems associated with radial mobility, as indicated by enrichment of tree rings formed prior to a labeled fertilizing experiment. Tree ring δ15N signatures failed to capture known changes in salmon abundance above a migration barrier on the Skykomish River but were related to historic escapement ranges at on Icicle Creek with tree-cores downstream from an historic migration barrier showing an elevated δ15N signature compared to upstream. While a relationship between tree-ring δ15N and salmon abundance may exist, thesis relationships may be obscured by the inter-radial mobility of nitrogenous compounds and confounded by local nitrogen cycle nuances, or by species specific processing of N