Relationships between salmon abundance and tree-ring δ15N: three objective tests

Quantification of a relationship between salmon escapement in rivers and riparian tree-ring delta N-15 could allow reconstruction of prehistorical salmon abundance. Unfortunately, attempts to quantify this link have met with little success. We examined the feasibility of the approach using natural abundance of delta N-15 in riparian tree rings formed before and after extirpation of salmon and N-15 tracer studies in a river and riparian soils. We concluded that (i) extractable (sap) N must be removed for interpretation of tree-ring delta N-15 because it contains up to 78% of the N in wood, is mobile, and differs from structural N in isotopic composition, (ii) no significant change in structural tree-ring delta N-15 was associated with salmon extirpation in a natural system, (iii) 500 parts per thousand (NH4+)-N-15 added to a stream was detected in riparian tree rings spanning at least 8 years, demonstrating interring movement of N that confounds detection of an annual signal, and (iv) addition of 28 000 parts per thousand (NH4+)-N-15 to riparian soils at a rate equaling 7.25 kg salmon.50 m(-2) resulted in maximum tree-ring delta N-15 of similar to 100 parts per thousand-600 parts per thousand. Thus, the calculated maximum signal possible from salmon was 0.08 parts per thousand-0.43 parts per thousand, which is within the range of natural variation. Evidence suggested that neither total nor structural tree-ring delta N-15 was useful for reconstructing salmon abundance.