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

Quantification of a relationship between salmon escapement in rivers and riparian tree-ring δ¹⁵N 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 δ¹⁵N in riparian tree rings formed before and after extirpation of salmon and ¹⁵N tracer studies in a river and riparian soils. We concluded that (i) extractable (sap) N must be removed for interpretation of tree-ring δ¹⁵N 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 δ¹⁵N was associated with salmon extirpation in a natural system, (iii) 500‰ ¹⁵NH₄⁺ 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‰ ¹⁵NH₄⁺ to riparian soils at a rate equaling 7.25 kg salmon·50 m^-2 resulted in maximum tree-ring δ¹⁵N of ∼100‰--600‰. Thus, the calculated maximum signal possible from salmon was 0.08‰--0.43‰, which is within the range of natural variation. Evidence suggested that neither total nor structural tree-ring δ¹⁵N was useful for reconstructing salmon abundance. (author)