Relationships between salmon abundance and tree-ring δ ¹⁵N: Three objective tests

Quantification of a relationship between salmon escapement in rivers and riparian tree-ring δ ¹⁵N 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 δ ¹⁵N 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 δ ¹⁵N 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 δ ¹⁵N was associated with salmon extirpation in a natural system, (iii) 500‰ 15NH ₄ ⁺ 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‰ 15NH ₄ ⁺ to riparian soils at a rate equaling 7.25 kg salmon·50 m ^-2 resulted in maximum tree-ring δ ¹⁵N 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 δ ¹⁵N was useful for reconstructing salmon abundance.