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Influence of spawning salmon on tree-ring width, isotopic nitrogen, and total nitrogen in old-growth Sitka spruce from coastal British Columbia

Coastal watersheds of the North Pacific benefit immensely from bear-mediated uploading of salmon nutrients, which increases aquatic and terrestrial productivity. To quantify the influence of spawning salmon on tree-ring signatures, we analyzed 543 rings from the heartwood of 13 old-growth Sitka spruce (Picea sitchensis (Bong.) Carrière) trees from five geographically separated watersheds in coastal British Columbia. In comparison with adjacent control trees, those receiving salmon nutrients (salmon trees) have rings that are, on average, 1.5 mm wider, 4.5‰ more enriched in isotopic nitrogen, and 0.021% more elevated in total nitrogen (P < 0.001, Mann–Whitney–Wilcoxon test). In this study, salmon nutrients enhance average stem growth by 19%. Furthermore, salmon trees show that increases in tree-ring width and nitrogen values lag sporadic, high salmon runs by 0 to 5 years. Using differences between control and salmon trees from the same site, our results collectively indicate that tree-ring width, isotopic nitrogen, and total nitrogen are valid, complementary tools for investigating historic, annual fluctuations in salmon abundance in coastal watersheds. We recommend their use in future, tree ring based reconstructions of past nutrient cycling over decadal to centennial time scales.