Each bear preying on the spawning salmon transported some 150 fish per day into the forest, where the roots of the trees foraged for the decaying protein and nutrients, the salmon flesh providing more than three-quarters of the tree’s nitrogen needs. The nitrogen in tree rings derived from salmon was distinguishable from the soil’s nitrogen because fish at sea get enriched with the heavy isotope nitrogen-15, which serves as a natural tracer of salmon abundance in the wood.
