The carcass zone: salmon contribution to tree rings in old‑growth Sitka Spruce (Picea sitchensis) throughout coastal British Columbia

Abstract

The contribution of Pacific salmon to riparian forest biodiversity is widely recognized, yet the direct influence on conifer
annual growth rings is less well-established. I examined broad spatial and temporal trends (1945–1999) in ring width, basal area increments (BAI), and nitrogen signatures in heartwood rings of 282 old-growth riparian Sitka Spruce (Picea sitchensis;
average age ~300 years) from 79 watersheds in three regions of coastal British Columbia. Several large yearly fluctuations in salmon biomass entering streams were positively but weakly correlated with tree growth, lagged one to four years. General linear models indicate that tree age and salmon carcass proximity were the major growth predictors, while tree distance to stream and riparian slope were not significant. Average annual BAI (marginal means) in carcass zones were 80%, 150%, and 55% higher than adjacent control sites on the Mainland, Mid-coast Islands, and Haida Gwaii, respectively. Nitrogen isotope signatures (δ15N) in heartwood rings ranged from –8.6‰ to 8.0‰ and were about 3‰ higher in carcass trees than control trees. Total nitrogen (TN) ranged from 0.03% to 0.15% and was largely independent of salmon carcass occurrence. Bivariate plots (δ15N against TN) indicate a geographical clustering of elevated TN in Haida Gwaii watersheds, lower δ15N and TN in the Mid-coast Islands, and elevated δ15N and TN in watersheds with exceptionally high salmon carcass transfer and bear activity. These cumulative data robustly quantify accentuated conifer growth from salmon-derived nutrients in riparian zones that are largely independent of climatic influences and tree age.