Effects of salmon carcasses on growth of riparian white spruce (Picea glauca) in Alaska: a long-term fertilization experiment

Pacific salmon (Oncorhynchus spp.) mature and accumulate biomass in the ocean before returning to their natal streams to spawn and die. Upon their death, salmon contribute marine-derived nutrients to rivers and riparian zones. Prior studies have found that riparian trees grow faster along salmon-bearing streams than along comparable streams without salmon. At Hansen Creek, a tributary of Lake Aleknagik in southwestern Alaska, spawning survey crews have been removing post-spawn sockeye (O. nerka) carcasses from the stream every day throughout the spawning season since the 1940s. Starting in 1996, a long-term fertilization experiment was enacted, whereby all carcasses removed from the stream were thrown to the left bank only, within 15 m of the stream channel. In the summer of 2016, foliage and increment core samples were collected from white spruce (Picea glauca) trees on both banks. Here we compare incremental growth rates of trees on the left (fertilized) bank and the right (unfertilized) bank to determine if salmon carcass fertilization has had an influence on basal area growth of white spruce growth. Accounting for variations in climate and age-related growth trends, we expect to see enhanced growth rates on the fertilized bank during the 20 years of fertilization (1996-2016). These findings will help to elucidate the importance of seasonal marine-derived nutrient subsidies to riparian forests and the time scales over which such subsidies influence riparian ecosystems