Fish and rivers play a vital role in nutrient distribution. Tree growth can be almost completely dependent on fish, especially in areas where soil nutrition is low. Young salmon swim out to the ocean where they spend their adult life growing and getting bigger. Salmon build muscle swimming around in the vast ocean avoiding predators and finding food. They store enough fat needed to survive their journey back to the stream they were born in. Salmon carry considerable quantities of nitrogen and phosphor.
During the salmon run, hungry hunters line the streams. When salmon are abundant, bears will selectively eat the fattiest parts of the salmon and leave behind the rest of the carcass. The discarded carcasses give opportunities to other wildlife, such as wolves, fox, birds of prey, and various insects. Wildlife help spread the salmon carcasses further into the forest, and the leftovers help fertilize the soil. Nitrogen is also spread through feces expelled from wildlife. Over 70% of nitrogen along streams comes from the ocean, which is transported by the salmon.
How do we know this? The key is nitrogen 15 (N15), a rare stable isotope of nitrogen. Here in the Pacific northwest, N15 is found almost exclusively from the ocean or in fish who have spent time there. So finding evidence of N15 in plants allows us to make a direct connection back to salmon. Trees eagerly wait to absorb N15 from the soil through their roots. Not all of the nutrients are absorbed, some of it flows back into the stream and back out to the ocean, where tiny organisms wait for their shipment of food.
The growth rings in trees are a historical archive of everything the tree experienced throughout its lifetime, narrow tree rings for years of drought and wider rings for years of ample rain fall, so of course, you can also work out the amount of nutrients available to that tree. So there is a direct connection to the number of fish in earlier times, and the amount of that special isotope N15 found in wood, and that’s how core samples from trees give us information of how many salmon once swam in our streams. As we know, salmon have declined dramatically over the last 100 years and there are many streams in North America that don’t have salmon left at all. If tree growth rings are a historical archive of information, so are the logs in our people’s caches, sod houses, smoke houses, and log cabins.
Article by: Michelle Quillin, Outreach Coordinator,TCC