Shooting Trees for Science Because of Salmon

A salmon carcass exerimentally placed in an Idaho streat tht once was home to naturally spawning salmon
A salmon carcass exerimentally placed in an Idaho streat tht once was home to naturally spawning salmon

I looked down the dark grey barrel of the shotgun, pulled the trigger, and watched a small branch tumble down to the ground through the canopy (maybe after a couple of shots if it had been a stubborn ponderosa pine). My assistant and I bagged the sample and put it with the others to be analyzed for nutrient content back at the lab.

Days later, as I relayed stories of my field work to a friend, she asked, “Wait, so let me get this straight; you’re shooting trees- for science- because of salmon?” She wasn’t wrong. The foliage samples I collected that summer were from trees in central Idaho, along streams that historically supported spawning populations of Chinook salmon and steelhead. These streams hadn’t seen spawning runs in about a century, and I was investigating how the loss of these fish affected forest productivity. The data from those slightly injured trees helped guide a model used to see the long-term story of how important anadromous fish were to this forest.

Salmon returned to their natal streams every year to spawn and died there, leaving behind a wealth of nutrients from their stinky, fishy carcasses (I know firsthand; I helped place hundreds of stinky, fishy carcasses into Idaho streams on a related experiment). According to conservative estimates of nitrogen deposited by these fish onto riparian soils (through transfer by bears, bugs, floods and other pathways), fertilization provided by salmon and steelhead, over the life of the trees, was comparable to that now applied in commercially grown forests. In our relatively nutrient-poor forests, this input was significant, and had unknown consequences for large, arid inland portions of the basin that once had anadromous fish.

Plenty of research has been done on the effects of salmon in coastal ecosystems but much less is known about their inland impacts even though salmon and steelhead can swim hundreds of kilometers away from the ocean. Historically, over half of the fall Chinook and summer steelhead in the Columbia river basin came from Idaho, much of which is now unavailable due to dams (PBS Nature did a show on Columbia River salmon).

The modeling project was an offshoot of a larger study tracing the influence of salmon nutrients through the ecosystem; hence the aforementioned hauling of thousands of pounds of dead, stinky, fishy carcasses into the woods. This broad experiment traced marine-derived nitrogen from fish (measurable because of its higher 15N isotope concentration) in great depth through the ecosystem for three years, and showed effects on water quality, stream-based producers and consumers, riparian soil nutrients and plants, and wildlife.

But how does losing salmon change whole forests over decades, affecting processes that are meaningful on a human scale? To answer this, I used ecosystem process modeling, using a computer to simulate carbon, water, and nitrogen cycles to create a forest model on which to run long-term experiments. I could customize it to the species and landscape of central Idaho (giving rise to my need to shoot at trees to make measurements on). The result: present day riparian forests would have 3 to 12 percent higher productivity if anadromous fish had not disappeared from the streams.

It’s not surprising that fertilizing the forest (from fish or any other source) makes it grow more; but healthy riparian forests are important for more that just the trees’ growth rate. Their shade provides cover for native fish (be they anadromous salmon, native trout, or others), their roots stabilize stream banks, their falling leaves add nutrients to the streambed and aquatic consumers, they provide habitat in the important transition zone between aquatic and upland ecosystems, and, of particularly modern importance, they sequester carbon dioxide from the atmosphere.

Simulations suggest these riparian forests could take up approximately 25% more CO2 if fish still spawned in their streams. Across an estimated 6714 km of potential spawning habitat above Hell’s Canyon Dam on the Snake River, that amounts to lost carbon sequestration of nearly 2000 metric tons of carbon per year. How will this effect play out in a changing climate? That’s a question for the next simulation experiment.

So, for now, I’ll put away that shotgun, pop a salmon fillet in the oven (yes, I can still manage to eat salmon), and think about that next question.

Written by: Andrea Noble Stuen
Affiliation: University of Idaho
Country: USA

This post is entry #33 in the #IUFRO2014 Blog Competition. The most popular entry will receive a certificate and 500 USD. The second and third most popular entries will receive a certificate and copy of the new book, “Forests and Globalization: Challenges and Opportunities for Sustainable Development”.

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