Riiven Wild
Salmon runs and old-growth forests
Salmon die in the rivers where they hatched, and their bodies end up feeding the spruce and cedar that line the banks.
The Scene
It is autumn on a salmon stream on the coast of British Columbia. The water is low and clear, and the gravel is crowded with spawning fish, their backs out of the water in the shallows. A black bear wades in, pins a salmon against a rock, and carries it up the bank into the forest. It eats part of the fish under a Sitka spruce and leaves the rest on the moss. By the end of the run, the ground beneath the trees is scattered with half-eaten carcasses. The bear eats what it wants and abandons the rest, and each trip up the bank carries another fish's worth of ocean nutrients into the roots of the forest.
The Creature
Pacific salmon spend most of their lives at sea, where they grow large on marine food. Then they return to the freshwater streams where they hatched, spawn in the gravel, and die. Their bodies are built from ocean nutrients, especially nitrogen, an element plants need to make proteins and grow. Coastal and interior forests are often short of nitrogen, so each returning fish brings a delivery the land cannot easily make for itself. Scientists call this marine-derived nitrogen, meaning nitrogen that began in the sea and reached the land inside an animal. The salmon do not carry it far on their own. Carcasses rot in the stream, wash onto gravel bars, or are hauled into the trees by bears and other scavengers. That last step matters most for the forest, since a carcass in the water releases its nitrogen into the current, while a carcass dragged into the understory releases it into the soil that feeds the trees.
How It Works
Once a carcass breaks down, its nitrogen and carbon move into the soil and the water. A study in central Idaho found that salmon carcass deposition produced large increases in soil nitrogen and carbon, with soil loading accounting for 31 percent of carcass nitrogen and 16 percent of carcass carbon. In forested streams of interior British Columbia, researchers found that once carcass biomass passed a threshold relative to stream flow, nitrate concentrations in the water rose to maxima 10-fold greater than ambient levels. Trees respond. In coastal British Columbia, where bears moved carcasses into riparian forests (the strips of woodland along stream banks), mature Sitka spruce roughly doubled their annual radial growth, the width they add to their trunks each year, compared with nearby control trees. A US Forest Service study in a Columbia Basin riparian forest modeled the same process: simulated marine-derived nitrogen additions raised annual net primary productivity, the new plant growth a forest produces each year, by 7 to 8 percent.
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The Ripple
The effect spreads beyond one tree. That same Forest Service simulation found that salmon nitrogen increased net ecosystem production, the carbon a forest stores after accounting for what it releases, by up to 47 percent annually. Bigger trees along a stream shade the water and drop branches and leaves into it, and the stream feeds insects that young salmon eat. Nitrate washing through the water after a run fertilizes the algae and plants in the channel. The bears that carry fish uphill are part of this chain, since the forest's growth depends partly on where they choose to eat. When runs shrink, the whole delivery shrinks with them. The Columbia Basin study estimated that if historic salmon runs had continued, present-day site carbon would be up to 3 percent higher, about 4.4 tons of carbon per kilometer of stream, carbon now missing from wood and soil that fish, bears, and decay once supplied.
Why It Matters
Salmon runs link the open Pacific to forests hundreds of kilometers inland. The nitrogen in a spruce ring along a British Columbia stream can trace back to plankton at sea, passed through a fish and a bear before reaching the soil. Where runs have declined, forests still stand, but they grow and store carbon with less of that ocean supply. Each autumn, the fish that die in the gravel leave part of themselves in the rings of the trees along the bank.
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