Trees grow on salmon

Categorie(s): Ecology

October’s Pacific rains in British Columbia create prime spawning conditions for wild Pacific salmon. It’s also a banner month for the towering trees of British Columbia’s coastal temperate rainforests. However, you might be surprised that it’s the salmon, not the rain, that make it such a great month for the trees.

This story of salmon, bears and trees illustrates the interconnected web of life, and has aptly spawned the name “The Pacific salmon forest”.

Every autumn millions of chum, pink, chinook, and coho salmon are leaving the Pacific Ocean to swim up rivers along the British Columbia coast. Waiting alongside these rivers are thousands of bears poised to fatten up for their winter hibernation. Each bear carries hundreds of kilos of fish out of the streams and into the forest to consume. With the abundance of fish, the bears can afford to eat only their favourite parts of the fish, the eggs (or roe), brain, skin and back muscles, before they move on to the next fish. Sometimes they eat as little as five per cent of the fish, leaving the rest to decompose on the forest floor. Many other animals feast on these rests: wolves, mink, marten, river otters, eagles, gulls, ravens, crows, shorebirds, etc. Bears and other mammals wander into the forest, and birds shelter in the trees, and they all leave nutrient-rich droppings as well as uneaten parts of salmon, which decay and fertilize the land .That’s where the relationship between the bears and the fish connects to the trees.

Tree growth in coastal rainforests is limited by the availability of the element nitrogen. The fish left behind by the bears are packed with nitrogen, which fertilizes the trees and helps them grow to their impressive size.

Several years ago, scientists, led by Dr. Tom Reimchen of the University of Victoria, tracked the nitrogen uptake in coastal forests and measured up to 80 per cent of their nutrients as salmon-based. They did this by following the nitrogen isotope 15, which is found almost exclusively in marine environments. It turns out that Douglas fir, sitka spruce, western hemlock and western red cedar owe a good deal of their girth to the nutrients carried from the ocean by salmon, then into the forest by the bears.

Just like there’s more to the birds and the bees than what they do with flowers, there’s much more to this story, too. Millions of insects also eat the leftover salmon, bolstering invertebrate populations, which in turn feed birds and other small forest creatures. Dr. Reimchen’s team found insects with more than half their nitrogen coming from an ocean source, and a greater abundance and variety of both insects and plants near salmon bearing streams.
Gulls, wolves, eagles, osprey, crows, pine martin and dozens of other species also take advantage of the rich bounty of salmon. Not a shred of nitrogen goes unused by nature; even the bears’ urine casts another shower of nitrogen into the forest ecosystem.

The rivers themselves distribute the salmon further. Remnants of salmon float downstream, where they are eaten by aquatic insects and smaller fish, finally drifting to the river’s mouth. In the estuary, crabs and other marine scavengers devour the final remnants. Just as the Pacific Ocean doesn’t end at its shores, the salmon forest doesn’t end at the edge of the wood. You can find it in the birds flying by your window, and in the breath you just took.

In a land of thin, relatively poor soils, these nutrient packets—brought from the ocean by salmon—are important to the life of the forest. Sitka spruce near salmon streams grow much faster than those on streams without salmon. Bears help to spread one of their own important foods, by carrying the seeds of berries they have eaten, and then depositing those seeds on the forest floor. Not only do the bear’s droppings deposit the berry seeds, but they simultaneously nourish the new seedlings that will grow into berry-producing bushes.
Adapted from: David Suzuki foundation 2012