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Dr Tom Reimchen can measure exactly how much these vast old trees need the fish. The secret is to look inside the tree, by taking a core sample. Written on it is the entire story of this 300-year-old tree.

Dr Tom Reimchen "The rings across, two, three, even 4 millimetres, which continues back to even the early parts of the 1800s, late 1700s."

Tom has taken similar samples from thousands of trees. An entire forest is lined up in his lab, each tree waiting to tell its own story. In this seasonal climate, annual rings are created as the tree grows. From these rings, he can determine not just the age of the tree, but also the amount of growth in each year. Some of the rings are thicker than others, showing that the tree has grown more. Like the bear fur, each annual ring can be analysed. Tom can search or the same type of nitrogen that's found in the bear's hairs. It comes from the ocean and its called nitrogen-15.

And the data will tell us just how much nitrogen in those trees comes from all those salmon.

I think this is really exciting. You see, the annual growth rings here show the presence of the stable isotope nitrogen-15, which significantly comes from the oceans. It could only have been carried here by the salmon. Now look, here is the present, the bark on the outside of the tree, so these rings represent perhaps the last 15 years and they're very closely packed together. But here, back in the 1980s, the rings twice as thick. The trees have been growing twice as much during the course of the year.

So, perhaps the salmon runs then were even more productive than they have been recently.

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But that's not the best thing. Come and have a look at this. By measuring the abundance of that nitrogen isotope in this material, I can tell you that majestic old giants like this beauty here are actually composed of up to 85% material that's derived from salmon.

Now, when I was a teenager, I remember learning that I was made of carbon, and carbon could only be formed when stars died. Effectively I was made of dead stars and that struck me as terribly romantic. But look at this. This is a forest made of the ocean! That's why the tree needs the fish.

Without this unlikely-sounding relationship, this magnificent ancient forest just wouldn't be the place it is today.

But there is one more relationship, crucial to seasonal forests all around the world, when it comes to surviving constant change. And its one that reaches its greatest intensity now, at the height of summer. It's the driest time of year, and the trees need water.

Fortunately, united with their fungal partners, the trees have massively extended their roots. Fungal threads in the soil are absorbing water and passing it to the tree. But what has only recently been discovered is the sheer scale of these fungal root networks. A single cubic centimetre of the soil here can have a mile of these white fungal threads running through it. They're called mycorrhizae. And for me, it's these organisms that are the real secret of the forest.

In the lab, the genetic fingerprints of individual mycorrhizae have been identified. By mapping and area 30m across, it's been discovered that individual fungi connect to more than a single tree. Just one fungus can be joined to 80% of all the plants growing here. And, amazingly, these physical links enable different species of plants to exchange nutrients. Older established plants and even nurturing younger weaker ones. It acts like an underground welfare system!

These giant webs connect all of the trees in this forest, and keep them, and all other things that are dependent upon them, alive. That's why scientists are calling this the Wood Wide Web

It's thanks to this natural phenomenon, the Wood Wide Web,. That, together, the trees in the forest ecosystems are resilient – resilient enough to cope with the dramatic changes the encounter every year.

And what's really amazing is how the web is built. It's thanks to hungry mammals like a flying squirrels that this essential life-support system is effectively maintained. It's actually fair to say that these trees wouldn't be standing here, wouldn't be thriving, unless a squirrel had eaten a truffle. And that is fantastic!

It is fantastic. It's fantastic to think that what animals do in one season influences the forest ecosystem throughout the year. It's almost as if all of these stories are choreographed. The arrival of the salmon at exactly the right time to fatten the bears for winter. Then, the emergence of the lush green vegetation fertilised by those salmon to sustain the bears when he emerged from hibernation. The squirrels – foraging for truffles in the autumn time, and sowing their spores throughout the forest to grow a fungal network that joins all of the trees and all of the plants, and provides them with nutrients. The budworm – chewing a hole in springtime in the canopy, so that in summer, sunlight floods down to the forest floor and produces the perfect hunting habitat for lynx. It's all in the timings. And it's this that makes these temperate forests such magical places.