“[At the time] birches were considered weeds. There was a huge program to spray and herbicide these trees to get rid of them because the foresters viewed the birches as competing with Douglas fir, competing for light especially. I was observing in these plantations, though, that when they weeded out the birches, when they sprayed them or cut them, that there was a disease in the forests that would just start spreading like a fire. It was called Armillaria root disease. I really thought, we're doing something wrong here. And so I wanted to know whether the birches were somehow protecting the firs against this disease and that when we cut them out it actually made it way worse.
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I had learned about these mycorrhizal fungi and how they could actually protect trees against diseases. And I'd also heard about David Reed's work in the U.K. where he had shown that in the laboratory that trees could be linked together by mycorrhizal fungi and pass carbon between them. So I tested this between birch and fir in my sick plantations.
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I knew that birch and fir were sharing carbon below ground — much against the prevailing wisdom that they only compete for light and also that the more that birch shaded Douglas fir, the more carbon was sent over to Douglas fir. So there was a net transfer from birch to fir that was sort of mitigating its shading effect. In this way the ecosystem was maintaining its balance — the birch and fir could coexist because of this collaborative behavior that was sort of offsetting some of the competition that was going on.”

“[Trees] get old. They do eventually decline. And dying is a process and it takes a long, long time. It can take decades for a tree to die. In the process of dying, there's a lot of things that go on. And one of the things that I studied was where does their energy — where does the carbon that is stored in their tissues — where does it go? And so we label some trees with carbon dioxide — with C13, which is a stable isotope — and we watched as we actually cause these trees to die. We stress them out by pulling their needles off and attacking them with bud worms and so on. And then we watched what happened to their carbon.

And we found that about 40% of the carbon was transmitted through networks into their neighboring trees. The rest of the carbon would have just dispersed through natural decomposition processes ... but some of it is directed right into the neighbors. And in this way, these old trees are actually having a very direct effect on the regenerative capacity of the new forest going forward.

This is a completely different way of understanding how old trees contribute to the next generations — that they have agency in the next generations. And our practices of salvage logging to get rid of dying trees, or trees that have just died or have been burned in wildfires — if we go in and cut them right away, we're actually short circuiting that natural process.

Our studies suggest it would have knock-on effects to the regeneration coming up. They're not going to be as well prepared for their lives coming forward. So I've been trying to tell people: Let's hold back on this salvage logging until trees have had the chance to pass on this energy and information to the new seedlings coming up.

What was revealed in the map was that the old trees were connected to almost every other tree in the forest, so they were the hubs of the network, and they were linked to all the little smaller trees and saplings and intermediates. And the reason that they were the most highly connected is because they had massive root systems with lots of growing points and lots of fungi on them.

And so these old trees were the hubs of the network, and when you start doing analytical work on networks like that using graph theory there are patterns that emerge. And what emerged out of the pattern is what we call a complex network, with a few large nodes - which are the old trees - and lots of small connected nodes. So those hubs...we started to do some experiments around them and realized that these old trees were actually facilitating the growth of the seedlings that were growing up underneath them.

And so that network pattern of the big old trees connected to smaller nodes was a neural network, it’s what’s called a biological neural network. And that pattern is a very similar pattern to our own biological neural networks in our brains. And in fact these kinds of complex networks are repeated in many many systems, they’re very efficient systems, they’re good at transmitting information. In our brains it would be thought patterns. And they’re very resilient.