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Beaver meadows are slow to reforest because they lack a soil fungus needed by black spruce. Photo: <a href="">Putneypics</a>, Creative Commons, some rights reserved
Beaver meadows are slow to reforest because they lack a soil fungus needed by black spruce. Photo: Putneypics, Creative Commons, some rights reserved

Natural Selections: Fungus and forest

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Tall trees may be the kings of the forest, but there is another kingdom of forest life that passes unnoticed. Dr. Curt Stager and Martha Foley talk about the arboreal network of fungus.

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Martha Foley: Two tidbits about fungus. One having to do with why beaver meadows, where the beaver pond has dried up and gone away, why that doesn't spring into woods all of a sudden. They generally stay grassy instead of being repopulated by the spruce trees or whatever.

Curt Stager: It's sort of a mystery because at first glance you'd say, okay, it should be pretty fertile soil because it was a pond and collected all this nice crud and organic matter. Okay, it's a little bit damp, so that would keep out a lot of trees, but there are trees that like that.

MF: Right, the spruce trees.

CS: The black spruces like it and if you have a beaver dam that eventually washes out and you get this meadow it can sit there for 50, 60, 70 years.

MF: Yeah they're beautiful, but why don't trees move in? I mean, if you had a blow-down in the forest or a place where it burned, in the next year you'd have raspberries, and birch trees, and all sorts of stuff.

CS: Yeah, and so there's got to be something fundamentally different about that whole issue. There was a recent study published in the journal Nature that looked into this and they think they've got it nailed down now. It's that there's something missing in the soil that the black spruce would need to let it survive besides just the nutrition, the dampness, and all that kind of stuff.

There are actually fungi in the soil that connect with the roots, which is really common for most forest plants. It's symbiosis going on. The technical term is mycorrhizae, where the plant roots and the fungi combine and they both share the water and the nutrition in the soil and help each other out. And in some cases it's actually quite specific, and black spruce trees actually have a specialized kind of mycorrhizae fungus they need connected to our roots.

MF: And it doesn't survive underwater, so it's not there when the beaver pond dries up.

CS: Right. But if you had something like a fire or a blowdown, normally, then the fungus is still in the soil and the seedlings can start up no problem.

MF: So eventually the fungus gets back in the soil, and then the spruce can come back?

CS: Right. But then the question is how do you get the fungus back into the soil.

MF: Little seed packets! You go out and you buy them at the spruce store and you spread them around.

CS: Well, sort of. Yeah, it was actually like little seed packets only it was rodents, these voles, red-back voles, that like these kinds of meadow habitats that eat the fungus that's growing out there in the woods. They get the spores in their digestive tracks, they poop in the meadows, and that's how the spores are apparently dispersed. So the idea is it probably won't turn into a forest until something like those voles will move in eventually. So maybe you should raise red-back voles.

MF: I know voles. Yes, thank you. I'm not going to be raising them. Tidbit number two: also having to do with fungus and trees. Cherry trees.

CS: In this case it would be fungi in the soil again, but its ones that are bad for the trees. In a lot of temperate zone forests where there are black cherry trees growing wild they usually don't form dense stands. They're usually pretty widely dispersed.

Some ecologists that study forests in the tropics have noticed that. Down in the tropics that's a common pattern for forest trees, for one to be over here and one way way off in the distance. The idea for that was that there's probably some sort of insect or other animal that attack the tree and if you drop your seeds right there, that your seedlings will all get killed.

So there's an advantage to dispersing over a large distance. These ecologists have been looking at the cherry tree story up here in the temperate zone and they think it might be similar, but have to do with fungi in the soil instead of above-ground attackers.

MF: You mean there's a fungi that discourages the growth of little cherry trees and so the only way they can survive is at a distance?

CS: Yeah, when they look around it turns out there is a fungus that grows and entangles in the tree roots and it doesn't necessarily kill the adult tree, but when the seedlings try to sprout it knocks them off and so they're dispersed out. It's interesting that what was normally considered tropical forest ecology is actually going on up here in the temperate zone maybe in a slightly different form with these soil fungi instead.

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