When people say that there’s life in dead trees, most of us picture what biologists call macrofauna — birds, bats, raccoons, squirrels, and all the other creatures that make their homes in the holes in trees.
Dead trees are certainly useful to these animals — around 40 per cent of the birds that live in the forest nest inside tree hollows, for example — but they’re just a very tiny part of the picture. A much more complex ecosystem is found on a much smaller scale.
You can get a glimpse of this world the next time you go for a walk in the woods.
Winter is the ideal time to explore this world: with no undergrowth, it’s much easier to leave the trail and go up to trees that look like they might have some holes worth examining. Equipped with a pair of snowshoes, you can really go just about anywhere in winter.
Shine your flashlight into a tree hollow, and look closely at the walls of the hole. It’s like peering into a cave. And just like a cave, there are creatures living here that live nowhere else.
The first thing you’ll likely notice is moss or lichen, lining the walls of the cave like shag carpet. Look closely and you will probably see that there is more than one kind of plant here.
Some holes will have fungi living inside, perhaps forming a miniature forest of mushrooms barely a few milimetres high.
What kinds of mosses, mushrooms or lichens are you seeing? It could be almost anything. Each hollow is a unique microclimate. The size of the hole, the species of tree, and even the side of the tree it is on will influence the microclimate, and so every hole can have its own unique mix of species.
There are mosses that exist only in the hollows in birch trees, and others that exist only in aspen trees, or white pines, or hemlocks. Some species are even more specialized than that, living only in holes of trees that are still alive, for example, or only in trees that are all but completely rotted.
And that’s just the beginning of the specialization, because plants aren’t the only things living inside these hollows. Tons of insects spend part or all of their lives inside these hollows, and they, too include a mix of generalist and specialist species.
There is a mushroom that grows on the dead birch trees, for example, and there are at least two species of beetle that only lives inside the fruiting body of that fungus. And then there are tiny wasps that specialize in hunting those beetles.
Drill down even further, and you would likely find specialist bacteria that rely on the tiny wasps. It’s like the old children’s song, There’s a Hole In The Bottom of the Sea, with “a hair on the flea on the fly on the wart on the frog on the bump on the log in the hole in the bottom of the sea.”
In some woodlands, 40 percent of the living creatures in the forest couldn’t exist without these microhabitats, and nearly every living thing in the forest makes use of them in some way.
The amount of life that gathers in dead and dying trees is truly astonishing, and actually increases the more the tree decays. Plants break down the wood fibres, creating homes for insects; predators, ranging from birds to miniscule flies, come to feast on the insects, in turn creating new habitat for other species.
When a tree is alive, around five per cent of its mass contains living cells; most of the rest is heartwood. Once the tree has died and begun decaying, though, more than 40 percent of its mass will contain living cells.
You can only see a tiny portion of this life when peering into a hollow tree. But with a flashlight and a bit of imagination, what you can see offers you a glimpse into a vast and complex world that scientists are still only just beginning to understand.