A surprising discovery shows co-operation, not competition, at work in forests.
When walking through a forest, not many of us would be aware that a large part of the mass of which a tree is made is out of sight, under the ground. The huge tangled network making up the subterranean root system of a tree is actually interwoven with an even larger one of fungus. This web of fungus threads often interconnects with other trees, even of different species.
The basic relationship between the trees and the fungus has been fairly well understood for a long time. The trees make carbon-containing compounds (such as sugars) from the air, which fungi can’t.
Thus, drawing carbon out of the tree roots is useful to the fungus. However, the fungi produce special enzymes that help liberate vital nutrients such as nitrogen and phosphorus from the ground. And this enables the tree to get more of these substances.
A relationship like this, to the mutual benefit of two different species, is common and is called symbiosis (‘together-living’).
Suzanne Simard, a forest ecologist with the British Columbia Ministry of Forests, Canada, had been taught to look at trees as individuals locked in an evolutionary struggle to get ahead of their rivals.
Trees strove to grow taller so they could get more light, which would block off the light to their less-fortunate neighbours.
However, Simard was puzzled. When you looked at the amount of variation between different species, it didn’t fit the predictions of this evolutionary ‘struggle’ theory. In fact, she says, ‘we could explain only 10 to 20 percent’ of this variation in terms of competition.1
However, she had noticed the way in which these fungal threads connected different species of trees. Simard planted seedlings of paper birch and Douglas fir, ensuring they were infected with regional fungi. A year later she put tents over some of the trees. Those trapped in the shade in this way should make less carbon (by photosynthesis) than those which were in the sunlight.
Six weeks later, she put sealed plastic bags over the trees and injected them with carbon dioxide with different ‘labels’2 so that she could tell afterwards which tree the carbon came from. A few weeks later, when she ground up the trees to look at these differing ‘carbon labels’, the results were staggering. Carbon compounds which had been made by one tree often ended up in another! Overall, trees that were in the shade took much more carbon from those that were in the sun than the other way around.
In short, what was happening was that the fungus was ‘managing’ the flow of carbon, taking it from healthy trees and giving it to ones in the shade. This was taking place regardless of the species—in other words, carbon from a healthy Douglas fir would end up helping a shaded paper birch to survive, and vice versa.
This would obviously explain why trees in a normal forest, deprived of much of their light by taller companions, do not suffer as much as one would have predicted.
As Simard says, ‘The survival of a group of plants may depend on an individual and its neighbours as well. From a strictly evolutionary perspective it may not make sense, but from an ecological one it does.’
This is just another example of amazing design, not only in living creatures, but in the complex and often unsuspected interrelationships they have with each other, even in a fallen world.
Imagine what must have been an even more amazingly designed harmony in the world before it was devastatingly affected by the rebellion of the first couple.
References and notes
- This item is based largely on C. Zimmer, ‘The Web Below’, Discover, p. 44, November 1997. All quotes come from that article.
- Different isotopes of carbon.