It all started with an account I was reading about a naturalist whose main job was taking care of displaced orangutans near Borneo. He became curious about small trees that were thriving around a large tree of the same species that was dying. He wondered if they were related and also why something that was killing the older trees seemed to be helping the younger ones. That goes against our standard idea of disease as something that hurts everything and in turn must itself be completely destroyed. He dug up the roots and traced them back between the trees. They were connected and the old mother tree seemed to be feeding the young ones with its last bit of energy and creating space in the canopy to bring light to the young trees.
I couldn’t find the reference again when I looked for it. But then I came across a TED [Technology, Entertainment, Design] talk by Suzanne Simard about trees. As a forest biologist Simard wondered if trees of different species shared information with each other.
She concocted an experiment using a little plantation of trees set in an older forest. Trees produce sugar carbohydrates during photosynthesis. She set up pairs of trees and covered a birch tree in clear plastic and a fir tree in black plastic so that the clear one would be photosynthesizing sugars and the other would be just using sugars. Then she introduces radioactive carbon-14 carbon dioxide gas into the clear plastic tent. She did that to 80 pairs of trees until a mother grizzly bear chased her back to her truck. She waited in her truck for an hour or so while the mother grizzly bear and her cub settled down in a nearby huckleberry patch and the birch tree under the clear plastic photosynthesized using carbon 14.
Then she tested the trees with a Geiger counter. The fir trees under the black plastic showed up as having carbon-14 that it could only have gotten from sugars created by the birch tree.
That was over 30 years ago. Many tests on trees of the same species, trees of different species and trees near their offspring showed that most shared sugars, particularly if they also had the same DNA. In fact fir trees sent sugars back to birch trees in the winter when the birch had lost their leaves. So yes trees could be mothers to younger trees and good neighbors to companion trees of different species.
Wanting to check this out in my own patch of forest, I walked up an old road. I saw lots of young trees near old fir trees on the wet side of the hill. As I got to the top of the grade a group of deer ran off with their white tails waving behind them. They had been standing in a grove of pine trees. But the ground underneath was bare except for pine needles, not even brush. It was a good place for deer to wait with clear views and open escape routes, but not really a forest of mother trees or different species.
Clearly more was going on here. I contacted my friendly local foresters for some advice. Jay Berube retired as a forest ecologist in 2003. This companion tree line of thinking was not well known back then. It was known that trees of the same species adapt to their local environment and matching seed stock to their locale was important for resistance to diseases such as root rot. Jay noted that pine trees do better in dry ground than fir trees. The fir trees were growing young to restock the wet side of the hill. The pine trees depend on fire to regenerate but must resist it somewhat to survive. So an open grove with no ladder fuels for fires but pine needles on the ground was a perfect for them. Berube said that pine trees are allopathic and actually prevent other plants from growing. There were younger pine trees growing in road cuts with some serviceberry trees for companions. And just a few hundred feet away was a dense stand of young pine that had regenerated after a fire or farming disturbance with no other species underneath.
Another finding from Suzanne Simod’s studies was that mixed species forests are more resistant to disease. The reason for this is not simply that the bugs are more spread out between their preferred host species but also that they share signals that certain pests are around and the trees develop resistance to them. The combination of spreading out the target species and help from the companion species must act a little like “social distancing” and give trees more time to “flatten the curve” of infection.
But it is not just trees that are involved. The pathway for this communication and exchange of material is not roots alone. (TED.com) Mycorrhizal fungi act as the highways between the tree roots. Several species of the hundreds of possible fungi may work with each tree. They live on the sugars from the trees but also bring water, minerals, sugars and signals from the rest of the underground biome to their hosts.
So the take away is that “as below, so above.” The plants in the air, like the millions of organisms in the soil help each other out. The hallmarks of a healthy environment are not only diversity and abundance, but also cooperation. Survival of the fittest means survival of the most cooperative. Approaches to management that attempt to eliminate “pathogens” using chemicals and mechanics that involve massive collateral damage ultimately make plants even more susceptible to drought and disease. Insects, animals and birds also cooperate in this biome.
Forester Bill Berrigan reminds us that “There is so much more to learn about tree’s interactions that we should not be making cutting decisions that may hurt the trees and the soils that they grow in.”