Trees that communicate, care for one another and foster cooperative communities have captured the popular imagination, most notably in Suzanne Simard’s much-praised book Finding the Mother Tree, soon to be a movie, and in other works like James Cameron’s Avatar, Peter Wohlleben’s The Hidden Life of Trees and Richard Powers’ Pulitzer Prize–winning novel The Overstory.
But many scientists like myself believe these depictions misrepresent ecosystems and harm the cause of conservation.
Do trees really talk? Sure. Plants emit hormones and defense signals. Other plants detect these signals and alter their physiology accordingly. But not all the talk is kind; plants also produce allelochemicals, which poison their neighbors.
Simard and others showed that carbon compounds made by one tree can end up in neighboring trees via the underground network of mycorrhizae, fungi that live on plant roots and exchange water and nutrients they gather from the soil for sugars plants make. They suggest that donor trees purposely and sacrificially send nourishment to others to help them grow and ensure the health of the community.
How would this work? Like other ecological interactions, cooperation must evolve by natural selection, in which traits increase in frequency because individuals who have them produce more offspring and pass on the traits.
Perhaps the simplest explanation is that the fungus shuttles carbon around to protect its own interests, cultivating multiple hosts to ensure its future supply of food.
Altruism can arise if a recipient is likely to reciprocate, ultimately benefiting the donor. Reciprocity among trees is possible, but many interactions are likely asymmetric, such as between mature trees and tiny seedlings.
Altruistic behavior can also evolve if it benefits relatives, who pass on the donor’s genes. Emerging evidence shows nutrient redistribution via mycorrhizal networks benefits kin more than unrelated plants. The mechanisms by which plants might recognize and respond to their relatives have yet to be fully worked out.
Unfortunately, the explanation most favored by popularizers, that trees send out resources to strengthen the community, is least likely. This would require natural selection to be countered by group selection—where groups that cooperate win out over groups that do not. When these forces conflict, natural selection almost always wins, because individuals are so much more numerous than groups and turn over much more rapidly.
Interestingly, when mycorrhizae transfer resources from a native grass to an invasive weed, this is interpreted as evidence of parasitism, not cooperation.
Overemphasizing cooperation is misleading. The forest floor is a forum of fierce competition. A mature maple tree produces millions of seeds, and on average only one will grow to reach the canopy. The rest will die, with or without help from mom.
Amid this struggle, trees can sometimes facilitate each other’s growth. But this does not mean that a forest functions like one organism. An ecosystem comprises an ever-changing diversity of organisms having an ever-changing variety of interactions, positive and negative.
After the last glaciation, different tree species migrated north at different rates and by different routes. The beech-maple forest, or the oak-hickory forest, did not move as a unit. In fact, trees currently live in combinations that may have no analog in the past or future.
Anthropomorphism is taboo in science because it deceives us more often than it helps. Trees are not people and forests are not human families or even republics. Suggesting that they are can only lead us to imaginary conclusions.
In interviews, Simard has said that she purposely uses anthropomorphism and culturally weighted words like “mother”—even though the trees in question are male as well as female—so that people can relate to trees better, because “if we can relate to it, then we’re going to care about it more.”
Do trees need to have human values and emotions for us to let them live? The science supporting conservation is compelling enough. New discoveries about the underground world are thrilling enough. The public deserves to hear the true story, without the confusion of personification and stretched metaphor.
These distractions keep us from confronting reality: facilitation may be real, but so is the Darwinian struggle for existence. We are moral creatures in an amoral world. Nature does not share our values, and mercifully, we may choose not to emulate all of nature’s ways.
Between treating plants as objects or as humans, I suggest a third way: let’s seek to understand plants on their own terms. Plants are fundamentally unlike us: mute, rooted and inscrutable. We need to meet the challenge of cultivating respect for organisms that are different from us—in their separate and complex bodies, in their sophisticated interactions, in their unfathomable lives.
This is an opinion and analysis article; the views expressed by the author or authors are not necessarily those of Scientific American.