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When trees have emotions, irrationality takes root

By Allen Greer - posted Friday, 5 October 2018

In 2015 German forester Peter Wholleben published a book on trees entitled Das geheime Leben der Bäume. The book was a best seller in Germany and was optioned for translation into 19 languages.  In 2016, the book was published in English as The Hidden Life of Trees. What They Feel, How They Communicate. Reviewers praised the book and the media accorded Wholleben celebrity status in interviews and profiles.

Wholleben’s goal with the book is to make us believe that trees are like sentient animals and to have us treat them accordingly. He says, “I, for one, welcome breaking down the moral barriers between animals and plants. When the capabilities of vegetative beings become known, and their emotional lives and needs are recognized, then the way we treat plants will gradually change as well.” In an ABC interview, he said he wanted “people to look at trees like elephants.”

He pursues two lines of argument to achieve his goal. He argues that trees are social beings that look after one another, and he ascribes a variety of human emotions to trees, ostensibly as to make his story more accessible to the general reader but, in fact, to implant the idea of human-like sentience to them. In Wholleben’s forest, every tree is an Ent.


Wholleben gives several examples of trees helping each other out. But all of his examples can be interpreted more prosaically in terms of individual trees or a fungus (more on this later) acting for their own benefit.

Take for example, what is perhaps his most oft-repeated example of trees helping each other out.  The African Umbrella Acacia, when browsed by an herbivore such as a giraffe, shifts toxins into its leaves and emits an airborne molecule, ethylene, that “warns” down-wind acacias that “a crisis is at hand” and causes the “forewarned” neighbours to pump toxins into their own leaves. Wohlleben describes this as an example of the “language” of trees and, by extension, their sociality. A simpler explanation is that the ethylene is a means of rapidly signalling other parts of the same tree, and the acacias downwind are responding fortuitously to the ethylene wafting onto them.

As another example, Wohlleben explains that the branches of neighbouring trees don’t grow in each other’s way because “a pair of true friends is careful right from the outset not to grow overly thick branches in each other’s direction. The trees don’t want to take anything away from each other, and so they develop sturdy branches only at the outer edges of their crowns, that is to say, only in the direction of ‘nonfriends.’”  The simpler explanation is that it is to each tree’s advantage to expend energy on growing branches toward the light of an opening rather than toward a shadow of a neighbour.

Wholleben gives other examples of trees helping each other out through apparent underground connections.

He describes an early experience of finding the living root system of a tree that had long since died above ground. He concluded the root system was getting “assistance from neighbouring trees” and the “… surrounding … [trees] were pumping sugar to the stump to keep it alive.” This example led him to conclude that trees can be “reluctant to abandon their dead” and might even be capable of “affection.”

In these examples, Wholleben doesn’t know the underground physical connections that trees use to “help each other out,” but he is aware of the two possibilities: root grafts and fungal connections. It has long been known from both nature and horticultural experience that trees can graft to each other through their roots or stems, leading to the two-way flow of carbon compounds, other nutrients and water. In nature, this may be merely a passive consequence of physical contact and the lack of a means, such as the vertebrate immune system, to distinguish “self” from “non-self.”


More recently, it has been appreciated there are hundreds of species of fungi that form extensive underground networks that enter or enwrap a tree’s roots and supply nutrients and water to the tree and gain carbon compounds from the tree, in a mutualist relationship. The filaments (mycelia) of these mycorrhizal fungi can also extend between trees and transport the above-substances from one tree to another, either of the same or different species.

Whatever the means, Wholleben assumes the transfer between two trees is for the benefit of the impaired tree. There are, however, two more mundane possibilities. In the case of a root graft between two trees, it may be that the intact tree simply takes over a useful bit of impaired infrastructure (roots with no functional crowns). And in the case of the fungal connection, either the transfer is simply passive from high to low concentration, or the fungi are actively making transfers that are advantageous to themselves as they expand into new areas.

This latter possibility offers an especially cruel alternative to Wholleben’s tree-centric view of the tree-fungus functional relationship. From a fungus-centric view, mycorrhizal fungi may have evolved a way to tap into a tree’s carbon production while the tree is still vigorous and to use the resource to partially foster young trees they will then exploit.

Wholleben’s views about the fate of “sick” trees in a forest provides a definitive example of how his view of trees as “social beings” fails. He thinks that when a tree becomes sick, healthy trees come to its rescue with nutrients and water via their root connections and mycorrhizal network, because if they ever become sick, they will be supported back to health by the recovered tree through the same means. Wohlleben says “even strong trees get sick a lot over the course of their lives” and “a lot of trees suffer injuries” [emphases added] that can lead to sickness. A little thought, however, suggests that a forest of trees all helping each other out whenever they are impaired will lead to a forest with an increasingly proportion of weak trees. In nature, impairment can be due to a genetic maladaptation and death is the usual consequence. When a tree in a forest goes down for whatever reason, competition among the young trees sprouting in its place is the natural mechanism that provides for the most genetically suitable replacement tree for that time and space - a process that will sum to the “healthiest” forest possible.

Wholleben says, “if every tree were looking out only for itself, then quite a few of them would never reach old age.” But reaching old age is not the point, maximum number of progeny in the next generation is, however achieved.

The long-appreciated theoretical problem of unrelated individuals within species or species among themselves “helping each other out” is that individuals or species that don’t reciprocate gain a short-term advantage (and evolution is only a “from one generation to the next” process), a process that ultimately undermines the altruistic arrangement. The exception that supports the fact is a situation within species in which the “help” is extended to close kin, in which case the individual is still advancing the immediate cause of its own genes. The other exception occurs within highly cognitive beings, such as ourselves, in which individuals who do not reciprocate are recognised and excluded from the benefits of the arrangement.

Wholleben wants us to believe trees share emotions and other attributes that most readers would think only apply to humans. Trees can “agree,” “approve,” “burnout,” “classify,” “count,” “cuddle,” “decide,” assume a “disguise,” “enjoy,” “practice denial,” plan their “futures,” form “intentions,” “give in,” “make mistakes,” “mourn,” ‘panic,” pine unto death, “pull rank,” “realise,” “resent,” “summon,” “take lessons to heart” and take “vows.” They can be “careful,” “cruel,” “forgetful,” “foolish,” “frugal,” “inconsiderate,” “keen” and “stupid.” They feel “anxious,” “bereft,” “desperate,” “malaise,” “relief,” “triumphant” and “unhappy.” They make “vain attempts” and “take advantage” and “precautions.” They have “goals,” “intuition,” “moods,” “positive feelings,” “priorities,” and “realisations.” They have “sex,” “families,” “children,” “aunts,” “uncles,” “buddies” and “friends.” They “help each other out.” And when they die they become a ‘cadaver,” a “corpse.” Throughout, he writes as if trees have volition and intent.

Two examples show how Wholleben deploys his anthroporphisms. Early in the book, he writes, roots are “conceivably… where the tree equivalent of a brain is located. Brain? You ask.” But he immediately qualifies, “Isn’t that a bit farfetched?  Possibly ….” But with that first mention of a brain the reader conjures up an image of the human brain and weds it indelibly with an image of a tree. Toward the end of the book, Wohlleben rebalances with, “large [?] plants do not have brains, ….” But by then, it’s too late. The association has taken root.

Wohlleben refers repeatedly to trees having “mothers.” Even standing alone, there would be no more emotive word than “mother.” But Wohlleben embellishes. He writes of: mothers and their children; “maternal instincts;” mothers “suckling their children,” “nursing their babies;” “mother’s tender care,” mothers not “approving” of their offspring and forests where “tree children can grow up under their mothers.”

Wohlleben said in a New York Times interview, "I use a very human language. Scientific language removes all the emotion, and people don't understand it any more [“any” more? what have we lost?]. When I say, 'Trees suckle their children', everyone knows immediately what I mean." Indeed, they do.

In a Public Radio International interview, he said, “When you pull all the emotions out of facts, no one is interested. It’s time to translate the research for lay people.” Pull the emotions out of facts? Facts don’t come with emotions; its people that attach emotions to them.

As his success has grown, his analogies have become more bizarre. In a May 2017 talk, he was reported by the Guardian as saying, “When you are walking through a forest in winter time you are walking through tree toilet paper,” and as asking rhetorically if killing older trees to make space for younger trees to grow wasn’t similar to killing human parents to make more room in the house for their children. Wohlleben clearly has other than pedagogical reasons for using emotional language and flouting standard usage. In our use of trees, he wishes to “… spare the trees unnecessary suffering…,” and allow at least some of them “… to grow old with dignity….”

He says, “... I suspect we would pay more attention to trees and other vegetation if we could establish beyond doubt just how similar they are in many ways to animals.”

Unwittingly, however, Wohlleben paints himself into a corner in the way he presents his case for a change in the way humans treat trees. Either he believes trees have all the human emotions he ascribes to them or he doesn’t. If he does, then he has deceived his audience about his use of these terms simply to be better understood. If he doesn’t, then his reasons for treating trees as sentient beings vanish.

Further, if Wholleben believes plants have the same emotions as humans, then he has an intriguing evolutionary question to answer. Plants and humans shared a common ancestor 1.6 billion years ago, and that ancestor was a single-cell organism. Wholleben must tell us, therefore, whether that ancestor had all the emotions of present day plants and animals and passed them on to all its descendants to the present day, or whether the two lines leading to plants and humans evolved those emotions independently in the course of their own separate evolution. And if the latter, whether the emotions are likely to be identical, of different, and if different, how he can possibly know what they are.

The public reception of Wholleben and his book is striking because it reveals not only a poor understanding of basic botany and evolution, but most alarmingly, an inability or unwillingness to think critically. Indeed, some reviewers/interviewers have thanked him for demonstrating what they long thought was true. Others have testified they will never again look at a forest in the same way.

The significance of Wholleben’s book is not for any contribution to the practice of forestry, the botany of trees or the welfare of sentient beings, but rather as a document of the gathering Unenlightenment.

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About the Author

Allen Greer is a biologist who writes about science and nature.

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