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.