Do trees scream or strategise? How do they live in forests?

In Roald Dahl’s short story “The Sound Machine”, a man invents a device through which he can hear sound at frequencies previously denied to the human ear. Much to his horror, he hears – or thinks he hears – the shrieks of flowers and plants as they are being uprooted.

He bent down and took hold of a small white daisy growing on the lawn […] From the moment that he started pulling to the moment when the stem broke, he heard – he distinctly heard in the earphones – a faint, high-pitched cry, curiously inanimate […] it wasn’t pain; it was surprise. Or was it? It didn’t really express any of the feelings or emotions known to a human being […] A flower probably didn’t feel pain. It felt something else which we didn’t know about – something called toin or spurl or plinuckment, or anything you like.

Dahl, whose birth centenary is being celebrated this September, was famous for his macabre, twist-in-the-tail stories, and the “The Sound Machine” contains the very particular mix of black humour and paranoia that he did so well. Yet it also touches on a subsidiary theme of his work: how much do we know about the inner lives of the countless other creatures that share our planet, including the ones we are barely willing to impute inner lives to? Isn’t it too easy to dismiss something as improbable, even irrational, just because it lies outside the purview of human experience?

I thought of the story again when I read the following passage in Peter Wohlleben’s The Hidden Life of Trees, which is a very different sort of book – a forester’s personal and professional journey of discovery that grows into a warm, accessible work of popular science:

There is research in the field that reveals more than just behavioural changes: when trees are really thirsty, they begin to scream. If you’re out in the forest, you won’t be able to hear them, because this all takes place at ultrasonic levels […] This is a purely mechanical event and it probably doesn’t mean anything. And yet? […] these vibrations could indeed be much more than just vibrations – they could be cries of thirst. The trees might be screaming out a dire warning to their colleagues that water levels are running low.

(Now back to Dahl for a moment, and a description of a tree’s response to an axe embedded in its trunk: “…a harsh, noteless, enormous noise, a growling, low pitched, screaming sound, not quick and short like the noise of the roses, but drawn out like a sob lasting for fully a minute…”)

“The Sound Machine” first appeared in The New Yorker in 1949, while The Hidden Life of Trees was originally published in German in 2015, with the English translation (by Jane Billinghurst) just out. Those 66 years would represent a tiny span of time to the large forest trees – often living for well over a thousand years – that Wohlleben writes so affectionately and insightfully about, but you’d expect huge advances in scientific development in the human world over such a period.

And yet, as Wohlleben repeatedly reminds us in his book, the pace of our knowledge and understanding has been creepingly slow when it comes to this vast and important subject. There is still so much we don’t know about trees, their complex co-dependent survival mechanisms, how they respond to stimulus, danger and opportunity, and this is especially true for the giants in natural forests, which have – in relative seclusion – gone about the business of interacting with countless other living creatures, while helping to maintain the stability of a whole ecosystem.

Wohlleben worked for the forestry commission in Germany for two decades, and admits that for years he himself looked at trees in the dispassionate way a butcher looks at the animals he is cutting up. When he did look more closely, his eyes were opened to a new world, one that he superbly reveals in this book.

Much of his narrative centres on research results – made available only in the past two decades – that have shown the presence of the “wood wide web”: a complex underground network facilitated by fungi, which serve a function comparable to that of fibre-optic internet cables, by helping trees transmit information to each other. Using this as a starting point, Wohlleben covers a number of issues related to trees growing in natural forests (his area of specialisation), but also making universally applicable points about plant life and its wider effects on the planet.

This book skillfully explores both the major and minor keys of its subject. On one hand, it offers a majestic view of the world’s big forests as water pumps that enable rain water to penetrate deeper into land masses than it otherwise would (which is why places located hundreds of miles from the ocean are hospitable to humans and other life forms). But there are also fascinating, ground-level descriptions of things that most people who don’t live or work near forests will never get to see, such as the “all-out drinking binges” engaged in by beeches during heavy rain:

[The rainwater] runs along the branches, where the tiny streams unite into a river that rushes down the trunk. By the time it reaches the lower part of the trunk, the the water is shooting down so fast that when it hits the ground, it foams up.

Or another roaring sound, this time caused by the excrement of millions of oak leaf roller caterpillars high up on a bare oak.

Thousands of black pellets were bouncing off my head and shoulders. Ugh!

Wohlleben has some fun along the way, and since he is writing for the layman, there are some cute chapter heads like “Street Kids” (for a wonderfully poignant section about the life cycles of trees that were planted as trophies in parks or on roadside kerbs, without adequate thought to their long-term needs), “Love” (for a discussion of procreation cycles), “Forest Etiquette” and “Ageing Gracefully”. Occasionally, the prose comes close to anthropomorphising in such a way as to give the impression that a plant’s behaviour is exactly comparable to that of humans. There are sentences like “Many Central European tree species have similar ideas about the ideal place to live…”, or “The yew, the epitome of frugality and patience, has decided to make the most of these conditions.” “The oak realizes it cannot beat this stiff competition,” we read, “and will never be able to grow tall shoots to overtake the beech. Perhaps in the face of rising panic, it does something that goes against all the rules…” And: “From then on, [the spruce] will also do a better job of rationing water instead of pumping whatever is available out of the ground as soon as spring hits without giving a second thought to waste. The tree takes the lesson to heart...”

Read closely though, and the context makes it clear that Wohlleben isn’t indulging in pseudoscience by attributing human-like agency or forethought to non-human creatures: he is trying to provide a sense of their behaviourial patterns – and how the survival-of-the-fittest theme plays out in their world – in language that we homo sapiens, limited as we are by our own consciousness and our own weird ways of doing things, might understand.

When I heard the title of this book, I immediately thought of Peter Tompkins’ and Christopher Bird’s The Secret Life of Plants, which developed a cult following after it was published in 1973. In his Introduction to Wohlleben’s book, Pradip Krishen warns of the dangers of such an association, pointing out that the Tompkins-Bird study verged “more on psychobotany than hard science”.

Most serious students of the subject are likely to have similar reservations. Having only skimmed through the older book, I have been both intrigued and bemused by some of its descriptions of obscure experiments conducted outside the realm of mainstream science, and in some cases, discredited over time. It is full of anecdotes such as the ones about plants responding to spooky stories being told in a dark room, and the authors breathlessly drawn on a wide range of sources and histories, from Jagadish Chandra Bose’s work in the realm of plant physiology (“which was buried during his lifetime by Western science and hardly ever cited since his death”) to Johann Wolfgang von Goethe’s conception of an “archetypal plant, a supersensible force capable of developing into myriad forms”. While this means the book is full of interesting vignettes, it doesn’t quite resolve itself into a cohesive narrative (and that may not have been the intention anyway). In comparison, Wohlleben’s book is meticulously organised, professional and focused.

And yet. Whatever you think about The Secret Life of Plants – with its seemingly outlandish claims and the goofily buoyant tone aimed at a New Age readership of the 1970s (the “superannuated hippie”, as Krishen drolly puts it) – it was rooted in a serious desire to extend the boundaries of a subject that had not been given enough attention. Wohlleben is more circumspect, and more in tune with rigorous, peer-tested science, but the starting point for his book too was a sense of wonder, a curiosity about life’s hidden mysteries. And he himself mentions that research has only just begun to skim the surface of how plant-life works. “So many questions remain unanswered,” he writes while discussing the ambiguities that still surround the movement of water from soil into a tree’s leaves, and the intriguing possibility that transpiration and capillary action aren’t as important as scientists have been thinking. “Perhaps we are poorer for having lost a possible explanation or richer for having gained a mystery.”

A running theme in The Hidden Life of Trees is that trees operate on very different time-scales from the ones we are used to, both in terms of their life-spans and in terms of how long it takes them to process information and translate it into action. Is this the main reason why we find it so difficult to recognise them as truly sentient beings with their own versions of personalities, Wohlleben wonders. (“Man merely thinks plants motionless and feelingless because he will not take the time to watch them,” the biologist Raoul Francé said once. The quote is included in The Secret Life of Plants, but it could just as easily have found space in Wohlleben’s book.) Or is it because we were cut off from these distant cousins so early in our shared evolutionary history that even the colour green – aesthetically pleasing though it is to our eyes – feels closer, as a “skin colour”, to alien beings than to a species we can identify with?

Whatever the case, Wohlleben believes that the distinctions usually made between plant and animal life are arbitrary. Among the many achievements of his book is the food for thought it provides on the staggering kinds of symbiosis in nature: from imperilled trees having to close their wounds after fungal invasions to deer using young trees (often with fatal consequences for the latter) as rubbing posts for their excess skin. There are reminders that nature is implacably cruel even while it is being jaw-droppingly beautiful, but there is equally a caution about the dangers of an increasingly anthropic world. In a moving coda, Wohlleben writes, “Forests are not first and foremost lumber factories and warehouse for raw material, and only secondarily complex habitats for thousands of species, which is the way modern forestry currently treats them. Completely the opposite, in fact.”

His book should convince many readers of this, even if they have never read Dahl, even if they aren’t fans of Tolkien’s melancholy Ents or Blyton’s The Enchanted Wood – or, for that matter, hippies high on the idea of a universal consciousness linking all things.

The Hidden Life of Trees: What They Feel, How They Communicate – Discoveries from a Secret World, Peter Wohlleben, translated from the German by Jane Billinghurst, introduced by Pradip Krishen, Penguin Allen Lane.