Peter Wohlleben’s “The Hidden Life of Trees”

Until recently I was quite sure that a broad difference between animals and plants was that animals, because they are mobile, readily interact with each other (flocking, pursuing, etc.) while plants, anchored to the ground, don’t do so because they can’t. Except to attract insect pollinators, plants, I thought, live a life of exquisite solo struggle, seeking only the sun and water.

I’ve been steadily learning how far off I was. German forester Peter Wohlleben’s popular book, The Hidden Life of Trees: What They Feel, How They Communicate, is the most compelling lesson yet.

Among his many descriptions of communication and mutual assistance is Wohlleben’s account of how trees defend not only themselves but also each other. Observers have noted, for example, that umbrella thorn acacias in the African savannah pumped toxins into their leaves when they felt giraffes nibbling on them. “The giraffes got the message and moved on to other trees in the vicinity. But did they move on to trees close by? No, for the time being, they walked right by a few trees and resumed their meal only when they had moved about 100 yards away.” They passed by the nearest trees because the trees being nibbled, in addition to pumping a repellent, “gave off a warning gas that signaled to neighboring trees that a crisis was at hand.” The giraffes knew these trees would not taste any better and kept walking.

hidden life of trees (

Many trees also have the ability to call in the air force. Reacting to bites from hostile insects, such trees emit scents that attract predators that devour the pests. “For example, elms and pines call on small parasite wasps that lay their eggs inside leaf-eating caterpillars.” The growing larvae devour the caterpillars from the inside.

The book brims with information and appreciations of this kind. Three more examples:

  • Trees that spend their lives in the forest fare much better than trees raised in one place and then transplanted to the forest. “Because their roots are irreparably damaged,…they seem almost incapable of networking with one another.” Like “street kids,” they “behave like loners and suffer from their isolation.”
  • Time for trees is slow and long. Internally, they, like animals, send alerts to parts of their body via chemicals and electrical impulses. But in a tree the electrical impulses move only about a third of an inch per second. (In our bodies, pain signals move  through our nerves about two feet per second, muscle impulses a hundred times faster.) No wonder it seems to us that plants are unresponsive.
  • Conifers (evergreens) “keep all their green finery on their branches” throughout the winter and have been doing so for 270 million years. Then deciduous (leaf-bearing) trees came along 100 million years ago, growing and discarding annually millions of delicate green solar panels. Was this an improvement? Why go to all that trouble? Wohlleben asks. Because “By discarding their leaves, they avoid a critical force—winter storms.” Between high winds, muddy soil, and a surface area equivalent to that of a large sailboat, tall evergreens take a battering in European winters. Growing and then dropping their huge surface area every year proved well worth while for the leafy new comers.

Wohlleben’s liberal use of human descriptors to explain the actions of trees delights many readers and annoys others. Andrea Wulf, in her review of the book, has both reactions.

I’m usually not keen on anthropomorphizing nature—and here trees are “nursing their babies” and having “a long, leisurely breakfast in the sun” while…fungus mushrooms are “rascals” who steal sugar and nutrients. These cutesy expressions make me cringe….But I have to admit that Wohlleben pulls it off—most of the time—because he sticks with scientific research and has a knack for making complex biology simple and thoroughly enjoyable.

I agree. While the vocabulary may bestow on trees a dignity and affection that we usually reserve for our own kind, it is scientists’ growing understanding of trees that creates the real story here. At a time of rapid environmental change, the book is as fascinating a revelation as one could ask for that life is even more intricate and purposeful than we knew.

Global Warming and the Cold War

When I was 14, my father brought me down to the furnace area in our basement to show me the water, food, and other supplies he had stockpiled to the ceiling. He gave me memory tips for finding my way to his hometown in North Dakota and the drug store where “they will take you in” if the East coast was bombed. I remember watching Kennedy on television in 1962 during the Cuban missile crisis the first year I was at college and thinking I might never get home. Into the 1980s, the nightmare was that if the shock waves and the fallout didn’t get us, the global chill from the atomic dust would.

The melting (

The melting Arctic (

Today the grand anxiety is of global warming instead of nuclear winter. Here in New Jersey, though, the concern is subdued. The state energy plan is all about natural gas and new pipelines, it’s a rare house that has solar panels, and people build new homes, on stilts at least, along the water.

The complacency about global warming comes in part from our disconnect from the environment. Rain and temperature are important to people according to how much or how little difference they seem to make in our lives. Perhaps early farmers and hunters, closer to the earth, would have reacted more readily to predictions of hotter summers or worse flooding. But in American suburbia, environment is mostly background. Weather is good or bad, a day is “beautiful” or not. The climate doesn’t feel closely connected to the food we buy or how well our houses keep us warm. Climate of course intrudes when storms hit or temperatures are extreme, but otherwise our human world is just that—a world settled and run by humans, not one bounded by nature.

We know better. We need to heed the data and find the will to act. We won’t have the fall of the Soviet Union to help get us through this time.

Evolution for a Warmer Climate

The climate of the next few centuries “is expected to change 10 to 100 times faster than in the recent geological past.” Slowing this disastrous shift is one struggle. Another is figuring out how to help earth’s organisms to survive it. Humans have long maneuvered evolution to breed plants and animals for their own use. But as the earth grows warmer, understanding and experimenting with how species adapt to change is taking on a more ambitious, long-term purpose.

Animals and plants are responding to current climate changes in two ways. They adjust within the capacity of their DNA—plasticity, as it’s called—and they adapt by changing genetically. They demonstrate plasticity when birds breed and plants flower earlier as spring comes sooner. Animals show flexibility when they migrate towards cooler poles and higher elevations. Many species that scientists had considered doomed have shown greater flexibility than expected, but such plasticity has its limits.

Genetic changes—true evolution—are profound but also less adjustable in the short term. These are changes that can enable a species to resist new diseases and to thrive in new environments. The species that have the best odds for making such  evolutionary leaps are generally those with short lives (and therefore frequent reproduction), many off-spring, genetic diversity, and mobility.

Copepods off the west coast show some flexibility to water temperature but don't migrate enough to make larger genetic adjustments. (

Copepods off the west coast show some flexibility to water temperature but don’t migrate far enough to make larger genetic adjustments.

A case in point: The shrimp-like copepod swims off the west coast all the way from Mexico up to Alaska. That range of water temperatures might suggest that copepods would have no trouble adjusting to warmer water if they needed to. But the local populations are acclimated only to their local temperature and they don’t travel far enough to breed with other copepods that can handle warmer oceans. They are stuck within a limited plasticity.

Purple sea urchins are a different story. They must cope with the increasing acidity in the ocean that is one effect of a warming climate. Like the copepods and temperature, sea urchins in different parts of the ocean tolerate their local acidity. But the sea urchins are better off than the copepods because, essentially, sea urchins are more sociable. They are more likely to migrate, interbreed, and adapt genetically to higher levels of tolerance.

Purple sea urchins move around and intermingle, making them good candidates for genetic changes that will help them tolerate the ocean's rising acidity.  (

Purple sea urchins move around and intermingle, making them good candidates for genetic changes that will help them tolerate the ocean’s rising acidity.

Many species of animals and plants, like the copepods, will be unable to adjust enough to survive. So humans are beginning to step in. Conservation groups are connecting wilderness areas so that migrating species aren’t cut off by highways, cities, suburbs. One example are the Yellowstone to Yukon Conservation Initiative corridors between wild landscapes that run from Yellowstone Park to northern Canada.

In more active interventions, scientists have moved the seeds of trees in order to acclimate them to new environments. Animals too have been transported in order to change their genetic strain (though not yet for climatic reasons). When Florida’s panther population dropped below 30 in the 1990s, eight females that were brought from Texas created enough genetic variation to overcome the effects of inbreeding and multiply the population.

For now, scientists are mostly gathering information. How much can organisms adjust before they die off? Which necessary changes are genetic and therefore relatively slow? And—a devilish question— which species should we try hardest to save? The ones most at risk? Or those most likely to survive? The popular animals like lions and elephants? The ones that carry out key functions in the survival of other species, like bees?

Such interventions and assessments reassure and unnerve me at the same time. As the climate worsens and the urgency grows, as the effort to save species scales up, imagine the conflicts that will emerge. We have seen the debates over GMOs, carbon capping, sea coast construction. Imagine the political feuds and violence of the future when temperature and rainfall have shifted so much that it’s time to commandeer new land for farming, to organize globally to select animals and plants for survival, to abandon regions that are turning to desert and floodplain, to allocate water to some communities of species and not others. As the stakes get higher, the human role on the planet will grow more urgent and potentially more dangerous.