Trees of Knowledge
Trees are smarter than we give them credit for, but they may not be smart enough for we’ve got coming next.
The UN Climate Change Conference in Madrid opened on December 2 by calling the climate crisis a “war against nature.” But trees have always been at war, fighting for their survival. While plants may seem passive in the environment, they can sense their environments, make decisions, and respond to threats—up to a point.
Every autumn holds terrible perils for plants. While many trees drop their leaves every year, the decision of precisely when to do so is a delicate one, as Peter Wohlleben, a German forest ranger, explains in The Hidden Life of Trees. Too soon and trees lose the chance to make food for the coming spring. Too late and an early snow or ice storm can weigh down leaves, tear off branches, and cause fatal injuries.
That decision may seem automatic to a human observer, but individual trees actually make different calls, which shows how subjective the event is. Wohlleben describes 300-year-old oak trees growing side by side. One always sheds its leaves earlier than the others. “The timing of leaf drop, it seems, really is a question of character,” he writes. “The tree on the right is a bit more anxious than the others, or to put it more positively, more sensible.” The two others are “bolder,” he writes, gambling on good weather.
Trees decide when to act based on day length and temperature, which they can easily sense. Rising temperatures mean spring; falling temperatures mean fall. “And what this proves as well,” Wollenben writes, “is that trees must have a memory.”
“This is quite different from the detailed and emotion-filled memories we recall every day,” writes biologist Daniel Chamovitz in the book What a Plant Knows. Yet plants use some of the same mechanisms we do to recall events. Epigenetics is one major way that past experiences stay with us, such as tolerance for cold climates—and with plants, too.
Trees decide when to act based on day length and temperature, which they can easily sense. Credit: Creative Commons
Epigenetic changes affect the way genes are expressed without altering the underlying DNA code. The DNA is wrapped around proteins called histones, Chamovitz writes, and events can change certain histones, which in turn affect which genes turn on or off. In that way, plants “remember” things like bad weather and attacks by insects. Furthermore, they not only remember environmental stress but they can pass those memories on, because epigenetic changes are heritable. Their seeds come prepared for the problems their parents encountered.
That means they can recognise droughts and recall ways to cope. Trees can prepare to close the pores (stomata) on their leaves to limit water loss or grow fewer stomata, and can be ready to bring up more water from their roots. With something called somatic adaptation, the growing tips of branches can use a different phenotype—that is, change their actual physical form—to cope with new anticipated growing conditions. This lets trees survive a wide range of environments. Trees, though we think of them as stationary and static, can use electrochemical gradients to move, as a Venus flytrap closes when an insect touches certain hairs in its trap. These gradients work like the electric signals in our nervous system.
But instead of a nervous system, any centralised source of decision-making, plants have what plant neurobiologist Stefano Mancuso, in The Revolutionary Genius of Plants, calls “collective intelligence.” Each part reacts to changes in its environment, including the changes in neighbouring parts of its own organism, much like a colony of bees. “Even though they have nothing akin to a central brain,” Mancuso writes, “plants exhibit unmistakable attributes of intelligence. They are able to perceive their surroundings with a greater sensitivity than animals do.”