The U.N. Climate Change Conference in Madrid opened on Dec. 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. 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 recognize 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 centralized 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 neighboring 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.”
Chamovitz writes, “intelligence is a loaded term,” but “plants are acutely aware of the world around them.” They can sense different kinds of light, evaluate chemicals in the air including those emitted by other plants, distinguish different kinds of touches to their leaves, and locate gravity. “And plants are aware of their past.” They don’t know us as individuals, but they know their environment, “and people are part of this environment.”
What they may not realize is how dramatically the human part of their environment is altering the rest. Winter comes later, spring comes earlier, storms come more erratically and cause more damage. Summers are hotter, and droughts occur more often and are more severe.
Trees know how to adapt, “but there is a limit,” says Andrew Mathews, a professor of environmental anthropology at the University of California–Santa Cruz. His work emphasizes forestry and sustainability. In Europe, the last ice age ended about 12,000 years ago, which is only 20 generations for trees, he says, so “adult trees are out of sync anyway” with the climate, but that’s not their most immediate problem.
“Trees don’t live in climate. They live in weather,” he says, and they can adjust several times in a lifetime to cope in ways that animals can’t, dropping leaves earlier or later, changing the shape and texture of their leaves, altering structures in their trunks, or growing deeper or shallower roots. Trees can even “migrate” to better climate zones by sending seeds carried by winds or animals to sprout in new areas. But can they move fast enough? Even if they do, new habitats are limited. Trees in mountainous areas can migrate to higher latitudes as temperatures rise, but even if they move fast enough, they may be pushed upslope “until they have nowhere else to go and will become extinct” in that location, Mathews says.
Even if plants do succeed in adapting to climate change by changing their location or schedule, they’re not alone in their biospheres. The flowers they may decide bloom earlier or later need pollinators. Will the bees, birds, butterflies, wasps, spiders, and flies move in tandem with the trees, and if they do, will they be on the same schedule? Scientists just don’t know.
We can debate the definition of “intelligence,” but we know that trees can identify and solve problems in ways that we can’t. They remember that spring is coming, and when it does they’ll be ready to sense the weather and make their decisions in response. But what they don’t know is that their decisions become more critical with every passing season and year. Our decisions are just as critical. But we know that we can foresee the future—and as smart as they are, the trees can’t.