When Trees Walk the Earth

Every June, the cottonwood trees of Anchorage, Alaska, burst into motion. After standing silent and immobile all year, the trees flush with long cottony bolls, and the air fills with their drifting seeds, little white puffs that swirl together like a snowstorm and drift into kitchen cabinets, between toothbrush bristles, and onto dark-colored folded laundry. It is an arboreal Feast of Fools, a bacchanalian revolt, the trees screaming that they, too, walk the Earth.

The residents of Anchorage, accidental bystanders to this orgy, complain bitterly. The cottonwoods are messy, they moan. They are unsightly, allergenic. “Cottonwoods aren’t good for anything but making more cottonwoods,” wrote Mike Doogan, a state lawmaker and longtime Anchorage Daily News columnist, summing up the general sentiment. “I think we should have a citywide program to eliminate them.”

The rancor seems out of proportion to any actual inconvenience the cottonwoods cause Anchoragers. It speaks, instead, to the way we understand trees, one of the most ubiquitous yet alien lifeforms with which we share our planet. Usually, trees are easy to ignore, in the same way that a stream or a mountain can usually be ignored. More than any other living thing, trees are a part of the landscape. We expect them to be nearly as unchanging. We often speak of trees as symbols of steadfastness, of reliability, of longevity; unspoken but perhaps assumed is that trees will receive our attention but not require it. To notice a tree on your own, to be struck by its size or beauty or the color of its leaves is one thing, we seem to think, but it is another to be forced into noticing it. Cottonwoods, in short, do not behave as we think trees ought to behave.

This view is more revealing of us than it is of trees. To us, constantly flitting around, moving our bodies so we are comfortable, directing our attention to whatever interests us, the life of a tree seems almost unfathomably still. It is true, an individual tree, once planted, rarely moves around. Married by root and fungal mycorrhizae to the soil and surrounding forest, it is wholly of its place, sculpted by local conditions, by sun and wind and ice. It cannot flee changing circumstances or new enemies, only endure them. But while cottonwoods might be unusually visible in their movements, they are not unique. Every type of tree—indeed, every living thing—is mobile, at least some of the time.

It is worth thinking for a moment about how to describe this movement. “Migration” is an uneasy fit. We use that word to describe the activities of individual fish, birds, and mammals, traveling back and forth with the seasons; also for the voyage of populations of monarch butterflies, from the mountains of central Mexico to Canada and back, a round trip that requires several generations. The journeys of trees, though, can only be understood as a collective action. A seed sprouts and grows up and produces seeds of its own, each moving a bit ahead of the last. The species grows more common in some parts of its range, less common in others. The migration of forests, should we call it that, is the work of many generations.

The travels of trees (and other plants) are different from the migration of animals in another way. Animals are guided by their by their senses—of taste, of smell, of sight, of Earth’s magnetic field. Trees, though, are guided only by conditions, by the fact of where each seed sprouts or fails to sprout. When conditions change, the forest follows.

I grew up in Anchorage. Last year, during a visit home, I saw the movement of trees firsthand. In early June, I set a seedling tray out on my parents’ porch, filled with soil. Most of the seeds I’d planted never came up, but within days, a whole forest’s-worth of tiny cottonwoods appeared. It seemed almost intelligent, although of course it was nearly the opposite. For the cottonwoods to have discovered the single square foot of habitable ground I’d provided meant that a similar number of seeds must have landed on every other square foot. It was conquest by attrition.

In this way, seed by seed, forests have traveled across continents, scaling mountain passes, jumping rivers and inlets. The fossil record shows that forests have often moved with surprising swiftness. As the last ice sheets of the Pleistocene Epoch retreated, some species seem to have galloped northwest across Canada at a mile or more per year. Now, it’s happening again. Around the world, scientists have tracked trees shifting poleward and upslope, expanding into space that has grown newly suitable.

Some scientists worry, though, that this expansion won’t keep up with the rate of modern climate change, and could face extinction. While few species of plants seem to have gone extinct during periods of climate change in the recent past, the pace of change today is especially fast. Animals and birds that once carried seeds from place to place are gone, and many routes are cut off by human development. For decades, some scientists and activists have argued that people should step in and make sure that rare species reach places where the climate suits them. Others, fearing unintended consequences, urge caution.

Some of the effects of climate change on the world’s forests will be slow and subtle. Seeds will sprout in new places, and forests will shift in composition, some species growing more common and widespread, others less so. But other effects will be harder to miss. A changing climate means that, in effect, mature trees are living in a place that is increasingly different from where they sprouted. Scientists in North America and Europe have observed that the rate of background mortality, the number of trees dying even in the absence of drought or other disturbance, has doubled in recent decades.

Warmer temperatures bring deadlier droughts, more of the insects that attack trees, and drier fuels; fires are burning earlier in the spring and later in the fall, spreading in places that haven’t burned in human memory. These effects will likely grow only more obvious. The moments in the past when Earth departed periods of climatic stability for periods of rapid change are marked in the fossil record by thick layers of charcoal and ash.

It is a strange thing, to abandon our old understanding of trees and forests as stable and unchanging. It was a comforting illusion. But I take heart in the cottonwoods. I didn’t mind when they took over my seedling tray. They were just doing, in an unusually visible way, what I know all trees are doing. With a million cottony fingers they reach into an uncertain future, searching for whatever new opportunities await.

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The Journeys of Trees by Zach St. George is available via W. W. Norton.