Walking through a temperate forest can be a magical experience. All of your senses are engaged. The air you breathe is moist and fresh thanks to the trees, bushes, ferns, and ground covers that take in carbon dioxide and release oxygen and water vapor. Look up and notice how the trees in the overstory share the sky, arranging their branches and leaves in relationship with their neighbors—a behavior called “crown shyness.”
Look around and see life at different levels in the understory. Younger hardwood trees grow slowly, thickening their trunks so that when a space opens above, they can climb quickly to claim a spot in the sun. Alders, poplars, birches, and willows with softer, faster-growing wood spring up at the end of winter, undaunted even in an already crowded forest. These trees are sometimes called opportunists or pioneers because after a forest burns, is wiped out by a flood or landslide, or is cut down, they are the first to fill the void.
Below the crown and above the ground are the harder-to-see species that live in and on the trees. You might hear singing or scurrying, and if you patiently follow your senses, spot birds or squirrels and their nests. But myriad species are nearly impossible to spy. In old, lush forests, entire ecosystems grow in soil held aloft by branches.
The understory’s bushes leaf out rapidly at winter’s end, absorbing the sun’s spring light before the big trees have filled the canopy. Over the summer, the bushes convert their sugars into nutrient-rich berries that hold hard seeds. Animals of all kinds are attracted to these colorful, delicious treats—you may notice hoof or paw prints near fruiting bushes. The animals metabolize the nutrients in the skin and flesh of the berries. The seeds, however, pass through their digestive tracts intact. Animal droppings act as packets of fertilizer for the seeds within. Paws, hooves, and feet will push some seeds into the soil, helping the bushes’ offspring set roots in new places.
These subtle complexities may be difficult to spot, but fallen trees are easier to notice. Look under or inside a fallen tree; you will find abundance in miniature. Trees decompose over years, providing a home, shelter, or platform for animals, fungi, lichen, and moss. As a tree disintegrates, its fibers contribute to the soft, spongy, aerated ground that holds moisture and space for seeds to germinate and take root.
Too small for your eye to detect and too numerous for our minds to comprehend are the forms of life just underground. Scoop out a teaspoon of soil, and you have lifted nearly one billion microscopic organisms from their home. The surface of the forest floor and the top layer of soil are full of busy organisms making new life out of fallen leaves, bark, animal droppings, and carcasses.
You may sense deeper, more connected forces at work around you, especially in an old forest. In the dark below the topsoil are systems whose intricacies and importance even scientists are just starting to comprehend. Burrowing creatures make tunnels and holes. Their digging for food, such as worms, small creatures, nutrient-rich bulbs, roots, or shelter, further aerates the soil and provides space for more kinds of life.
The holes and tunnels are crossed by the roots of trees and networks of tiny tube-like fungi that carry water, sugars, minerals, and other nutrients between the trees. Through the network of fungi, trees even alert their community to incipient threats, increasing the chances that the forest survives.
Over thousands of years of climate stability, the forest’s animals, plants, insects, and organisms adapted to this specific location. They learned to thrive within the timing of the forest’s seasons, its high and low temperatures, its rainfall and snowfall, the depth and duration of the winter freeze, the intensity and frequency of summer rains, and the other species that live there, from hosts to rivals and predators to prey to parasites.
Some living things traveled there by wind, on their own legs, or in the stomachs of other animals to what turned out to be an ideal home. Others evolved through mutation, with each generation experimenting to uncover traits better suited to the specific location. The long stability of the Holocene climate enabled wondrous complexity.
Humans are relative newcomers to Earth. Almost all species in this forest—and every other ecosystem—have been living on Earth longer than we have. Certain ancient species of plants, trees, and other organisms have survived climatic changes on Earth over hundreds of millions of years—will they survive into the future? The answer depends on the speed of climate change and whether we can help other species adjust.
Nature needs space and time to adapt and evolve.
Over thousands of years, pollen and seeds traveling successfully to a new suitable location allowed even the sturdiest plants to migrate, sometimes over great distances. Now, pollen and seeds are extremely likely to arrive on land controlled by humans who would classify any unfamiliar plant as a weed. Animals and insects can explore new areas when their homes become unsuitable, but enormous, single-crop fields cannot support the biodiversity that wild spaces can.
The faster we change the climate, the less diverse and abundant the world’s lifeforms will be.
Carbon-rich biomes, and the species that thrive in them, take time to develop. Small forms of life with shorter life spans, like bacteria and insects, are more adaptable, in part because they produce hundreds or thousands of generations of offspring before larger species can reproduce once. In contrast, it will be harder for slow-growing species like trees and large animals to survive.
At present, the atmosphere is warming faster than it has at any time in at least tens of millions of years—too fast for many species to either adapt or change where they live. If we can dramatically slow warming, we increase the chances that most of the plants and animals with whom we live—and on whom we depend—will adapt with us.