The Secret Life of Trees
On the outside, forests look like calm, peaceful places. But hidden among idyllic, moss-covered branches, lies a constant branch-to-branch contest to grow taller and faster than everyone else.
Trees grow by converting water from the soil, carbon from the air, and sunlight from the sky into their food, sugar. We call this process photosynthesis. Leaves capture carbon dioxide gas by opening small holes called stomata. But here’s the problem: if carbon can enter the leaf, then water can also evaporate away. As one carbon dioxide molecule moves into a leaf, hundreds of water molecules are simply lost to the air. To make four sugar cubes, a tree must spend about a gallon of water.
Forests are a business operating on razor thin margins. They must conserve water as much as possible, only opening stomata when sugar can be made quickly. If it’s too dark, leaves won’t be able to combine water and carbon dioxide. If it’s too cold, the chemistry of photosynthesis moves too slowly. And trees can’t just hunker down and conserve water either. If a tree doesn’t take every chance possible to capture carbon, their neighbor will make more sugar, grow taller, and block out the sun.
Forests have some amazing talents to make a living in such a difficult business. First, each leaf on a tree is closely tuned to its surroundings. If the conditions are right for photosynthesis, a leaf can open its stomata and start making sugar within seconds. And the leaves are not working alone. The roots will send chemical signals throughout the entire tree, providing each leaf with constant updates on how much water is available and whether the time is right to make sugar.
But there’s another problem: gravity. Photosynthesis happens in leaves, but water they need comes from roots in the soil. During summer, a healthy tree may consume over 20 gallons of water in a day. For a tree that’s as tall as a building, how do you lift all that water to somewhere you can actually use it?
The solution to moving water hundreds of feet upward is beautifully simple: use a straw. You can think of trees like giant bundles of bendy straws. These small tubes, which scientists call xylem, carry water slowly but surely up the tree with the same exact physics that lets you and me enjoy a glass of lemonade. Remember how trees lose lots of water through their leaves? As that water evaporates, it pulls up on water lower down in the xylem and roots, literally sucking up more water into the leaves. Even though water is evaporating, there is a steady flow of water up the tree that prevents the leaves from drying out. In this way trees not only move water upward but use very little energy to do so.
Trees are bona fide experts in making sugar and moving water. But they may not be ready for the changes humans are causing on our planet. Climate change is changing the business of being a tree. Warmer and drier weather means that trees are losing water to the air faster and have less water in the soil to work with. Because of this water stress, some forests may become grasslands in the next century. This means less habitat for animals, less timber for humans, and a little less beauty in the world.
So, next time you find yourself in the woods, think for a second about the dance happening around you. Every tree built itself out of thin air - literally. The choices we make in the future will decide whether this dance will continue.
Keenan Ganz is a graduate student in Remote Sensing at the University of Washington. He uses specialized cameras on satellites and drones to study forest health and wildfire. One day, Keenan wants to build an improved forecasting system to understand when and where wildfire will burn next.