Trees get their carbon from carbon dioxide (CO₂), a gas that makes up about 0.04% of the atmosphere. Every carbon atom in a tree’s trunk, branches, leaves, and roots was once floating in the air as part of a CO₂ molecule. On average, 47.4% of a tree’s dry weight is pure carbon, all pulled from this single atmospheric source.
How CO₂ Enters a Tree
Trees absorb carbon dioxide through tiny pores on their leaves called stomata. Each stoma is formed by a pair of guard cells that can swell or shrink to widen or narrow the opening. When stomata open, CO₂ from the surrounding air diffuses into the leaf’s interior, where it dissolves into the moist surfaces of the cells inside.
Opening stomata comes with a tradeoff: the same pores that let CO₂ in also let water vapor escape. Trees constantly balance their need for carbon against the risk of drying out. Light, humidity, and even the concentration of CO₂ itself all influence how wide the stomata open. When CO₂ levels in the air are higher, trees can afford to keep their stomata partially closed, taking in enough carbon while losing less water. When levels drop below normal, stomata open wider and the leaf surface develops more pores to compensate.
As atmospheric CO₂ has risen in recent decades, researchers have observed that many plant species are responding by reducing both the number and the size of their stomata. This lets them maintain the same rate of photosynthesis while conserving water.
Turning CO₂ Into Sugar
Once inside the leaf, CO₂ reaches the chloroplasts, the tiny green structures where photosynthesis happens. The overall reaction is straightforward: six molecules of CO₂ combine with six molecules of water, powered by sunlight, to produce one molecule of glucose (a simple sugar) and six molecules of oxygen. For every CO₂ molecule a tree absorbs, it releases one oxygen molecule back into the air.
The key step is carried out by a single enzyme responsible for more than 90% of all carbon fixation on Earth. This enzyme grabs a CO₂ molecule and attaches it to an existing sugar molecule inside the cell, splitting the result into two smaller molecules that the cell then rebuilds into glucose. It’s a surprisingly slow enzyme, processing only a few reactions per second, but trees compensate by producing enormous quantities of it. By some estimates, it’s the most abundant protein on the planet.
The glucose produced in this process is the tree’s basic building block and energy currency. It can be burned for immediate energy, shipped to growing tips and roots, or converted into the structural molecules that make up wood.
From Sugar to Wood
A tree doesn’t stay made of glucose for long. Most of that sugar gets transformed into two main structural materials: cellulose and lignin. Cellulose is a long chain of glucose units linked end to end, sometimes thousands of units long, forming strong fibers that give wood its tensile strength. Lignin is a very different kind of molecule, a complex, cross-linked structure that acts like a glue binding cellulose fibers together and making wood rigid and water-resistant.
Together, cellulose and lignin are the dominant components of wood. Every bit of carbon locked in these molecules traces back to CO₂ that once drifted through the atmosphere. A mature tree can store hundreds of kilograms of carbon this way, and that carbon stays locked in the wood for as long as the tree stands. According to the USDA Forest Service, the carbon fraction of wood varies widely by species, ranging from about 18% to 75%, with the average across U.S. tree species sitting at 47.4%.
A Small Bonus From the Soil
While the atmosphere is overwhelmingly the main carbon source, roots can also absorb small amounts of inorganic carbon from the soil. Soil air contains far more CO₂ than the open atmosphere, sometimes reaching concentrations of 1,000 to 23,000 parts per million compared to roughly 420 ppm above ground. Dissolved bicarbonate in soil water can also be taken up by root hairs.
Lab experiments have shown that carbon absorbed through roots gets incorporated into sugars and transported up to the leaves. This pathway may offer an efficiency advantage: roots can take in CO₂ from their damp surroundings without the water loss that comes from opening leaf stomata. Still, the contribution is minor compared to what the leaves pull from the air. For practical purposes, the atmosphere is the tree’s carbon supply, and CO₂ is the molecule that delivers it.