How Do Trees Absorb CO2? The Science, Simply

Trees absorb CO₂ through photosynthesis: their leaves take in carbon dioxide from the air, use energy from sunlight and water drawn up through the roots to convert it into sugars, release oxygen as a by-product, and lock the leftover carbon away in new wood. A growing tree is essentially carbon pulled out of the sky and stored as solid trunk, branch and root — though how quickly and how much it captures depends heavily on the species, its age and the local climate.

How photosynthesis captures carbon, step by step

• Leaves breathe in. Tiny pores called stomata open and draw carbon dioxide from the surrounding air.
• Sunlight powers the reaction. Chlorophyll captures light energy, which the leaf uses to split water and rearrange the carbon.
• Sugars are built. Carbon, hydrogen and oxygen recombine into glucose — the fuel and the building material for new growth.
• Oxygen is released. The oxygen left over from splitting water is returned to the air we breathe.
• Carbon is locked in. The tree turns those sugars into wood, so the carbon stays put for decades or even centuries.

The counterintuitive part: a tree is mostly made of air

Ask most people where a tree's mass comes from and they say the soil. It mostly comes from the atmosphere. The carbon in a five-tonne trunk arrived as carbon dioxide gas, one molecule at a time, through pores on the leaves. Soil supplies water and mineral nutrients, but the bulk of the structure is assembled from air and sunlight.

This is why a forest works as a carbon store at all: growth is the mechanism. A tree that is not adding wood is not adding carbon, which is also why a mature, slow-growing forest stores an enormous amount while absorbing relatively little each year, and a young, fast-growing one does the opposite.

Where the carbon actually goes

Most of the captured carbon becomes the physical structure of the tree — the trunk, branches, bark and roots — while a meaningful share also enters the soil through fallen leaves, dead roots and the fungi living around them. Roughly half of a tree's dry weight is carbon, so a mature, heavy tree represents years of patient, compounding absorption rather than a one-off deposit. That is also why healthy soil matters as much as the canopy: undisturbed ground can hold as much carbon as the trees standing on it. The trees you can follow on your Evertreen forest keep sequestering more each year as they add wood, instead of capturing a fixed amount once and then stopping.

How much CO₂ does one tree absorb?

Honestly, it varies a lot. A commonly cited range is ≈ 10–40 kg of CO₂ per year for an established tree, but the real figure swings widely with species, growth rate, rainfall, soil and how much room the tree has to spread. The timing matters as much as the total: young saplings absorb very little in their first years — sometimes well under a kilogram — then accelerate sharply as their leaf area and wood volume expand, before tailing off again in old age. That early lag is why treating a freshly planted sapling as an instant 20 kg saving is misleading, and why any single one-tree-equals-a-number claim is best read as a rough long-run average, not a first-year guarantee. The sizing arithmetic is in how many trees to offset your carbon footprint.

What happens to the carbon when a tree dies

This is the question that separates a carbon store from a carbon sink, and it deserves a straight answer: much of it comes back. As deadwood decays, microbes break down the carbon compounds and release CO₂ back into the atmosphere. If the tree burns, the release is immediate.

Some carbon does persist — in stable soil compounds, in long-lived timber products, in the next generation of trees that grow using the nutrients released. But permanence is a real constraint, not a technicality, and it is exactly why certified forestry projects hold a share of credits in a shared buffer pool against fire, disease and clearance, and why monitoring matters more than planting volume. A forest that is protected for a century stores carbon; a forest that is planted for a photograph does not.

Trees versus other carbon removal

• Trees are cheap, immediately available, and deliver biodiversity, water and community benefits alongside carbon — but they are slow and their permanence must be managed.
• Soil carbon is enormous in aggregate and genuinely difficult to measure, which is why it is harder to certify.
• Engineered removals such as direct air capture store carbon far more durably, but cost an order of magnitude more per tonne and are limited in scale today.

These are complements, not competitors. Planting is the option that is available now, at a price most organisations can act on, with visible co-benefits.

How Evertreen estimates this — conservatively

Rather than quoting one flattering figure, Evertreen uses a transparent method for estimating tree CO₂ that deliberately leans conservative and accounts for species and real growth over time; you can read the full approach in how we estimate tree CO₂. Because every tree is geolocated and traceable — with GPS coordinates and progress photos — the growth behind those estimates is something you can actually verify rather than take on trust. And if you want to size your own footprint before planting from £1.5 per tree, our free CO₂ calculator gives you a grounded, no-obligation starting point.

Frequently asked questions

Do trees absorb CO₂ at night? Photosynthesis needs light, so active carbon capture happens during daylight hours. At night trees respire and release a small amount of CO₂, much as we do, but across a full 24-hour cycle a healthy, established tree remains a clear net absorber.

How much CO₂ does a tree absorb in a year? Often somewhere around 10–40 kg once it is well established, but this varies hugely by species, age, soil and climate, and is far lower — sometimes near zero — in a tree's first few years.

Do older or younger trees absorb more? Fast-growing, established trees usually absorb the most each year because they are adding the most new wood. Very young saplings capture little at first, and very old trees slow down again, so a tree's strongest years for absorption are typically in its productive middle age.

Where does a tree's mass come from? Overwhelmingly from carbon dioxide in the air, combined with water. Soil supplies water and minerals rather than the bulk of the structure.

What happens to the carbon when a tree dies? Much of it returns to the atmosphere as the wood decays or burns, though some persists in soil, in timber products and in subsequent growth. This is why permanence and monitoring matter.

Do forests store carbon in the soil too? Yes, and in many ecosystems the soil holds as much carbon as the trees above it, accumulated through leaf litter, dead roots and fungal networks.

Which trees absorb the most CO₂? Fast-growing species in warm, wet conditions accumulate biomass quickest, and mangroves are exceptional because of the carbon locked into their sediment. The species suited to the site usually beats the fastest grower on paper.