How forests store carbon: photosynthesis and the carbon cycle

How can a forest store something that was floating in the air? The answer begins with photosynthesis, when a tree takes in carbon dioxide. Carbon becomes part of living plants, returns to the atmosphere through respiration and decomposition, and can remain in dead material and soil. Following these movements helps us understand what forests do for the climate and what India's forest carbon figures measure.

From air to plant matter

In photosynthesis, plants use energy from sunlight to turn water and carbon dioxide into carbohydrates, releasing oxygen. Chlorophyll, the pigment that makes plants appear green, captures energy from sunlight for this process. Sunlight supplies the energy, while carbon dioxide supplies the carbon that becomes part of the carbohydrates.

Trees use that carbon to make the material in their roots, stems and leaves. NASA describes this as turning carbon dioxide into the building blocks of the plant. The tree stores carbon as part of its plant matter.

This is one route through the carbon cycle. As NASA's Terra programme explains, carbon moves among the atmosphere, land, oceans and living organisms through biological, chemical, geological and physical processes. Forests take part in that wider cycle, with photosynthesis as one of the processes moving carbon.

Plants release carbon too

A living tree also needs energy for its life processes. Through cellular respiration, it breaks down glucose to release usable energy. Aerobic respiration uses oxygen and produces water and carbon dioxide. The Royal Horticultural Society's explanation of plant growth sets out both processes: plants make food through photosynthesis and release energy from it through respiration.

Plants, like animals, respire. They continue to do so while photosynthesis takes place, so carbon can move into and out of a living plant. To work out how much a whole forest has added to its stores, we need to count both the carbon taken up through photosynthesis and the carbon released.

The cycle continues after a leaf dies

Dead plant material remains part of the carbon cycle. During decomposition, organisms such as bacteria and fungi break down organic matter, returning carbon dioxide and mineral nutrients to the environment. Some carbon in dead leaves and wood remains in the soil after the vegetation dies and becomes buried, rather than immediately returning to the air.

Conditions also matter. NOAA explains that organic matter can release methane as it decomposes in environments with little oxygen. The gases released during decomposition therefore depend on the conditions in which it happens.

Measuring forest carbon therefore means looking beyond living trunks. The Food and Agriculture Organization identifies five pools: above-ground biomass, below-ground biomass, dead wood, litter and soil organic matter. These categories cover carbon stored in living vegetation as well as in dead material and soil. All of these stores count towards the forest total.

A stock and a sink answer different questions

A carbon stock is the amount of carbon stored at a particular time. A carbon sink describes the balance of movements over a period: more carbon is absorbed from the atmosphere than is released. If releases exceed uptake, the forest acts as a carbon source instead.

Natural Resources Canada uses this balance to explain forest sinks and sources. A sink can still release carbon through respiration and decomposition. What makes it a sink is that uptake exceeds release over the period being measured. We need to measure that balance to know whether a forest is a sink.

The distinction helps when reading a report. A stock figure tells us how much carbon a forest holds at a particular time. To see how quickly that store is growing, we need to track gains and losses over time. A change in stock tells us how much the stored quantity has risen or fallen between assessments. Keep the period covered by those assessments attached to the figure.

What India's forest report says

The Forest Survey of India publishes the India State of Forest Report, or ISFR, every two years. ISFR 2023 was released on 21 December 2024. Its edition year differs from its publication year. When using the figures, identify the assessment they belong to, so readers know when the carbon stock was assessed.

ISFR 2023 estimates carbon stock in India's forests at 7,285.5 million tonnes of carbon. The environment ministry's official release reports an increase of 81.5 million tonnes from the previous assessment. The report's comparison is with 2021. The increase is the change between those assessments, not an annual figure.

Check the units as carefully as the number. Carbon and carbon dioxide have different masses: a carbon dioxide molecule contains one carbon atom and two oxygen atoms. Keep the reported unit, tonnes of carbon, when quoting the stock. The figure of 7,285.5 million tonnes measures stored carbon, not carbon dioxide removed each year.

These are national estimates. To find out whether a particular state or forest gained carbon, look for figures at that level. Keep the date, area covered and unit attached to any figure you quote.

Why these movements matter for climate

Rising concentrations of atmospheric carbon dioxide warm the atmosphere. Moving carbon between the air and forests therefore matters for climate. Carbon in wood is in a different place and form from carbon dioxide in the atmosphere. As respiration and decomposition show, stored carbon can return to the air.

Human activities also alter the wider cycle. NASA identifies burning fossil fuels and clearing land as ways people disturb carbon exchanges. Continuing fossil-fuel emissions still cause harm, and forests have limits on how much they can absorb. To understand their role, keep track of both the carbon they hold and the balance of carbon entering and leaving them.

How to read the next carbon claim

When a report says a forest stores or absorbs carbon, ask what is being counted, which unit is used and what period the claim covers. Check whether the number describes a stored total or a change, and whether it includes soil and dead material. These questions connect the biology of a leaf with the meaning of a national statistic.

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