Forests are major carbon sinks because they remove carbon dioxide from the air and store carbon in living plants, dead organic matter, and soil. This matters because carbon dioxide is a greenhouse gas that affects Earth’s climate. A mature forest tree acts like a carbon pump, taking in CO2 through its leaves and moving carbon into its trunk, branches, roots, and surrounding soil.
When many trees grow together over decades, the forest can hold a large amount of carbon.
Key Facts
- Photosynthesis stores carbon: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- About 50% of dry wood mass is carbon, so 100 kg of dry wood contains about 50 kg of carbon.
- Carbon dioxide and carbon are related by mass: CO2 stored equivalent = C stored x 44/12.
- Forests store carbon in aboveground biomass, belowground roots, dead wood, leaf litter, and soil organic matter.
- Respiration, decomposition, wildfire, and logging can release stored carbon back to the atmosphere.
- Net carbon storage increases when photosynthesis removes more CO2 than the forest releases through respiration and decay.
Vocabulary
- Carbon sink
- A carbon sink is a system that absorbs more carbon from the atmosphere than it releases over a period of time.
- Photosynthesis
- Photosynthesis is the process by which plants use sunlight to convert carbon dioxide and water into sugars and oxygen.
- Biomass
- Biomass is the total mass of living or recently living plant material, such as trunks, branches, leaves, and roots.
- Soil organic carbon
- Soil organic carbon is carbon stored in decomposed plant and animal material within the soil.
- Decomposition
- Decomposition is the breakdown of dead organisms and waste by microbes, fungi, and other decomposers, often releasing carbon dioxide.
Common Mistakes to Avoid
- Assuming trees store carbon only in their leaves is wrong because most long-term carbon storage is in wood, roots, dead organic matter, and soil.
- Confusing carbon with carbon dioxide is wrong because carbon is one element, while CO2 is a molecule with extra oxygen that has a greater mass.
- Thinking all forests always remove CO2 is wrong because fires, drought, disease, logging, or rapid decomposition can make a forest release more carbon than it absorbs.
- Ignoring soil carbon is wrong because forest soils can store large amounts of carbon for years, decades, or even centuries.
Practice Questions
- 1 A tree gains 80 kg of dry wood in one year. If dry wood is 50% carbon, how many kilograms of carbon did the tree store that year?
- 2 A forest stores 1200 kg of carbon in new biomass. Using CO2 stored equivalent = C stored x 44/12, how many kilograms of CO2 were removed from the atmosphere?
- 3 A mature forest has high photosynthesis but also high respiration and decomposition. Explain why it might store carbon more slowly than a young growing forest.