Carbon sequestration by forests is one of the most important ecosystem services for the stability of Earth's climate. Without forests, atmospheric CO2 concentration would be significantly higher than it is today. But the global forest system is under pressure: deforestation, wildfires, drought, and pest infestations are reducing absorption capacity, while climate change itself alters the balance between photosynthesis and respiration.

How trees capture CO2: the mechanism

Photosynthesis is the process by which plants capture atmospheric CO2 and convert it into solid organic matter (sugars, then cellulose, lignin, starch). The simplified equation: CO2 + H2O + light → CH2O (organic matter) + O2. The captured carbon is incorporated into the tree's structure: wood (cellulose and lignin: the majority of carbon), bark, roots, leaves. How captured carbon is distributed: roughly 50% of the carbon fixed by photosynthesis is respired by the tree itself (autotrophic respiration: the tree uses part of the energy for its own metabolic functions, releasing CO2). The remaining 50% is Net Primary Production (NPP): carbon accumulated in biomass and soil. Of this 50% that goes into the soil as roots, exudates, and litter: about 60–70% is mineralized by soil decomposers and released as CO2 (heterotrophic respiration). The remaining 30–40% becomes stable humus (long-term sequestration in soil). The net ecosystem balance (NEE: Net Ecosystem Exchange): the difference between total photosynthesis and total respiration (trees + soil). In a young, rapidly growing forest: the NEE is negative (the forest absorbs more CO2 than it releases). In a mature old-growth forest: the NEE can be close to zero or slightly negative. Not because the old forest isn't important for climate: but because its contribution is maintaining an enormous carbon stock (not adding new carbon). If the old forest is destroyed, all that carbon is released rapidly into the atmosphere.

How much CO2 does a tree capture: the real numbers

Estimates of CO2 sequestration per tree vary enormously depending on species, age, climate conditions, and soil type. Some representative estimates for common trees in Italy: mature oak (Quercus robur, 50 years, lowland forest): annual sequestration of 10–22 kg CO2/year. Over its lifetime (100 years): total carbon stock of 500–2,000 kg C = 1,800–7,300 kg CO2. Mature beech (Fagus sylvatica, 50 years, mountain forest): annual sequestration of 8–18 kg CO2/year. Plantation poplar (Populus x hybrida, 10 years, intensive cultivation): much higher annual sequestration due to rapid growth: 25–50 kg CO2/year. But short lifespan and low final stock (the tree is felled at 10–15 years). Scots pine (Pinus sylvestris, 50 years): annual sequestration of 6–15 kg CO2/year. Olive tree (Olea europaea, 50 years): annual sequestration of 5–12 kg CO2/year. Factors influencing sequestration: growth rate: fast-growing trees (poplars, eucalypts, black locust) sequester more CO2 per unit of time than slow-growing trees (oak, beech, yew). But durability matters: a poplar felled after 15 years and used as biomass releases the carbon; an oak that lives 200 years retains it. Size: the sequestration rate increases with size up to a maximum that depends on species and environment. The largest trees (80–150 cm diameter) sequester more carbon per year than medium-sized trees (Stephenson et al., Nature, 2014). This has important management implications: protecting large trees is more effective for CO2 sequestration than felling and replacing them with young plants.

The climate threat to forest sequestration: a system in distress

The global forest carbon sequestration system is under growing pressure from multiple fronts. Drought and heat: as the climate warms, the frequency and severity of droughts is increasing in many regions. Droughts reduce photosynthesis (stomata close to limit water loss), increase tree mortality (xylem cavitation: water columns in the xylem break under excessive tension), and increase vulnerability to pests (e.g., bark beetles in spruce forests stressed by drought). Tree mortality in Europe has doubled in temperate forests between 1990 and 2020 due to combined effects of drought and extreme heat. Wildfires: forest fires (increasing sharply due to climate change) rapidly release accumulated carbon. In Europe, the 2022 fire season burned approximately 750,000 hectares (the worst in 15 years), releasing hundreds of millions of tonnes of CO2. Pest infestations: the bark beetle (Ips typographus) in Alpine spruce forests, the pine processionary moth in pine forests, Xylella fastidiosa in Apulian olives: pests favored by climate change that kill millions of trees, transforming CO2 sinks into sources. Saturation of tropical forests: recent research (Brienen et al., Nature, 2015) suggests that Amazon tropical forests are showing signs of reduced sequestration capacity due to drought and increased mortality. From reliable sink to a system in distress.

The carbon market and forests: opportunities and risks

The economic value of forest CO2 sequestration has generated a rapidly expanding forest carbon market, with significant opportunities but also serious risks. Forest carbon credits: mechanisms like REDD+ (Reducing Emissions from Deforestation and Forest Degradation: UN), national carbon credit programs, and the Voluntary Carbon Market allow forest owners to sell CO2 sequestration credits to companies wanting to offset their emissions. In theory: a mechanism that pays for forest conservation and creates an economic incentive to maintain them. Risks of the forest carbon market: additionality: the carbon credit should represent sequestration additional to what would happen without the project. But many carbon credit projects certify forests that weren't at risk of deforestation anyway. permanence: the carbon sequestered by a tree is "permanent" only if the tree doesn't burn, isn't felled, and doesn't die for other reasons. Wildfires can erase decades of sequestration in days. greenwashing: some companies use forest carbon credits to communicate "carbon neutrality" while continuing to emit without real reductions. This risk has discredited the voluntary forest carbon market. The scientific position (IPCC, 2022): forest sequestration is an important climate mitigation tool, but cannot replace fossil emission reductions. Forest carbon credits are a useful tool if rigorous, but not a license to keep emitting. In Italy: the European ETS (Emissions Trading System) doesn't yet include the forest sector. Regional programs exist (e.g., Lombardy Region, Tuscany) with incentives for afforestation recognizing carbon sequestration. FSC (Forest Stewardship Council) certification includes carbon sequestration criteria in certified forests.

How to measure a forest's carbon: tools and methods

Measuring carbon sequestered by a forest is a technical field with increasingly sophisticated methods. Ground-based methods (in situ): forest inventory is the basic method. The diameter at breast height (DBH: diameter at breast height, at 1.30 m from the ground) and height of every tree in sample plots are measured. Allometric equations are applied (correlating biomass with DBH and height for each species) to estimate total biomass and contained carbon. Soil carbon is measured by taking soil samples and laboratory analysis of organic content (loss on ignition, elemental analysis). Remote sensing methods: satellite imagery (Sentinel-2, Landsat: free; WorldView, Pléiades: commercial) provides data on forest cover and above-ground biomass (using the NDVI index: Normalized Difference Vegetation Index). Aerial LIDAR (Light Detection And Ranging) measures the vertical structure of the forest (height, canopy, structure) with high precision. Radar satellites (Sentinel-1, ALOS PALSAR) penetrate the canopy and estimate total biomass. Monitoring networks: eddy covariance networks (meteorological towers with CO2 and water vapor sensors installed in forests) directly measure CO2 exchanges between the forest and atmosphere in real time. The European ICOS network (Integrated Carbon Observation System) includes dozens of forest sites across Europe, with some in Italy (San Rossore, Collelongo, Renon).