Tree Carbon Sequestration

At a glance:Tree carbon sequestration is the amount of carbon a tree fixes from atmospheric CO₂ by photosynthesis and stores in woody tissue (trunk, branches, leaves, roots). Forests and urban trees thus act as 'carbon sinks', an important natural mechanism against climate change; quantifying tree carbon is used for carbon accounting, urban-greening mitigation-benefit assessment and afforestation tracking. The core is two conversions. Step 1: carbon stock from biomass: carbon = dry biomass × carbon fraction. 'Dry biomass' is the water-free dry weight (live trees contain water, fresh weight would overestimate), and by scope may count only above-ground or include the root system (often via a root:shoot ratio). Biomass itself is usually not weighed directly but estimated by allometric equations from easily measured diameter at breast height (DBH), tree height and wood density, with species- and region-specific equations. The carbon fraction is carbon's share of dry biomass; because wood is mainly cellulose, hemicellulose and lignin, carbon content is fairly stable, mostly 0.47–0.5 across species, IPCC defaults to 0.47 or 0.5; this tool defaults to 0.5. Step 2: carbon stock to equivalent CO₂: CO₂ equivalent = carbon × 44/12. The 44/12 (≈3.667) comes from molecular weights — CO₂ molecular weight is 44 (carbon 12 + two oxygens 2×16), carbon atomic weight is 12, so every 12 mass units of fixed carbon corresponds to 44 mass units of CO₂ removed from the atmosphere. Example: a tree with 1,000 kg dry biomass and 0.5 fraction → carbon = 1000 × 0.5 = 500 kg C; CO₂ equivalent = 500 × 44/12 ≈ 1,833 kg CO₂, meaning this tree's woody tissue removed about 1.83 t CO₂. Correct use and interpretation need notes: first, this is 'carbon stock' — the carbon cumulatively fixed now — distinct from 'annual sequestration rate', which looks at biomass increment over time; mature large trees store much but their annual increment slows, while fast-growing middle-aged trees have higher annual rates. Second, when a tree dies, is felled or burned, the fixed carbon is released again, so the sink is 'reversible' — accounting must consider permanence and emission timing. Third, biomass estimate accuracy depends on whether the allometric equation fits local species and site; use measured carbon fraction if available. Fourth, a full forest carbon pool also includes litter, standing dead wood and soil organic carbon; this tool focuses on living-tree carbon from biomass. In short, tree carbon uses two concise conversions — carbon = biomass × carbon fraction and CO₂ = carbon × 44/12 — to quantify a tree's mitigation contribution into comparable, additive numbers, a foundation tool for greening and carbon-sink management.