Why trees are not weighed directly
The only way to know exactly how much a tree weighs is to cut it down, dry it and put it on a scale. That is done for a few trees in research studies, but it is obviously not practical for a whole forest, and it defeats the purpose when the goal is to keep the forest. Instead, scientists measure trees that are standing and estimate their mass using relationships built from the trees that were weighed.
The quantity being estimated is usually biomass: the dry mass of the living tree material. Carbon is then derived from it. Biomass is also the starting point for tree-based figures reported in carbon accounting and used in the study of the global carbon cycle.
Step 1: measure the tree
The inputs are simple field measurements. Trunk diameter at a standard height (often called diameter at breast height) is the most important. Tree height is also often used, and so is species, since different species have different wood density. These are the measurements described in the article on measuring tree height and diameter.
Step 2: apply an allometric equation
Allometry is the study of how the size of one part of an organism relates to another. For trees, researchers cut and weigh a sample of trees of different sizes, then fit an equation that predicts the mass from measurements such as diameter, height and wood density. A basic form uses diameter alone. More detailed forms add height and wood density.
These equations are called allometric equations. Because the trees in the sample came from a particular region and forest type, an equation is most reliable for similar trees. Using an equation built for one climate on a very different forest can give misleading results. Some equations cover many species across wide areas, which makes them convenient, but their errors can be larger for any single stand.
Mass tends to rise faster than diameter: a tree twice as wide is usually much more than twice as heavy. That is why a few big trees often hold a large fraction of a forest’s biomass, a point that matters for old forest carbon.
Step 3: add the parts you cannot see
A first estimate often covers only the above-ground parts: trunk, branches and leaves. Roots hold a substantial share of a tree’s mass, but they are difficult to measure. Methods typically estimate roots as a proportion of above-ground mass, and the proportion varies by forest type. Dead wood, leaf litter and soil are counted separately. Soil carbon is a large store and is the subject of the article on soil carbon storage.
Step 4: convert biomass to carbon
Wood is mostly made of carbon, hydrogen and oxygen. Roughly half of the dry mass of wood is carbon, and a common simplification is to multiply dry biomass by about 0.5. More refined values for particular species or tissues are often slightly different, typically a little below or around that figure.
To express this as carbon dioxide equivalent, which is the unit used in many climate reports, multiply the carbon mass by 44/12, or about 3.67. This is the ratio between the weight of a molecule of carbon dioxide and the weight of its carbon atom.
A hypothetical worked example
Suppose an allometric equation estimates that one tree has a dry biomass of 400 kilograms above and below ground. This number is made up purely for illustration.
- Carbon at an assumed 50 percent: 400 × 0.5 = 200 kilograms of carbon.
- Carbon dioxide equivalent: 200 × 3.67 ≈ 733 kilograms.
If a hypothetical plot of 0.1 hectare contains 20 such trees (and nothing else is counted), the plot holds 20 × 200 = 4,000 kilograms of carbon, which scales to 40,000 kilograms, or 40 tonnes, per hectare. Real forests contain trees of many sizes and species, so each tree gets its own estimate before the totals are added. The scaling from plots to a larger forest follows the logic described in the article on forest inventory plots and sampling.
Where uncertainty creeps in
Every step adds error, and good reports show it.
- Measurement error. Slightly mismeasured diameters are magnified because the equations are steep.
- Equation choice. Different equations can give quite different answers for the same tree, especially for large or unusual trees.
- Wood density. Assuming an average density for a species can be off for individual trees.
- Missing parts. Roots, dead wood and small stems are often estimated indirectly.
- Sampling. The plots may not represent the rest of the forest.
- Change over time. Carbon stock is a snapshot. Growth, death and harvest change it, which is why repeated measurements matter.
Newer tools
Remote sensing can supplement field data. Airborne laser scanning gives canopy height and structure across a wide area, and models link those measures to biomass at plots. This is the idea behind forest lidar, and some satellite sensors are designed to estimate biomass at a coarse scale. These methods still need field plots for calibration, because the sensors measure structure, not carbon itself.
What the numbers mean
Biomass and carbon estimates are best read as ranges rather than exact values. They are useful for comparing forests, tracking changes over time and informing planning, as long as the method is consistent. A tree carbon figure also says nothing about how long the carbon will stay stored, which depends on fires, harvest and management, and that is the subject of carbon permanence.