How much carbon does a forest contain?

Question

Trees in a forest, alongside the fungi, animals and plants that live there in, capture atmospheric carbon dioxide and store it inside their tissues and in the soil as humus.

It is possible to estimate the amount of carbon stored in some portion (say 100 square meters) of a climax forest?

This should depend on the specific biome, for example Eucaliptus and Pine forests seem to contain less organic matter than broad-leaved tree forests in temperate regions.

PS: A slightly more speculative aspect of this topic is to evaluate weather reforestation can contribute to lower the amount of carbon dioxide in the atmosphere.

Answer 1

There are some well established methods for this. For example, estimating forest C stocks and fluxes is done for national C accounts which are used by the Intergovernmental Panel on Climate Change (IPCC) for global reporting.

The usual method is to measure the diameter of all trees in a defined area (a plot, e.g. like you suggest of 100 square meters), then use allometric equations to estimate either the volume or biomass of each tree, then convert this to C (which is simply 50% of the total dry biomass). This gives you the C stock of all trees in your plot, which can then be scaled up to the entire forest if you wish (you would need lots of plots to make a credible estimate).

Allometric equations vary between tree species due to their differences in shape and wood density. However the general proportion of C varies little between species, it is always close to 50% of biomass. It is better to use allometric equations specific to the tree species you are measuring but if none are available you can use a generic equations:

For live tree biomass, diameters of a sample of trees are measured and converted to biomass and carbon estimates using allometric biomass regression equations. Such equations exist for many forest types; some are species-specific, whereas others, particularly in the tropics, are more generic in nature (e.g., Alves et al., 1997; Brown, 1997; Schroeder et al., 1997). Cutting and weighing a sufficient number of trees to represent the size and species distribution in a forest to generate local allometric regression equations with high precision, particularly in complex tropical forests, is extremely time-consuming and costly and may be beyond the means of most projects. The advantage of using generic equations, stratified by ecological zones (e.g., dry, moist, and wet; see Brown, 1997), is that they tend to be based on a larger number of trees (Brown, 1997) and span a larger range of diameters; these factors increase the precision of the equations. A disadvantage is that the generic equations may not accurately reflect the true biomass of trees in the project.

If you already have wood volume data, which is often available for production forests, you can use a Biomass Expansion Factor to convert volume to biomass, and then to C.

Another method is to use lidar (laser scanning) to generate a 3D model of the trees in your plot and then calculate their volume -> biomass -> C.

Since trees are the easiest component of the forest to measure, there are methods to estimate the other C pools (litter, soil, roots, dead wood) because it is generally too difficult and time consuming to measure these. E.g. dead wood is generally 5-40% the C of live trees, but ideally you would use an estimate based on a similar forest type.

You specifically mention climax forest - though the method is the same for all forests it is likely to be less accurate for climax forests because they tend to be more variable in tree size and most allometrics have been developed for smaller trees (e.g. in plantations) and are usually not as accurate for larger trees. To improve accuracy you would need to cut down some trees and measure all of their components to derive your own allometrics.

BTW eucalypt and conifer forests in temperate regions have the highest C stocks per unit of land area of any forests.

Answer 2

This question is very broad and this would require a lot of calculations, I will focus on the carbon dioxide in the trees, hopefully some other users can add details in respect to the organism and their carbon dioxide storage.

I found an article(R.Jandl et al.,(2007)) which did some calculations, this site does a really good job in explaning these calculations so I will cite that:

As trees photosynthesise they use carbon dioxide (CO₂) from the atmosphere with water from rain or irrigation and nutrients from the soil to form carbohydrates, which make up the tree’s biomass, but how much carbon is made by a tree in this process? Researchers at Ecometrica have worked it out! The amount of carbon stored by a tree depends on its size, which in turn is influenced by factors, such as species, local environmental conditions and the way it is managed. In an attempt to find a simple answer to this question, researchers at Ecometrica have broken down approximately how much carbon is stored in each element of a typical tree (the branches, the leaves, the stem and the roots) by percentage for a quick and simple calculation.

They used standard forestry practices to estimate the amount of carbon contained within the stem, branches, roots and leaves of a mature sycamore (Acer pseudoplatanus), found near their office in Edinburgh, with a diameter of 52 cm and a stem height of 12 m.

First they measured the various parts of the tree and, from these measurements; they calculated their volumes, which were then used to calculate the biomass (mass of living matter in plant tissues): The radius of the stem at 1.3 m above the ground (r1 = 26 cm) and at the top of the stem (r2 = 20 cm), and The height of the stem (h = 12 m) They used known relationships between the biomass of the stem and the biomass of roots, branches and leaves to estimate the total biomass of the tree: Stem volume was estimated using the equation for the volume of a truncated cone:           

which gave an estimated stem volume of 2.0 m3 The wood density of sycamore is approximately 620 kg/m3, so using the equation:

Mass = Density × Volume the stem biomass was 1243 kg, or approximately 1.2 tonnes The biomass of the roots, branches and leaves of a sycamore tree are known to be around 26%, 11%, and 1% of the total biomass, respectively. These proportions were used to estimate the total biomass of the tree without having to dig up roots or cut down branches. In summary, the sycamore tree had a stem with a biomass of 1.2 tonnes, roots of 0.5 tonnes, branches of 0.2 tonnes and leaves of 0.02 tonnes, giving a total biomass of 2 tonnes.

The carbon content of woody matter (stem, branches and roots) and that of leaves is approximately 50% of  their biomass. Multiplying the total biomass by the proportion of that biomass which is carbon gives an estimate of the total amount of carbon stored in the tree, which in this case was exactly 1 tonne! Each tonne of carbon equals to approximately 3.67 tonnes of CO₂.

Then the further calulations really depends on the density of the trees in the forest (if you know the exact number let me know). There are a lot of atricles which gave some numbers the range is based on these articles somewhere around 30-400(and some even 1200 trees). I used the data from this article, which measured the optimal density for growing. I assumed that the climax forest is just naturally so I just picked 400 trees per acre (this is just guessing offcourse and I just generalized this for all trees, but I saw 400 in an other article):

Then we can just do some simple math for the carbon dioxide for the trees:

total CO_{2 for the trees} = 400 x 3.67 = 1468 tonnes

Let me know if someone can find the tree density of a climax forest

Answer 3

It is certainly possible to make an estimate, and other answer have already elaborated on the methods used. As to the underlying question, how much this contributes to store carbon: Lewis et al have published a study (in Nature) estimating the amount of carbon stored over long time periods in African tropical forests. They combined this with other publications and estimated that 1.3 Pg C yr-1 is stored in the tropical forests of our planet.