We visit each planting one time per year. Our Monitoring and Reporting specialist leads a team from our partner or local office, together with interested farmers.
First we count the number of surviving trees. Then we take out a tape measure and measure around the trunks of one of one in every 10 or 20 trees; 20 if it is a very large stand. We also measure height
The above ground (dry) biomass (AGB) of standing trees is calculated using the following formula:
AGB = pi/4 * f * d * DBH2 * H
Where DBH is ‘diameter at breast height’, H is the height of the tree and d is the wood density, which is variable among tree species; i.e. some wood is heavier than other. The value for many species can be found in databases. pi/4 ‘translates’ the sqaure (DBH2 ) to a circle, f is a facor for the sahep of the tree.
We then calculate using the ‘rule of thumb’ that 1 ton of dry wood biomass = 0,5 tons of carbon. Each tonne of carbon equals 3,6 tonnes CO2, because the molecular weight of CO2 is 3,6 times the atomic weight of C (Carbon)
This sounds relatively easy, yes? It is not that easy, actually….
How do you, for example, measure the DBH? That is straightforward in a plantation on flat ground with trees each of which have only one stem/trunk. You take a tape measure and measure around the tree. Then you convert this circumference, as measured with the tape measure to diameter. But not all trees stand in plantations, and not all on flat ground. This picture shows what you need to do, if the situation is more complex:
And how do you measure height? When the trees are small we measure by a measuring stick (and show you the stick next to the tree, on our pictures on the webpage). When the trees are larger, you can use an app on our smartphones. It can do the trigonometry. We will spare you the equations here, but feel free to ask!
And all of this can only happen when the trees are growing, on trees that are already some years old.
So how do we know how many trees are needed to offset e.g. 5,000 tonnes of CO2?
The short version is we assume an average number for the tree growth each year over 20 years. The assumption is based on (very conservative) estimates by experienced tropical foresters. If you want the whole explanation, please ask.
The longer description is: We are getting quite a lot of data from our plantings, so we know how many species grow, at least in their younger years. For many species we can et information about average height and diameter at maturity. From that we get a pretty good idea how much carbon they will contain whne they are fully grown. We know, for example, that the largest tree we plant (and indeed one of the largest known species) the ‘Muvule’ (Milicia excelsa) will contain in excess of 30 tonnes CO2 equivalents when it is fully grown.