How do you know how much CO2 a tree captures?

This depends on many factors. Therefore, one can only calculate with average values here. For our planting area, we refer to a study on trees in Latin America. According to this study (Poorter et al. Nature, 2016), a tree there binds about 200 kg of CO2 during the first 20 years, i.e. about ten kilograms per year. This figure refers to the average uptake of trees in Latin America. Of course, this study does not address each individual tree, but calculates by hectare.

And, also important to know: Of course, the absorption of CO2 increases with the age of the trees: a 35 meter high Manilkara zapota (approx. 80 cm trunk diameter at 1.30 height), for example, stores 215 kilograms of CO2 per year, i.e. 21 times more. And this is exactly where the great potential of old trees lies. That’s why Plant-for-the-Planet is also stepping up its efforts to protect existing forests. Considering that half of the CO2 of a forest is only bound in 5% of the trees, forest protection is a very meaningful approach.  

Cf. also: https://doi.org/10.5751/ES-02781-140131