The distribution of the sizes of individual trees in a forest matches the distribution of the sizes of individual branches of a single tree from that forest.
By measuring the thickness of each branch and the distance to the next smaller branch you find out the fractal branching rule of the tree.
With this rule you can calculate how much CO2 the tree is taking in.
By extrapolating the fractal branching rule to the whole forest you are able to calculate the CO2 intake of an entire forest.
(Excerpt from PBS-documentary: Fractals – Hunting The Hidden Dimension)