The easiest way to do it would be using porousSimpleFoam and adding the tree canopy geometries as porous zones.
Precise values of porosity, across types of trees and species, are very hard to come by. You can imagine the complexity, if we consider movement of the canopy alone and porosity on 3dimensional space.
What you could do is play with the porosity coefficients until you achieve the % of reduction you expect from a tree you are including in your model.
We are hoping to be adding the porosity recipe in the next BF update. It is an idea in the works for some time now.
I also plan to share an example of simulating tree canopies in OF in the last of the 3 Butterfly webinars. I will share the case folder in the forum as soon as I have it prepared.
Kind regards,
Theodore.