What is the best material to approximate a tree

I am curious as to what material is best to represent a tree.

I am calculating the radiation in a terrain in order to understand where certain uses may be better off in a building. the simulation is composed of: the terrain (which I assigned ground material) and the trees.

The trees are represented as a extruded cross in order to save computer resources.

i would like what material do you recommend in order to simulate those trees.

Here is an Image of the model.

Regards.

Diego