the material file for the oak trees is called "WombatOak.material" there are only four textures as all of these mats are oak trees and the one called "OakComplete" is used for pretty much all of them. "OakFoliage, OakFoliageFall, OakFoliageSummer" are all optional materials for if you wish to change ONLY THE LEAF TEXTURE of each of the trees to fit a seasonal type map. there is also "OakBark" in there as one of the trees did not use the OakComplete properly.
Hope this helped.