The trees are plants, thus they are part of the biosphere. With their roots, the trees manage to create lot of pressure on the rocks, thus contributing to their weathering through breaking them up. Even though this weathering is happening because of the usage of physical force, because it is force created by living organism, it is considered as biological process. The mechanical weathering in this situation is exactly that, the physical force used, so the tree roots manage to create two different types of weathering at the same time.
