The trees that you see below were made in Maya and to make them, I simply first made a texture file for the leaves in photoshop and applied them to individual faces of the tree. I then made the tree stump from a cylinder and extruded it. Afterwards, I attached the leaves to the tree.
For the grass, I used a particle system in Maya to generate the individual strands. I then covert it to a polygon and applied a green texture to it.