Sorry, but this is a limitation of Forest. We have done some tests using horizontal textures, but the result is not very good... mainly because the transition effect between the vertical and horizontal images while the camera is moving.
Although if you need only an image, not an animation, i'm thinking on a work around:
- Create a Forest object, using the horizontal shadow plane to paint the tree, with a material NOT based in Planes Material (because it hides the shadow planes). As texture, you can use any greenish texture + the horizontal shadow masks included with Forest (as opacity map).
- Create other Forest, overlapped with the first, only for the vertical shadows. It must use Planes Material (to draw the shadows), but the material may be transparent (opacity: 0%), or we will get some strange effects because the tree texture.
Well, this is a bit hard to explain with words... i've attached a sample scene.
You can also use some tree meshes and use the Custom Mesh feature, but this may be a pain if the scene have a lot of trees.