There is at least 2 major forests. Millersweald and then the Ridge Forest. I believe there is also an alpine forest up in Altwood but its speculation. I've spent a bit of time discussing them with the VN team to try and figure out a description for them if we wanted to use it.
Now also remember if the engine can't handle a forest map, there is a grassland and salt flats. Unfortunately these are probably flat areas and wouldn't look all that interesting for gameplay.