I don't know what look you're going for, but have you tried making the forest tiles that aren't just one tree over and over, but a tiling pattern of multiple trees, possibly indistinct ones? That could look more like a forest. Tiling the same tree with all its detail inevitably starts to look like an artificial pattern rather than a forest, I think. That's not necessarily a bad thing though, it can work very well for a game where the maps are meant to be symbolic rather than representative.
Among your individual trees, I think the last one has the best sense of volume, but it's too symmetrical. If you tweak that, it should look pretty good!