Forest 1 and 2 are found in a branch off Grassland 7. Forest 3-6, however, are found after Beach 4. All Forest levels are shrouded by fog, which thins out with altitude. The Forest terrain tends to be rather level. The Forest stages use the same landscape as the Opening Street, but with different tilesets:
While Forest 3-6 maintains the landscape, it has a different tileset:
It's possible that Forest 1 and 2 are intended to be pine forests and Forest 3-6 are intended to be rainforests.
List of Forest stages
There are six stages in the Forest series: