Forest is the home of animals, in Little Alchemy 2 you could combine Animal, Fog, Grave, Orchard, Park, Wild Boar, Wolf, Nad Woodpecker with the element Forest. The process of making Forest is not harder than Tree, you just need to make Life, Plant, Tree, and then Forest. Check out the detailed walkthrough and try it yourself.
Fire + Earth = Lava
Lava + Earth = Volcano
Water + Water = Puddle
Water + Puddle = Pond
Water + Pond = Lake
Lake + Lake = Sea
Earth + Sea = Primordial Soup
Volcano + Primordial Soup = Life
Earth + Life = Soil
Life + Soil = Plant
Plant + Big = Tree
Tree + Tree = Forest