To make a tree in Little Alchemy 2, you need to make plant first. Then you could combine Plant with Big to create Tree. See the detailed instructions below.
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