What type of biome is this? (and how do I make I make it a forest?)

Forest.

that is not a biome its a layer ,there are 5 layers (sky/space , surface , underground , caves , hell) thats the surface and its very very similar to forest basicaly the only thing that changes is the background but if you want to make it forest put 10-20 trees in the same place and make sure the dirt has green grass and itshould work