Greenbull can’t have the wood fruit because that would give him the Moku Moku no Mi, which can’t be the case because that’s the name of smoker’s fruit.
So forest or nature or something more general than wood, but it’s difficult to think of the name.
Maybe the Kojo Kojo no Mi (plant plant fruit)? But plants aren’t a natural “element.”
We also saw him grow mushrooms which are not plants.