When you generate your world you can change the amount of layers per layer zone. I don't know the minimum off hand but it is probably around 6 normal stone or

When you generate your world you can change the amount of layers per layer zone. I don't know the minimum off hand but it is probably around 6 normal stone or cave levels, minus the levels used for open sky space and magma sea which I don't know the minimum for. Trees are multilevel now and I don't know that they will grow on a single layer.