Quote from Captain_Chaos
I don't see how you get that from my answer. You can erase forest layers, namely by right-clicking with the forest layer selected.
But perhaps you mean something else? Are you talking about erasing existing trees from an existing map? That has nothing to do with layers (that's a WorldPainter term), but in that case you're right, it's not yet possible to do that with WorldPainter. You can do it relatively quickly with a tool llike WorldEdit or MCEdit, by selecting an area and changing all wood and leaf blocks in that area to air.
What the hell, right clicking was one of the first things I thought I tried before I even asked that question but it turns out I guess I didn't. Now I just feel retarded :/
Thanks for the kind reply though!