I tried using the filler set on flatten or clear, but I only get a broken filler. I did manage to make the area around the tree flat though.
However, I actually LIKE the trees. The worst thing about them is that they give enough shade for mobs to survive daylight. However, the rubberwood is useful and incredibly plentiful when compared to regular, puny trees that barely give. Also, the rubber they give is a superior alternative to amount than trying to collect rubber resin.