Is it possible to edit the tree billboards generated by a terrain?

The title says it all. I've created a group of trees, eucalyptus trees in particular, which are quite bushy at the top. The problem occurs when in billboard mode, the nice bushy tops/sides are cut off, which looks a little wrong.In short, is it possible to tweak the billboards? Or, possible to change the parameters of how the billboards are generated?

Thanks and cheers, Daniel