What you can do is chop out your forests into smaller pieces, and do a mask for each individual forest.
that's how I've been using the forest generator, works wonderfully.
If one or more forests are larger than 1024x1024, you can still do it, just chop the mask into 2 or more pieces with overlap, and then mask/erase as needed to stitch them together. the WIP thread for my New Empire map may be helpful to you..
and here's where we started talking about multiple files for masks in the forester thread..