If you draw two forestry designations with a one-square gap between them, you'll be unable to designate squares in that gap (as if they were mining/dumping designations with incompatible elevations). When erasing a one-square space in an existing designation to create such a gap, the problem doesn't occur.
Sadly, they do carry height information ... you can see it when you try to place a mining deignation next to a forestry one. on slopes the height value can be tramatically different from what one would epxect.
Would be good to be improved.
Ideally mining/dumping and forestry get properly separated (separate overlays!). Though having them together can be useful, as they are kind of mututally exclusive.