This looks stupid - and you can see if from ground level
Short of selecting and manually moving trees do I have any options here? it's not connected to landscape tile boundaries - just looks like they're using a grid based distribution and neglected to deal with the edges of each cell
for what I'm sure is a "very good" reason this is caused by enabling the Collision with World option in the placement settings.
This is all painted - I'm doing that for the main forests because procedural spawners don't give me enough control to carve specific paths through the trees etc. and like you say, covering large areas is quick.
I'll probably use foliage spawners for distributing undergrowth and sparser stuff like bushes in meadows as placement is less important - finding the balance is a little tricky but it'll work itself out as I go.