With a lightmap resolution of 32x32, you aren’t really dealing with that much quality anyways. For a box, or something relatively sharp angled, they might work just fine. For a tree, that’s going to be even the size of the player? It’s going to blow those bad lightmap artifacts right out in the open. Again, you’d be better off with either adding in a greyscale channel for faked AO/cavity or you could vertex paint the tree model’s shadows in. Either way, you could easily do away with their lightmaps entirely. Then from there, you could still project their shadows onto the world lightmap. Win:win.
And no, it doesn’t drastically increase anything if you’re vertex painting in on the landscape; with layers. Just darken the ground in that area. You could manually go around and do that in 5x5km map in the matter of hours. Just focus on areas where players will be. Either way, I’d suggest what I said in the previous paragraph.
EDIT: Looking at the pics you posted, yeah, I’d honestly do away with lightmaps on them. The resolution is definitely so low that it looks like vertex painting anyways.