The backroute turns every lix in the first tree with an exploder.
The 3 trees have different treetops. This is necessary to fix an almost-backroute where the lixes get bunkered in the rightmost tree.
The intended route digs in the first tree to turn the last lix. This must remain possible. Forestidia's br1 turns everybody in this tree. This must be prevented. It's very delicate to allow one and not the other.