New trees always pop neer other trees but never disappear. That's why after a while (not that long) forest became impossible to cross.
What i propose is that for low quality trees (< 20 ?) they must fade / disappear after a while.
For the moment, we have to cut lot (really a lot) of trees around towns, and ask server's admin to use a script to remove trees in forests, otherwise we can't move from one town to another.