It would be nice that trees would fall to the opposite direct that you were chopping it at, so you can control the direction of the fall. Additionally, the tree segments check if they will hit anything on the way down. If they do hit something, they do damage to it, if the damage doesn't destroy the object, the tree segment is destroyed and it spawns a bunch of randomly spread wood blocks.
Tree trunks could take environmental damage, after two hits the tree trunk is destroyed, leaving room for future trees to grow. You could do that idea you had for how they respawn with looking to see if a tree is near and looking to see if a tree isn't to close then spawning a sapling if before the new map reset you plug variables into a simulator and get back what the tree density that the effect would create, so that the new map has said tree densities. Additionally, if there isn't any tree remotely close, the tree tile could change to something different.
Hemlocks: As they grow instead of radiating forest, they radiate dirt. The dirt should turn back when not near a hemlock. Hemlocks would create hemlock groves, which I totally love.
Lastly, with environmental damage, why not have it "damage" only things that have their origin on the tile? This would mean even kilns would be "hit" the same amount as say, baskets. More importantly this would solve the problem of things being hit more often if they are off-grid.
Edit: I forgot: Buildings damage: When a house takes enough damage, it no longer protects the things inside (say, a hole in the roof), so things start taking damage inside the house. The cellar could flood becoming a low water tile with no chance of fishing. When all the things in the house are destroyed, the house could go through it's final steps of being destroyed. This also applies to destroying it by hand, you have to empty it before you can do the final hits to end it. When you repair it enough to stop it damaging things inside, then you can take a bucket to the cellar and empty it with say ten buckets full of water before things in there stop taking environmental damage.