Trees with trunks missing because of the "semi-grief" of some careless players... There are a bunch of mods that try to prevent it in various ways.
I have created yet another one for my game: it prevents players from cutting a tree node if it is not on top, that is if it is under another tree node.
This kind of works in my game because players have a way to "jump" on top of even tall pine trees and cut it down from there. There are also additional benefits for them to cut the whole tree (leaves are the only way to get sticks, so cutting down a whole tree is the fastest way to get a few sticks thanks to leaf decay). In Less Awesome Games (tm), I guess players can still build scaffolding to get to the top of the tallest trees or use specific gear to climb the tree.
One feature of this mod is that it is extremely light: it just checks if there's another node which is in the "tree" group over it when a member of the "tree" group is dug.
This is not a perfect solution though: apple trees are small enough to easily be cut down the tree from the top of the trunk, so that I've seen hanging "branches" here and there.
Also note that you should add the corresponding dependencies if you have mods that add tree types (moretrees etc.) because my mod overrides tree nodes at init time.