Tree Autopsy: How do trees "Heal"?

Today I had the privilege to work with Wood Saver Milling. We milled a large black walnut tree that was removed because it was threatening a neighbor's home due to structural issues most likely due to it being topped in the past and had already lost some or its large braches at thier week connection points due to topping.

Something that supprised me and may supprise you is that trees do not heal in the same you and I do. Trees are able to encapsilate wounds but the scars, dead wood and weekness is permanet. In the above three images you can see examples of this. These images show a 6"+ cut that was created in a process known as topping. This cut was made using a heading cut, one of the most damaging to trees. You can also see the new growth encapsalated over the stub over a period of years.  It can grow around all sides yet as you can see the material is permanetaly dead and forms a permanet structural weekness. Also, in the year it takes to heal a large wound it opens the tree up to bugs and decay. The new outside growth surounding the old cut piece is all that is supporting any tree growth above. If you were to clip the wood on the edge of the board, parallel and in line with the old cut, the remaining wood would just fall apart.  Let us look at other less catastrophic pruning and the trees response to the damage.

In the the images directly above you can see an example a a proper pruning cut and how the tree was able to encapsilate the damage with minimal damage to the structure and tree health. The cut appears to have been made at the bark ridge and branch collar making it as easy as possible for the tree to encapsilate.

Trees not only stuggle from damage caused by humans through pruning and topping but from the lack of proper pruning. The images directly above are from a steep branch angle causing the tree to include the bark in its center. The tree is basically growing into itself and what is in between the two branches, or the branch and base, is encapsilated, leaving a permentant weakness in the wood. This would be the equivalent of a perforated edge in paper, substantialy weaker and more likely to rip. The steep branch angle and included bark shown was over two feet deep!