JimT wrote:

Decks and other exposed wood which has been cut and milled
into lumber is no longer alive...

Wood on a tree is still alive and growing...

As has already been mentioned, the wood inside a tree under the bark is
actually dead wood. The only difference between it and the lumber you
buy at Lowes is that the wood in the tree hasn't been milled.

It is best to leave tree wounds alone and allow them to heal
naturally... Exceptions to this logic are rare...

If the exposed surface is large enough, the wood can rot and/or be
attacked by insects before the tree has a chance to grow bark to cover
it.

I think that different climates can be more problematic than others. In
the north-east and great-lakes area, you have a shorter growing season
(takes more time to cover exposed cuts with bark) and lots of humidity
and freeze-thaw action in the winter, both of which is hard on untreated
exposed wood.