There is no roof in woods...
But I understand you are questioning the logics of the situation.
Mortar (and Indirect Fire) against woods create Air Bursts (i.e. the shells hit trees before reaching the ground, thus creating more destructive results).
Why should they be protected from exploding, "going up", if they are not "coming down"?
I am no military expert, but don't mortar shells gave some "timing fuse" preventing them to explode prematurely?