From what I've seen with trees, the entire tree isn't a barrier to shells. There are spots near the bottom where the shot will go through them. Around the trunk and maybe a few branches will stop the shells, but if you go low enough or possibly to the sides, there seems to me to be areas where the shell gets through if it's far enough away from the solid parts.
The branches seem to be the barrier, the leaves just seem to me to be concealment, not cover.
Wiley.