Long story short the people of the forest use trees to 'bury' their dead instead of actually burying them into the ground. They use plant-manipulation magic to accomplish this, making the tree open up a hollow inside itself where they put the bodies into and then use magic to seal the tree back up, making the area where the body is stored bulge out a little bit because of the space that has been made inside it. Think of it like gradually turning a forest into one giant above ground crypt with the trees acting as living wooden sarcophagi.
The seal is not always air-tight and would sometimes allow air or fluid leakage. The surface of the inside of the hollow is covered in bark due to how the magic works and is as protected from damage as the tree is from the outside so it's not like the tree's innards are entirely exposed to all of the processes of bodily decay but it still concerns me enough to think about how this practice would affect the trees and in turn the forest's health as a whole.
How would using trees as sarcophagi affect the forest?
They've been doing this for hundreds of years, if that helps. The forest's trees are similar to oak trees.