Variables over 8000 are completely temporary. Simple as that. Some have special functions, like one (the number escapes me at the moment) keeps track of the last sprite you talked to, and 0x800D is used as LASTRESULT which is the main var used by ASM to return temporary items.
The flag for the tree goes in its Person ID to keep it hidden after you cut it, but once you leave the map, the flag gets reset and the tree appears again.
I see. Thank you!
And this might be a stupid question, but would it be possible to change the person id of a tree or rock and thus change the flag it corresponds to?