Do you have that glass hive finished yet? Focus, focus, focus. :-D
With 100+ watts of heat per sq foot from the sun, a tree would quickly burn up if it didn’t have a way of shedding that incoming solar heat. Like most beasts, trees are cooled by the evaporation of water. When you’re as large as a tree, you need to pass a lot of water to accomplish that cooling. However the active cooling is in the leaves, not the trunk.
What happens in the trunk is probably a combination of cool water drawn up from the roots and heat conducted into the truck from the outside air. My guess is the inside cavity would be cooler than the outside air, but not by a large amount, but I don’t know.
The more interesting question to me is what effect all that water has on the insulation properties of the tree cavity. People assume that because an Apis cavity is surrounded by 4” or 6” thick walls it is well insulated. In fact that is probably not the case in the spring and summer when lots of water is saturating the wood. Wood is only a descent insulator when those xylem cells are filled with air (not water). Air is the real insulator in most things (including foam).
BTW… I might need some of those magic pills from Finland too. :)