Scholars agree that the official art of Sinnoh clearly illustrates the premises leading up the Old Chateau contains no overarching foliage. We can therefore deduce that the dark overcast in all of Eterna Forest is not due to trees (see Figure B), as this would be illogical.

(Jannies and moderators, this is a relevant Pokémon related discussion. Please see fit to not delete intellectual discussion and to stay the hell out of it. Thank you).