MrA says:
Back to Topic ; Is this the difference between in trees and on trees?

As an American, I may use words differently differently than other native English-speaking countries.

'On trees' is used mainly for something that's within reach. For example, lights and ornaments go on a Christmas tree.

'In trees' is for something that sits closer to the trunk, and generally speaking, cannot be reached so easily. For example, there's a cat stuck in that tree.