You can put a little Shiva Lingam, or stick a small Ganesha statue on the tree then it is okay. That is one of the remedies in Vastu Shashtra. Or just write ‘Om Namah Shivaya’. You do not have to cut the tree. I too am not in favor of cutting a beautiful maple tree, no!
North east corner means the Divinity is there, so it should be a place of prayer, worship. You can put a yoga mat or a small bench where you can sit and meditate.
The place for meditation is north east and that is why they say that it is better if nothing is obstructing.
It is okay to have a temple in the north east. A temple is usually long with a big steeple high up. But don’t worry about it. Any little imperfection according to Vastu can be corrected by one simple thing – Om Namah Shivaya.