If they are not clearly defined, you need a lawyer.
If they are clearly defined, you may need a lawyer anyway - based on the question.
This is one of the problems with a condex and/or a small condo complex.
Us vs them.
Hopefully we stay friendly and agree on everything - everytime.
(Please note: when you choose an answer as a Best Answer, or at least give a thumbs up, it helps those who answer questions here.)
Some condexes in town are set up with older condo documents that state each owner owns their individual side lf land. This is unusual but I have run into it on a handful of Haverhill condex sales. In this scenario the owner with the tree on their side is responsible for removal.
Some condexes are set up with both owners being responsible for all of the land. The scenario should be clearly described in the association documents.
If you own the property you should have been given copies of the relevant information at closing. If you no longer have the documents the registry of deeds should have access.
Feel free to call or email with any additional questions.
Regarding tree removal, have you asked your insurance agent? The master insurance policy may cover it. It sounds like you may need to consult with an attorney to get more specific answers to your situation.