Who is responsible for tree removal?

If the inspection didn't flag the trees as an issue, the buyer's request for tree removal is a negotiation ask, not a contractual obligation. You are not required to remove trees that weren't identified as a defect in the inspection report. That said, whether you accommodate the request depends on the overall deal and how motivated you are to keep this buyer. If the buyer is asking for tree removal as their only request and the deal is otherwise clean, it might be worth getting a quote and deciding whether the cost is worth keeping the transaction on track. If it's one of several requests and the total is adding up, you have every right to push back. Respond through your agent. Decline the request, offer a small credit if you want to compromise, or get a quote and decide from there. Tree removal costs vary widely from a few hundred dollars for a small tree to several thousand for large ones, so knowing the actual number helps you make a business decision rather than an emotional one.

When a tree is encroaching on a neighbors property, Seller is generally responsible for removal. If that is going to be a deal breaker, you may want to address that concern during the contract negotiations.

Everything is negotiable! It sounds like you are under agreement already, so any concessions you make should provide equal value back to you as the seller.

If the buyer is requesting tree removal, you can decide how to respond. Is there a safety issue? Is it a deal breaker? Discuss with your agent and negotiate the best solution.

The buyer or seller is responsible for tree removal.

This is something to discuss with whomever is representing you (Your Agent). In this case it sounds like the buyers are making it a part of their negotiation.