You can include such a condition in the agreement of purchase and sale, but the seller may not want or be legally able to oblige you. To answer your question further, we’d need to know who owns the trees. Are they on the seller’s lot, the boulevard, or overhanging the property?
Your first step is to request a property survey. Trees located on Woodstock boulevards belong to the city, which plants, maintains, and prunes them. So the short answer to that question is no, not without the city agreeing.
Trees on private lots are the homeowner’s responsibility, provided their roots are on the lot you want to purchase. If any part of a trunk or visible roots extends to a neighbour’s lot, you’re dealing with boundary trees. In that case, the seller and neighbour have joint ownership, and a legal obligation to talk before doing anything more.
Likewise if branches from a neighbour’s tree overhang the seller’s lot, they are the neighbour’s responsibility. Any effort you make to alter or prune such trees can land you in court. While Woodstock bylaws protect your right to keep your property safe, you can’t trespass on a neighbour’s property or damage their trees to get the view you want.