Under 23 MRSA 3101, "'Maintenance' includes, but is not limited to, snowplowing, snow removal, sanding and ice control; grading and adding gravel and surface material; installing, cleaning and replacing culverts; creating and maintaining ditches, drains and other storm water management infrastructure; creating and maintaining sight distances on curves and at intersections; and cutting brush, trees and vegetation in the right-of-way."
So cutting trees and brush is specifically included so long as it is within the right of way (as opposed to the traveled way) or is for visibility on curves and at intersections. There is also the clause, "but is not limited to" at the beginning of the definition, which leaves a little wiggle room. I would say that anything related to safe travel on the road could be considered maintenance, so signs could be included. Another consideration would be to compare the duties of the road association to the duties of a town road commissioner or the DOT. Whatever they would have authority to do, a road association should also have the authority to do.