Monterey Tower Controller reported a visibility issue with trees blocking the view of certain spots at MRY airport.

Narrative

There is a major safety issue at Monterey airport with trees blocking the view from the Tower of the FBO ramps; and left base to Runway 28L/R. This has been a problem for many years and has been brought up to the ATM several times; which has been passed on to the airport authority several times and has been completely ignored or brushed off every time. The trees on the south side of the airport completely block the view of the ramp area of FBO X and FBO Y. The area that is blocked now was completely visible a few years ago. Now even large aircraft cannot be seen at all; and there have been many incidents when aircraft are moving in these ramps and the Tower cannot see them and aircraft moving into the ramp have potential conflicts with them. Aircraft moving into these areas also completely disappear from view. Ground crews towing aircraft also cannot be seen. It causes potential dangerous situations as the Tower cannot see aircraft or vehicles and give control instructions and/or pass along traffic conflicts to them. This also causes delays and slows down traffic flow significantly because we cannot see traffic or potential conflicts to keep aircraft and vehicles moving or hold them in position when needed. There are also many incidents with helicopters departing or arriving to the ramp that have potential hazards with vehicles or aircraft since we can't see the ramp area. Another area is trees blocking the view to the southeast ramp; where we cannot see aircraft or vehicles until they are almost up to Taxiway Alpha; which causes potential safety hazards and slows down traffic flow.The view of the left base to Runway 28L/R is also blocked by trees. Aircraft in these positions completely disappear from view; which again is a safety hazard and slows down traffic flow; since we cannot see their position from the runways.The solution is simple but must not be ignored any longer before it gets even worse. Simply top the trees down to a level where they do not obstruct the view of the Tower from the affected areas mentioned above. The trees do not have to be removed or cut down; simply topped and have some annual maintenance on them and the problem will be solved and should never resurface. We've been asking for this for years; back when it would have been a smaller issue and the trees only partially blocked the Tower's view. Now the views of the affected areas are completely blocked and we are still being ignored. Please top these trees and restore our view of these areas before something really bad happens that could have easily been prevented if the Tower could simply see the areas we are tasked with controlling and protecting.