We have some farm land and investigated this when we considered selling it. For example, I can add a covenant restriction of "no clear cutting" and receive a tax advantage of some amount. But the buyer is someone else - like a developer, another farmer, a forester (who can still select cut), etc.
The land trust people do not have the money to buy the land, they just enforce whatever covenants are made. Any covenant is between buyer and seller - just like exists today on other restrictions.
For example of how this works now - The local power company has an easement across our land. Any buyer has to decide if they want to buy property with that easement - or restriction if you will. It is up to the power company to enforce that easement. With a covenant restriction of "no clear cutting", or "no development within 25 feet of river",the land trust people would enforce the covenant.