Olmsted talks about "invisible forest" refers to forest lands -- and, for that matter, any other land types -- protected by a perpetual conservation easement, the existence and location of which are concealed from the public, whether deliberately or because of the opaque nature of the easement process. Because easements, like other forms of deeds, must be recorded at the local land registry or recorder's office, they can never be made undiscoverable. But, despite the efforts of some states and conservation organizations to compile conservation easement data for public consumption, there are few functional systems that comprehensively track and provide easy access to conservation easement data.
James L. Olmsted,
The Invisible Forest: Conservation Easement Databases and the End of the Clandestine Conservation of Natural Lands ,
74 Law and Contemporary Problems
Available at: https://scholarship.law.duke.edu/lcp/vol74/iss4/4