I’m not sure, but in the grand scheme of things the government is actually most likely the Trustee of the land under legal definitions. The Trustee has a fiduciary obligation to the “beneficiaries” in that circumstance, and to squirrel away large tracts of land as “mitigation banks” probably violates that responsibility to the beneficiaries if they are defined as the general citizenry.
In this case, the government owns the land per the Treaty of Guadalupe Hildago 1844 which was signed at the end of the US Mexico war.