Commercial, industrial or mixed-use. Common open space totaling not less than 33% of the total tract of a commercial, industrial or mixed-use Planned Development District shall be provided in perpetuity, and not less than 25% of each area or division within a commercial, industrial or mixed-use Planned Development District shall be provided in perpetuity. This land shall be exclusive of any land area used primarily for vehicular modes of transportation, including parking areas and other features. The developer, a small manager, tenants' association or similar mechanism shall be provided for the long-term ownership and maintenance of the common open space, subject to the approval of the Planning Board, and the grant of a conservation easement to further ensure the protection of this open space may be required.