Section 31. No action for the recovery of land shall be commenced by or in behalf of the commonwealth, except within twenty years after its right or title thereto first accrued, or within twenty years after it or those under whom it claims have been seized or possessed of the premises; but this section shall not apply to the province lands in the town of Provincetown lying north and west of the line fixed by section twenty-five of chapter ninety-one, to the Back Bay lands, so called, in Boston, or to any property, right, title or interest of the commonwealth below high water mark or in the great ponds; provided, further, that this section shall not bar any action by or on behalf of the commonwealth, or any political subdivision thereof, for the recovery of land or interests in land held for conservation, open space, parks, recreation, water protection, wildlife protection or other public purpose.