Any increase in value of privately owned lands resulting from the planting of trees shall not be taken into account in determining the actual value of such lands for a period of thirty years from the date of planting such trees. This section shall apply to all lands so planted; however, in the event that any trees become sufficiently mature as to be of economic use and value prior to the expiration of thirty years, any increase in use and value shall be thereafter taken into account in determining the actual value of such lands.