Section 12. If, by conveyance or other action of the owner thereof, a portion of land which is valued, assessed and taxed under the provisions of this chapter is separated for a use which does not qualify as recreational land, the land so separated shall be subject to liability for conveyance or roll-back taxes applicable thereto, but such separation shall not impair the right of the remainder of such land to continuance of valuation, assessment and taxation thereunder; provided, however, that such remaining land continues to so qualify.