When any tract has been classified as forest land for purposes of taxation under sections 12-96 to 12-100, inclusive, the classification shall be continued as long as proper forest conditions are maintained thereon except as herein provided. Use of such land for pasture, destruction of the tree growth by fire and failure of the owner to restore forest conditions, removal of tree growth and use of land for other purposes, or any changed condition which, in the opinion of the State Forester, indicates that the requirements of said sections are not being fulfilled, shall be sufficient ground for cancellation of such classification. When requested to do so by the assessors, or whenever he deems it necessary, the State Forester shall examine classified forest land and, if he finds the provisions of said sections are not complied with, he shall forthwith cancel the classification of such land, sending notice of such cancellation to the Secretary of the Office of Policy and Management, the assessor of the town in which the land is located and the owner of such land. Such land shall thereafter be taxed as other land. Whenever a classification has been cancelled, the assessors shall deduct the valuation of both the land and the timber thereon as established at the time of classification from the then value of such land and timber as assessed for future taxation and, on the excess value thus determined, a tax shall be collected at the rate of five mills per annum for the entire number of years that such land has been under classification. This tax shall be in addition to any annual tax or yield tax which has been paid or is collectible. The past value of timber which has been destroyed by natural agencies, or which has been cut and upon which a yield tax has been paid, shall not be considered in fixing the present assessable value.