An assessment of forest land for purposes of taxation shall be held to be in excess of just value by any court of competent jurisdiction, upon proof by the owner that the tax burden imposed by the assessment creates an incentive to abandon the land, or to strip the land, or otherwise to operate contrary to the public policy declared in section 563. In proof of his contention the owner shall show that by reason of the burden of the tax he is unable by efficient operation of the forest land on a sustained yield basis to obtain an adequate annual net return commensurate with the risk involved.  

For the purposes of this section forest land shall be held to include any single tract of land exceeding 25 acres in area under one ownership which is devoted to the growing of trees for the purpose of cutting for commercial use.