For the purpose of encouraging forestry when lands are devoted exclusively to such purpose, and the reconstruction, redevelopment, and rehabilitation of obsolete, decadent, or blighted areas, the general assembly by general law may provide for such partial relief from taxation of the lands devoted to any such purpose, and of the improvements thereon, by such method or methods, for such period or periods of time, not exceeding twenty-five years in any instance, and upon such terms, conditions, and restrictions as it may prescribe; provided, however, that in the case of forest lands, the limitation of twenty-five years herein described shall not apply.