Whenever it is found necessary to obtain by purchase or condemnation any right-of-way for the Natchez Trace highway, parkway or scenic public park, and there is upon such land a growth of standing, merchantable timber, the highway commission and the counties acquiring such land are hereby authorized, in their discretion, to pay a premium of not more than twenty percent (20%) in addition to the cash market value of such standing, merchantable timber for all of said timber which is left standing on the said right-of-way on the date of its acquisition.