The ash, black cherry, black walnut, butternut, catalpa, coffee tree, the elms, hackberry, the hickories, honey locust, Norway and Carolina poplars, mulberry, the oaks, sugar maple, cottonwood, soft maple, osage orange, basswood, black locust, European larch and other coniferous trees, and all other forest trees introduced into the state for experimental purposes, shall be considered forest trees within the meaning of this chapter. In forest reservations which are artificial groves, the willows, box elder, and other poplars shall be included among forest trees for the purposes of this chapter when they are used as protecting borders not exceeding two rows in width around a forest reservation, or when they are used as nurse trees for forest trees in such forest reservation, the number of such nurse trees not to exceed one hundred on each acre; provided that only box elder shall be used as nurse trees.