During his stay in Hokkaido, Clark examined the local flora and introduced new species of plants and trees from Japan to the United States. He sent to Massachusetts a large assortment of seeds, many of which proved of special value to his own state, on account of the high latitude from which they were selected. In Teine-ku, Sapporo, he discovered a new lichen on the side of Mt. Teine, at an elevation of , which was named "Cetraria clarkii", in his honor, by Edward Tuckerman.