The species that is causing the most impediment to restoration is the Old World climbing fern (Lygodium microphyllum), introduced in 1965. The fern grows rapidly and thickly on the ground, making passage for land animals such as black bears and panthers problematic. The ferns also grow as vines into taller portions of trees, and fires climb the ferns in "fire ladders" to scorch portions of the trees that are not naturally resistant to fire.[54]