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any of a group of relatively primitive ferns in the order Cyatheales, most of them characterized by ascending trunklike stems and an arborescent (treelike) habit. Tree ferns are conspicuous plants of humid tropical forests around the world. Species found at lower elevations are often widespread colonizers of disturbed or successional habitats. A large number of species are restricted to very small ranges on islands or at higher elevations, often in more-mature forests on isolated tropical mountaintops.