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Pin's variety theorem for positive varieties of string languages and varieties of finite ordered semigroups is proved for trees, i.e., a bijective correspondence between positive varieties of tree languages and varieties of finite ordered algebras is established. This, in turn, is extended to generalized varieties of finite ordered algebras, which corresponds to Steinby's generalized variety theorem. Also, families of tree languages and classes of ordered algebras that are definable by ordereddoi:10.1016/j.tcs.2005.07.026 fatcat:7bjt35s6zrdx3cxeoxo2g7gx6q