The Parmeliaceae is a large and diverse family of Lecanoromycetes. With over 2700 species[2] in 71 genera, it is the largest family of lichen-forming fungi. The most speciose genera in the family are the well-known groups: Xanthoparmelia (822 species), Usnea (355 species), Parmotrema (255 species), and Hypotrachyna (262 species).[3]
Nearly all members of the family have a symbiotic association with a green alga (most often Trebouxia spp., but Asterochloris spp. are known to associate with some species).[4] The majority of Parmeliaceae species have a foliose, fruticose, or subfruticose growth form. The morphological diversity and complexity exhibited by this group is enormous, and many specimens are exceedingly difficult to identify down to the species level.
The family has a cosmopolitan distribution, and is present in a wide range of habitats and climatic regions.[5] This includes everywhere from roadside pavement to alpine rocks, from tropical rainforest trees to subshrubs in the Arctic tundra. Members of the Parmeliaceae are found in most terrestrial environments. Several Parmeliaceae species have been assessed for the global IUCN Red List.
Based on several molecular phylogenetic studies, the Parmeliaceae as currently circumscribed has been shown to be a monophyletic group.[6] This circumscription is inclusive of the previously described families Alectoriaceae, Anziaceae, Hypogymniaceae, and Usneaceae, which are all no longer recognised by most lichen systematists. However, despite the family being one of the most thoroughly studied groups of lichens, several relationships within the family still remain unclear. Phylogenetic analysis supports the existence of seven distinct clades in the family. The Parmelioid clade is the largest, containing 27 genera and about 1850 species – about two-thirds of the species in the family.[7][8]
Many Parmeliaceae genera do not group phylogenetically into any of these clades, and these, along with genera that have not yet had their DNA studied, are classed as "genera with uncertain affinities".[7]
The Parmeliaceae has been divided into two subfamilies, Protoparmelioideae and Parmelioideae.[9][10] The diversification of various Parmelioideae lineages may have been a result of gaining innovations that provided adaptive advantages, such as melanin production in the genus Melanohalea.[11] Diversification of the Protoparmelioideae occurred during the Miocene.[12] The Parmelioid clade is the largest in the Parmeliaceae, with more than 1800 species and a centre of distribution in the Southern Hemisphere.[13]