[7][8] The species Calicium hemisphaericum, described as new in 1955 by Grace E. Howard from specimens collected in Washington state,[9] was placed into synonymy with A. adequatum in 1975 by the pin lichen specialist Leif Tibell.

[2] In an analysis of the systematics of the Caliciaceae using molecular phylogenetics, Prieto and Wedin discovered that the lichen was one of several Calicium species that did not group with other members of the genus.

[12] Allocalicium typically has a thallus that is immersed in its substrate (endosubstratal),[3] although some specimens have been recorded as having thalli with warty to almost squamulose (scaly), greenish-grey areoles.

[14] The appearance of Phaeocalicium polyporaeum is reminiscent of Allocalicium adaequatum due to its small stature, dark head, and lighter olive-brown stems.

This similar species can be distinguished by its uniquely obconically shaped head and its substrate preference: the fruiting bodies of polypore fungi.

[16] Allocalicium adaequatum grows on thin branches and twigs of the species Alnus incana, Populus, and Salix.

[18] It has also been recorded in South America,[19] Europe[20] (including Finland,[21] Germany,[22] Switzerland,[13] and Ukraine[23]) and the Russian Far East.

[26] In a study conducted in the Wells Gray Provincial Park, located in east-central British Columbia, Canada, Allocalicium adaequatum has been observed exclusively in forest stands that are older than approximately 300 years.

[27] A previous study conducted in Northern Europe similarly concluded that A. adaequatum could be used as a reliable indicator species of forest continuity.