The Cladonia gracilis group is a monophyletic group of species that all are morphologically similar to C. gracilis. In this group, the delimitations of species is difficult due to the morphological similarity between taxa, and the fact that many of the characters used to classify species are influenced by environmental factors such as light exposure, temperature or humidity.[6]
^Brodo, Irwin M.; Sharnoff, Sylvia Duran; Sharnoff, Stephen (2001). Lichens of North America. Yale University Press. pp. 256–257. ISBN978-0-300-08249-4.
^Pino-Bodas, Raquel; Burgaz, Ana Rosa; Martín, María P.; Lumbsch, H. Thorsten (2011). "Phenotypical plasticity and homoplasy complicate species delimitation in the Cladonia gracilis group (Cladoniaceae, Ascomycota)". Organisms Diversity & Evolution. 11 (5): 343–355. doi:10.1007/s13127-011-0062-2.