T.  Tõrra and Randlane, T., ?The lichen genus Usnea (lichenized Ascomycetes, Parmeliaceae) in Estonia with a key to the species in the Baltic countries?, Lichenologist, vol. 39, pp. 415-438, 2007.

T.  Randlane, Tõrra, T., Saag, A., and Saag, L., ?Key to European Usnea species?, Bibliotheca Lichenologica, vol. 100, pp. 419-462, 2009.

C.  Truong, Divakar, P. K., Yahr, R., Crespo, A., and Clerc, P., ?Testing the use of ITS rDNA and protein-coding genes in the generic and species delimitation of the lichen genus Usnea (Parmeliaceae, Ascomycota)?, Molecular Phylogenetics and Evolution, vol. 68, pp. 357-372, 2013.

L.  Saag, Tõrra, T., Saag, A., Del-Prado, R., and Randlane, T., ?Phylogenetic relations of European shrubby taxa of the genus Usnea?, Lichenologist, vol. 43, pp. 427-444, 2011.

P. W.  James, Clerc, P., and Purvis, O. W., ?Usnea?, in Lichens of Great Britain and Ireland, London: British Lichen Society, 2009.