Usnea longissima Ach.
Methuselah's Beard Lichen
Basis for Listing
Usnea longissima (Methuselah’s beard lichen) has a nearly circumboreal range, with three major strongholds in North America. The species is most abundant along the Pacific coast, can be found along the northern Atlantic coast, and has a limited range around the Great Lakes, where it appears to be fairly uncommon. In Minnesota, U. longissima was first documented in 1897 by Bruce Fink from a site near Grand Portage in Cook County. Since its original detection in the state, U. longissima has been documented a number of times (always in the Northern Superior Uplands Secction). All but a handful of these are from small isolated populations. Usnea longissima has suffered major declines throughout its range due to its high sensitivity to air pollution (Hinds and Hinds 2007; Wetmore 2002; Brodo et al. 2001). Because of this, and the fact that U. longissima primarily reproduces through fragmentation and wind dispersal, it was listed in Minnesota as a species of special concern in 2013.
Usnea longissima can be a large and charismatic lichen over much of its range, particularly along the West Coast. In fact, Brodo et al. (2001) says that it can reach 3 m (10 ft.) in length, and Hinds and Hinds (2007) say it can reach 10 m (33 ft.) in length (though this may be an aggregate of separate thalli)! Usnea longissima does not typically reach such great lengths in Minnesota; however, the label on Bruce Fink’s original 1897 collection from Grand Portage states “this specimen 5 feet long”. The long thalli of U. longissima were likely wiped out in our area by the logging of old-growth forests at the beginning of the last century. Because of this, most modern collections of U. longissima in Minnesota are less than 30-60 cm (1-2 ft.) in length.
The species is characterized by the long, thin, pendent, growth form of its thallus (lichen body), which consists of a fairly thick, central branch and numerous, perpendicular, side branches of similar length, giving the lichen a “pipe-cleaner” or “bottle brush” appearance. The central branch typically lacks a cortex, which gives the surface of this structure a white, cottony appearance; the side branches have a cortex, which appears as a smooth greenish surface. This two-tone structure can give the lichen a slightly lighter coloration than other similar Usnea (beard lichen) species. Apothecia are unknown from North American specimens of U. longissima.
In Minnesota, U. longissima is a very distinctive and unique species. Although there are about 20 species of Usnea in Minnesota, only a handful of these have pendent growth forms and lack a holdfast; and within this group, U. longissima is the only species that lacks an outer cortex along the main branch. In fact, this trait is so unique among Minnesota’s lichen flora that the species can be identified by small fragments found on the ground.
In Minnesota, U. longissima can be found in a wide range of habitat types, including forested peatland, forested acid peatland, fire-dependent forest, wet forest, and mesic cliffs. Apparently the most important features in each habitat are high humidity and air quality. Although the species can be found in a wide array of habitats, the species is perhaps most predictable in forested peatland systems, where it typically grows on conifers, particularly Abies balsamea (balsam fir).
Biology / Life History
In North America, U. longissima is not known to produce apothecia. The species can occasionally produce soredia on old branches (Hinds and Hinds 2007), though this appears to be infrequent. It is likely, therefore, that U. longissima reproduces primarily through the fragmentation of its thallus and the wind dispersal of these fragments. This method of dispersal relies on fairly continuous forested systems, which were more common before widespread European settlement.
Conservation / Management
Anything that opens up the canopy in old forests and forested peatland systems can have a major effect on the microhabitat upon which U. longissima and many other rare lichen species depend. Opening the canopy allows more sunlight to reach the ground, raising the temperature while lowering the humidity. Usnea longissima also relies upon the trees themselves as substrate. Efforts should be made to protect old-growth forest communities, particularly along the shore of, or within close proximity to, Lake Superior.
Best Time to Search
Searches for U. longissima can be conducted year-round, whenever they are not covered by snow or ice.