The wonders of hair ice and needle ice

While hiking in the Carolinas of the southeastern US, I was baffled by curious strands emerging from the forest floor, covered in fresh snow. Later, I learned that I had observed “needle ice” (see image), a form of ice that is related to a different phenomenon known as “hair ice”.

Hair ice, also called ice wool or frost beard, originates from the fungus Exidiopsis effusa. In 1918, Alfred Wegener (who earlier had proposed continental drift theory) suggested a fungus as the suspected source of the delicate strands of hair ice. However, E effusa was not identified as the responsible fungal agent until 2015 by Hofmann et al., whose seminal work (Biogeosciences 2015; doi.org/10.5194/bg-12-4261-2015) provides an excellent review of discoveries related to hair ice, including: (i) E effusa's gossamer-like hair ice is fragile (disintegrating if handled) and ephemeral (lasting for hours or a few days under ideal conditions); (ii) E effusa emerges only from decomposing branches of broadleaf trees and in temperatures at or below 0°C; (iii) the fungus has been reported in multiple countries between the latitudes of 45° and 55°N; (iv) the mycelium of E effusa appears to provide the supporting structure for the hairs of ice, which have a diameter of around only 0.02 mm; (v) hair ice forms from a dense concentration of mycelium, drawing water from the porous substrate of the wood; (vi) a recrystallization inhibitor is likely responsible for the stabilization of the fine hairs; and (vii) E effusa's fruiting body typically appears macroscopically weeks later on the wood surface as a thin, white rot coating.

The specimen featured in the accompanying photograph is needle ice. Often mistaken for hair ice, needle ice is a related ice type that grows from soil instead of wood, has slightly stiffer needles, and forms columns (Mätzler C, Wagner G, Preuss G, and Hofmann D. 2013. Enlightening the mystery of hair ice. IAP Research Report 2013-01-MW. Bern, Switzerland: Institute of Applied Physics, University of Bern). The source of needle ice formations is not associated with a fungus; rather, it is groundwater, which rises to the surface by capillary action and freezes. The phenomenon occurs in areas that experience frequent thaw–freeze cycles (Cold Regions Sci Technol 1988; doi.org/10.1016/0165-232X(88)90076-6). Needle ice is also recognized as a cause of soil disturbance, though its unusual appearance, like that of hair ice, is especially captivating.

Special thanks to Diana Hofmann for help with identification.