Hypogymnia

Hypogymnia is a genus of lichenized fungi in the family Parmeliaceae. Members form foliose lichens that typically grow on tree bark, wood, or occasionally on rocks in temperate and boreal regions.

The thallus of Hypogymnia lichens is usually broadly lobed and relatively flat, with a gray to greenish upper surface and a darker lower surface bearing rhizines. Some species show a more rounded, sometimes inflated or hollow-edged appearance in the thallus when viewed in cross-section. Reproduction occurs via sexual fruiting bodies (apothecia) in some species, while others reproduce asexually through fragmentation or propagules such as soredia or isidia. The chemistry of Hypogymnia often includes secondary metabolites common to the Parmeliaceae, such as usnic acid and atranorin, which can contribute to coloration and protection from environmental stress.

Habitat and distribution are broad, with Hypogymnia species found across many continents, especially in moist forests of temperate and boreal zones. They commonly inhabit the bark of deciduous and coniferous trees, and less frequently rock surfaces. Because lichens are sensitive to air quality, Hypogymnia species are frequently used as bioindicators of environmental health, responding to pollutants such as sulfur dioxide and nitrogen deposition.

Notable species include Hypogymnia physodes and Hypogymnia tubulosa, among others. As with many lichens, their presence reflects long-term ecological conditions and contributes to ecosystem functions such as habitat for microfauna and nutrient cycling.