FAQ

A lichen is a composite organism formed by a symbiotic partnership between a fungus (the mycobiont) and one or more photosynthetic partners: either a green alga, a cyanobacterium, or both. The fungus provides the structure and protection, while the photosynthetic partner produces food through photosynthesis. Neither partner can form the lichen body alone. Recent research has also found yeasts and bacteria living within lichens, making them even more complex than previously thought.

No. Lichens are not plants, animals, or even a single organism. The main body of a lichen is formed by a fungus (which belongs to Kingdom Fungi, not Kingdom Plantae). The photosynthetic partner is either a green alga (Kingdom Protista/Plantae) or a cyanobacterium (Kingdom Bacteria). Together they form something entirely unique: an organism that doesn't fit neatly into any single kingdom of life.

No. This is one of the most common misconceptions about lichens. Lichens do not parasitize trees; they simply use bark as a surface to grow on (they are epiphytes). They don't send roots into the wood or steal nutrients. In fact, the opposite is often true: lichens on trees are a sign of good air quality. When you see lichens disappearing from trees, it usually means pollution is increasing, not that the lichens were causing harm.

Some lichens are edible, but caution is essential. Bryoria fremontii (horsehair lichen) was a staple food for Indigenous peoples of western North America, pit-cooked into a nutritious bread. Cladonia rangiferina (reindeer lichen) has been used to make flour and even alcohol. Parmotrema perlatum (black stone flower) is used as a culinary spice in Indian cuisine. However, some lichens contain toxic compounds like vulpinic acid, and many are too bitter or acidic to eat without processing. Never eat a lichen you cannot positively identify.

Most lichens grow extremely slowly, typically 0.5 to 5 mm per year, depending on species and conditions. Some crustose lichens like Rhizocarpon geographicum (map lichen) grow as little as 0.1 mm per year. Foliose lichens tend to be slightly faster, and some fruticose lichens in optimal tropical conditions can grow up to 20 mm per year. This slow growth is why lichens can be used to date rock surfaces (a technique called lichenometry).

Some lichens are among the oldest living organisms on Earth. The crustose lichen Rhizocarpon geographicum (map lichen) has been estimated at over 8,600 years old in some alpine and arctic locations. Even common foliose lichens on trees can be decades old. Lichen communities in undisturbed old-growth forests may have been developing for centuries.

Yes, and very reliably. Lichens absorb water, minerals, and pollutants directly from the air (they have no roots or protective waxy coating), making them exquisitely sensitive to air pollution, especially sulphur dioxide and nitrogen compounds. Scientists use lichen diversity, species composition, and the health of indicator species to map air quality across entire regions. The general rule: more lichen species (and more sensitive species) equals cleaner air.

Despite often growing side by side, mosses and lichens are fundamentally different. Mosses are true plants (Kingdom Plantae) with stems, leaves, and chlorophyll. Lichens are composite organisms: a fungus partnered with an alga or cyanobacterium. Mosses are always green. Lichens come in many colours: gray, green, yellow, orange, brown, black. Mosses have tiny leaves visible to the naked eye; most lichens have a smooth, crusty, or leafy surface without obvious leaf structure.

Lichens reproduce in two main ways. Asexual reproduction: Small fragments containing both fungal and algal cells (called soredia (powdery) or isidia (tiny finger-like outgrowths)) break off and establish new lichens elsewhere. This is the most common method. Sexual reproduction: The fungal partner produces spores in structures called apothecia (disc-shaped) or perithecia (flask-shaped). These spores must find a compatible algal partner after germination to form a new lichen, a risky strategy that explains why asexual reproduction is more common.

Nearly! Lichens have been found growing on bark, rock, soil, leaves, glass, metal, plastic, bone, old leather, and even on other lichens. Some can survive the vacuum of space. However, each species has preferences: some only grow on acidic rock, others only on basic (calcareous) rock; some prefer smooth bark, others need rough bark. Very few surfaces are truly lichen-proof; even Antarctic rocks host small crustose lichens.

Yes. Many lichen species are threatened by habitat loss, air pollution, and climate change. Lobaria pulmonaria (lung lichen) is legally protected in the UK and several European countries due to the loss of old-growth forests. Usnea longissima (Methuselah's beard) is endangered in many regions. The IUCN Red List includes numerous lichen species. Because lichens grow so slowly, once a population is lost, recovery can take decades to centuries.

No. Lichens have no roots, stems, or vascular tissue. Some foliose lichens have root-like structures called rhizines that attach them to their substrate, but these are purely for anchorage; they don't absorb water or nutrients like plant roots do. Lichens absorb everything they need directly from the air and rain through their surfaces, which is why they are so sensitive to air pollution.

Lichens are classified by growth form into four main types: Crustose — flat crusts fused to rock or bark (like paint). Foliose — leafy lichens with distinct upper and lower surfaces, loosely attached. Fruticose — three-dimensional, shrubby or pendant lichens (like Usnea beards or Cladonia cups). Gelatinous — jelly-like lichens with cyanobacterial partners that swell dramatically when wet. There are also intermediate forms like leprose (powdery) and squamulose (scaly) lichens.

Yes, both historically and in modern research. Usnea produces usnic acid, which is used in commercial wound creams and has demonstrated effectiveness against MRSA and other antibiotic-resistant bacteria. Lobaria pulmonaria was used for centuries to treat lung diseases (and modern research has found anti-inflammatory compounds in it). Various lichens have been found to contain anti-cancer, antiviral, and antioxidant compounds. However, some lichen compounds are toxic, so self-medication with lichens is strongly discouraged.

Lichen colours come from a variety of pigments and chemical compounds. Green or gray-green comes from chlorophyll in the algal partner showing through the cortex. Orange and yellow come from anthraquinone pigments (as in Xanthoria) or usnic acid. Black or dark blue in gelatinous lichens comes from cyanobacterial pigments. Bright yellow-green in map lichen comes from rhizocarpic acid. Many of these pigments serve as UV sunscreens, protecting the delicate algal cells from radiation damage.

Remarkably, yes. In 2005, the European Space Agency exposed Xanthoria parietina and Rhizocarpon geographicum to the vacuum, extreme temperatures, and cosmic radiation of space for 14 days aboard the International Space Station. Both species survived and resumed normal photosynthetic activity afterward. This result has implications for theories about how life might travel between planets (panspermia) and suggests that lichens are among the toughest multicellular organisms on Earth.

Start with growth form: is it crusty (crustose), leafy (foliose), shrubby or hanging (fruticose), or jelly-like (gelatinous)? Next, note the colour, substrate (bark, rock, soil), and habitat. A hand lens (10x) reveals features invisible to the naked eye, like soredia and isidia. Simple chemical spot tests (K, C, and P reagents) can confirm many species. A good regional field guide is invaluable. Visit our Identification page for a step-by-step guide to getting started.

Lichenometry is a dating technique that uses the slow, predictable growth rate of certain lichens, especially Rhizocarpon geographicum (map lichen), to estimate the age of rock surfaces. By measuring the diameter of the largest lichen on a surface and applying known growth rates for the region, geologists can estimate when the rock was first exposed. It has been used to date glacial moraines, landslides, archaeological ruins, and even the age of Easter Island statues.

Lichens with cyanobacterial partners do. Cyanobacteria (like Nostoc) can convert atmospheric nitrogen gas into biologically usable forms, a process called nitrogen fixation. This includes all gelatinous lichens (Collema, Leptogium), Peltigera species, and tripartite lichens like Lobaria pulmonaria (which has both algae and cyanobacteria). In old-growth forests, these lichens can contribute 1–5 kg of nitrogen per hectare per year, a significant nutrient input.

Several factors contribute. Lichens can only photosynthesize when moist, and many environments only provide moisture intermittently. They also lack roots, so they cannot actively seek water or nutrients. The symbiotic relationship, while providing resilience, is metabolically less efficient than free-living organisms. And they invest heavily in producing defensive chemical compounds (like usnic acid and other lichen acids) rather than growth. The trade-off is extraordinary longevity: what they lack in speed, they make up in endurance.