Humble lichens have been credited as sentinels of air quality.
Lichens are the mosaics of fungi that speckle tree bark and rocks at the ocean. In those precarious perches, lichens absorb their food from the fog the wind and the rain. With no roots but with very absorbent tissue, but no roots lichens are exquisitely vulnerable to gases released from any pollutants carried by the wind and rain. It is this sensitivity that makes lichens powerful sentinels of forest health.
Pollution builds up inside lichen tissues in proportion to its concentration in the wider environment. Anything poisoning lichens also accumulating more broadly in their environment.
Despite their plant-like form, lichens are not plants often called tiny ecosystems, they are actually compound organisms made up of two, or even three, very different partners, none of which is a plant. The dominant partner in a lichen symbiosis is a fungus, a colony of algae, supplying food by photosynthesis. Separated from its partners, the fungus itself would be a shapeless glob.
Lichens to dye for
An arsenal of nearly 600 chemicals unique to lichens helps them survive in marginal environments and ward off attacks by bacteria, other fungi and grazing herbivores. Many lichen species contain bitter compounds that may discourage feeding by invertebrates The pigments, toxins and antibiotics have made lichens very useful to people in an array of cultures as a source of dyes and medicines.
The warm brown rugs made by Navajo Indians. come from boiling the vagrant lichen, Xanthoparmelia chlorochroa. Known as a vagrant lichen because it grows loose on the ground, “ground lichen” is free to wander on the wind. Before weaving a rug or blanket, members of the Ramah Navajo Weavers Association raise and shear their own sheep, spin the yarns, and dye them with vegetal dyes “ground lichen” shown loose in the basket.
The lichens once used to dye Scottish Harris Tweed contain substances that gave the fabric an earthy aroma and reputedly made it moth repellent.
Shrubby grey lichens scraped off coastal rocks and soaked in ammonia-rich stale urine, yielded some of the famous royal purple dyes of antiquity.
The most widely used dye lichen among Native Americans was the eye-catching wolf lichen, Letharia vulpine. A tribe in Alaska traded coastal commodities such as fish oil for wolf lichen from the inland to colour their prized dancing blankets. Though famed as a wolf poison, the wolf lichen often becomes one of the prized medicines in tribal pharmacies. Medicinal tea was made from it or used externally to treat skin problems. The lichen substance usnic acid has been used in some European antibiotic creams.
As if painted by a mad hand, a tombstone encrusted with lichens.
The brightly coloured crustose lichens used as medicines such as Pleopsidium oxytonum translates literally as “lizard semen”– alluding the push-ups that lizards do during courtship displays.
Almost any stable surface makes suitable turf for lichens. Under favourable conditions, lichens will find a home on the stained-glass windows of cathedrals, even on the backs of Galapagos tortoises!
Growing imperceptibly for centuries – even millennia- some lichens are amongst the world’s oldest living things; this makes them useful to scientists for dating archaeological artefacts and tracking geological events such as the retreat of glaciers.
There are rich lichen floras in the intertidal zone in many places, and they are accompanied by a correspondingly complex set of invertebrates.
Lichens are prominent role players in transforming landscapes by slowly chipping away and dissolving rock into soil, adding organic matter when they die. In the Negev Desert, in Israel, two species of snails that eat lichens growing under the surface of limestone rocks (“endolithic lichens”) were discovered to be converting rock to soil at the amazing rate of 0.7 to 1.1 metric tonnes per hectare per year!
The action was due to the fact that the snails pass significant amounts of rock through their digestive tracts in the process of consuming the lichen. In addition, the snails were taking nitrogen from the lichens and leaving it behind in the new soil; this was found to be a principal component of the nitrogen cycle of this desert.
Far from the rain-drenched forests of the Pacific Northwest, on the grey streets of 1860s Paris, a botanist named William Nylander was one of the first scientists to notice a peculiar pattern. More lichen species grew in the oasis of the Luxembourg Garden than elsewhere in the city. The park was less polluted than the rest of Paris.
Nylander made the connection: Better air quality meant higher lichen diversity. Protect them and everything else is safe.
Because of their extreme sensitivity, lichens are useful indicators of air quality. Thriving in pristine environments lichens are fast disappearing in regions of air pollution and habitat disturbance. Lichens act like sponges, taking up pollutants that come their way. By analysing lichens chemically, scientists can tell what’s in the air.
After winter rain, leafless forest trees come alive with lichens, including oakmoss lichen, an important ingredient in many fine perfumes. Virtually dormant when dry, lichens become bright, plump and metabolically active when damp.
Everything notwithstanding, lichens will keep growing and changing in step with the changing planet. They’ll breathe in the mountain air, soak up water as it drips down the trees and swell out from the mist rolling in from the sea.
Lichens will forever stand as a beacon of the air we breathe.