The canary on the tree trunk
The small, grey, leafy organism you might see clinging to tree bark in London is a plant-like growth; it records of the city's air quality. This is Physcia tenella, a type of foliose (leafy) lichen. Lichens are composite organisms, a symbiosis between a fungus and a photosynthetic partner, either an alga or a cyanobacterium. The fungus creates the structure, and the alga provides food through photosynthesis. This partnership allows lichens to colonize bare surfaces, but it comes with a vulnerability. Because they absorb water and nutrients directly from the atmosphere, they also absorb airborne pollutants.
Physcia and its relatives are exceptionally sensitive to sulfur dioxide (SO2), a gas produced by burning fossil fuels like coal. When SO2 dissolves in water, it forms sulphurous acid, which damages the algal partner's ability to photosynthesize and harms the fungal structure. Sensitive lichens begin to die off when annual average SO2 levels are as low as 8-30 micrograms per cubic meter (µg/m³). During the Industrial Revolution and into the mid-20th century, London's air was saturated with SO2 from coal fires, creating a "lichen desert" in the city center where only the most tolerant species could survive.
A city's air mapped by life
The near-total absence of lichens like Physcia was a biological confirmation of London's severe air pollution, which culminated in the Great Smog of December 1952. This event led directly to the passage of the Clean Air Act of 1956, which established smoke-free zones and restricted coal burning. The effects on the city's lichen flora were not immediate, but they were significant.
As SO2 levels began to fall (UK emissions have decreased by 80% since 1962), lichens began a slow process of recolonization. By 1970, surveys could find only a handful of lichen species in central London. A 1988 study of northwest London, however, found 49 species on trees as SO2 levels continued to drop. The reappearance of foliose lichens like Physcia and even more sensitive, bushy (fruticose) lichens mapped, showing where and how much the air had improved.
Today, while SO2 is less of a problem, nitrogen compounds from vehicle exhaust and agriculture present a new challenge. Citizen science projects, like the Open Air Laboratories (OPAL) survey which ran from 2007 to 2019, have used the presence of nitrogen-tolerant species (like Physcia and the bright orange Xanthoria parietina) versus nitrogen-sensitive species to map this new form of pollution. The types of lichens you see on a single tree trunk can indicate the primary pollutants in the local air.