Often mistaken for moss or dirt covering the surfaces of roadside trees and concrete, lichens are unique and mysterious organisms where fungi and algae exist symbiotically. Sensitive to environmental changes, they are now attracting attention as tools to grasp the effects of atmospheric pollution and the urban heat island phenomenon.

In lichens, fungi depend on algae to survive. The fungi combine with algae to use the nutrients the algae produce by photosynthesis. Many types attach themselves to tree bark, but this does not mean that the organism is feeding on the tree for nutrients.

Many of the species that can be seen around urban areas are those like Parmotrema tinctorum, which spreads over surfaces like a wrinkled sheet of thin gray paper, or Candelaria concolor, which looks like yellow paint from far away, to name a couple.

According to Yoshihito Ohmura, a research group leader at the National Museum of Nature and Science, lichens absorb water, oxygen, carbon dioxide and other substances directly into their structure, so they are more susceptible to changes in the atmosphere than plants that grow roots in the soil. A research team including Ohmura has summarized the results of the change in the distribution of Parmotrema tinctorum in 56 locations across Shizuoka's Shimizu Ward over roughly 30 years starting in the 1970s. When the level of the atmospheric pollutant sulfur dioxide reaches a yearly average concentration of over 0.02 parts per million, this particular species reportedly gradually weakens and its distribution decreases.

Candelaria concolor (Photo courtesy of the National Museum of Nature and Science)

Around 1973, there were many factories and other industrial facilities operating around the Port of Shimizu, and the lichen was not found in any of the 18 locations near the heavily industrialized area. However, when factories adopted restrictions on sulfur dioxide emissions and other measures, the concentration of the substance in the air decreased, and the lichen was discovered in 10 locations in 1994.

In contrast, the number of locations along National Route 1 where Parmotrema tinctorum can no longer be found increased from the late '70s into the '90s. "The source of atmospheric pollution shifting from factories to automobiles matches the changes in the lichen's distribution patterns exactly," said Ohmura. In recent years, with restrictions on diesel vehicles in the greater Tokyo area and other such measures, air quality has significantly improved even in large cities. The national museum identified 57 species of lichens around the Imperial Palace grounds in the heart of Tokyo in 1995 and 1996, but that number grew to 98 species in a survey conducted from 2009 to 2013.

There are also lichens whose distribution has shifted for other reasons. In October 2015, the tropical species Glyphis cicatricosa was discovered by the National Museum of Nature and Science's Tsukuba Botanical Garden in Tsukuba, Ibaraki Prefecture. Up until then, the northern limit of the lichen had been the Izu Peninsula in central Japan, but due to the effects of global warming and the heat island phenomenon caused by urbanization, there is a possibility that its distribution may spread further northward in the future.

Ohmura operates a website introducing information about lichens that he compiles from platforms like Twitter and other sources. "In areas where lichens have been wiped out by air pollution, there is a possibility of previously unseen southern varieties growing in their place due to improved air quality and rising temperatures," he said. "I hope to further explore the relationship between air quality in urban areas and changes in lichen distribution."