1. What are lichens, the moss-like things on tree trunks?
Lichens are organisms that survive by living together: a fungus (mostly related to molds and mushrooms) holds algae (green algae or cyanobacteria) inside its body. Because they look very similar to moss plants, they were confused for a long time [1]. As of 2018, about 1,800 species have been recorded in Japan [1]. The National Museum of Nature and Science describes lichens as "fungi living in symbiosis with algae" and holds about 250,000 specimens. In an exhibition held from December 2017 to March 2018, one section was dedicated to "lichens reflecting changes in the atmospheric environment" [2]. The relationship between lichens and air is significant enough to be a museum exhibition theme.
2. How do lichens take in moisture and minerals from the air?
Plant leaves have a cuticle (a protective layer on the surface), stomata (openings for gas exchange), and tubes that carry water from the roots. Lichens do not have these three things [3]. Instead, they take in moisture and minerals from the air, rain, and dust directly through their entire body surface [4]. It is like standing in the rain without a raincoat or boots. However, unlike a person getting wet, lichens absorb substances into their bodies along with the water. If pollutants are in what they absorb, those pollutants enter the lichen too. According to the U.S. National Park Service, sensitive species may show reduced photosynthesis, lose color and turn pale, or have their algae die, sometimes causing the entire lichen to die. Strength varies by species, and some are resistant to pollution [4].
3. Why did all the lichens in a Paris park disappear between 1866 and 1896?
Wilhelm Nylander, a Finnish lichenologist, studied lichens in the Luxembourg Gardens in Paris. He recorded that there were about 30 species (some records say 32) in 1866, but they had all disappeared by 1896 [3]. Coal heating and industry release sulfur dioxide (SO2), and the loss of the lichens has been linked to it. Nylander is said to be one of the first to point out the relationship between sulfur dioxide and lichens [3]. Usually, records of living things note what was "found." Here, the fact that things that were there before were "gone" became a clue to air changes. However, this is a record from one park, so it cannot be said that sulfur dioxide was the only cause of the disappearance.
4. Why did lichens return to the Paris park trees about 100 years later?
At the end of the 1980s, when sulfur dioxide in the air decreased significantly, lichens appeared again on the trees in the Luxembourg Gardens. A survey in 1991 by Seaward and Letrouit-Galinou reported that at least 11 species had settled there [5]. Nylander’s record ended in 1896, and this survey was in 1991, so the gap is about 100 years. "Returned after about 100 years" is an estimate comparing the 1896 record and the 1991 survey. The 11 species that returned are fewer than the about 30 species from 1866. This record shows that lichens can return when the air improves, but not everything returned to exactly how it was.
5. How did the absence of Umenokigoke lichen match high sulfur dioxide in Japan?
*Umenokigoke* is a leaf-like lichen commonly seen on stones in Japanese cities and graveyards, and on tree trunks. A study published in the Journal of the Ecological Society of Japan in 1976 by Sugiyama, Kurokawa, and Okada investigated five cities: Sendai, Tokyo, Fuji, Shizuoka, and Yokkaichi. They reported that areas without *Umenokigoke* roughly matched areas where sulfur dioxide in the air exceeded 0.020 ppm [6]. This 0.020 ppm is a guideline for the conditions of this study and is not the same line everywhere. In 56 graveyards in Shimizu Ward, Shizuoka City, the distribution of *Umenokigoke* was surveyed in 1972–1978, 1994, and 2003. It was reported that the distribution recovered after sulfur dioxide pollution improved. However, near the intersection of the Tomei Expressway and National Route 1, it began to disappear around 1978, and the blank area expanded. This is linked to changes in the atmospheric environment, such as increased traffic. However, no direct link between lichen distribution and individual pollutants was found, and other factors such as drying may also be involved [7]. *Umenokigoke* is also sensitive to car exhaust and is known as a species that reflects the state of urban air [1].
6. How can lichens show the type of air pollution, such as nitrogen?
When sulfur dioxide decreases, nitrogen-containing pollutants like nitrogen oxides and ammonia become more noticeable. Then, species that prefer nitrogen (such as the yellow lichen *Xanthoria parietina*) increase, and species that prefer acidic environments decrease [8]. This means the combination of lichen species can show changes in the type of pollution. In the UK citizen science project "OPAL Air Survey," participants counted lichens on the trunks and branches of deciduous trees. From which nitrogen-sensitive, resistant, and intermediate species appeared, they worked out a rough guide to nitrogen pollution. A later evaluation reported that the simplified method could detect large differences in pollution but sometimes missed subtle ones [9]. Lichen observation is not a replacement for measuring instruments but is a helpful clue. Air quality is not determined by lichens alone.
7. How can you observe and record lichens on tree trunks?
Observe tree trunks in parks or along roads with an adult. If you are near a road with cars, stand on the inside of the sidewalk. Do not climb the trees, and do not peel off the lichens by hand; just look at them. Sketch the colors (white, green, yellow) and shapes (round or leaf-like). Writing down the tree type, distance from the road, and sunlight makes it easier to compare later. You cannot evaluate air quality based only on the presence or absence of lichens. Observation is a starting point for thinking, "Why is it different?" The National Museum of Nature and Science exhibition page introduces lichens and the atmospheric environment [2].
Safety Note: With an adult, record the color (white, green, yellow), shape (round, leaf-like), tree type, sunlight, and distance from the road of lichens on nearby tree trunks.