87 lichen species on Buddhist cemetery stones mapping acid rain history. PH-sensitive organisms record 400 years of atmospheric pollution.
A living record of pollution
In the sacred Mount Koya (Koyasan) lies Okunoin, Japan's largest and most revered cemetery. Its two-kilometer path is lined with over 200,000 tombstones set among ancient cedar trees. These stones, some dating back over 400 years, are memorials and unintentional scientific instruments. The surfaces of these dated monuments host a diverse community of lichens, a living, growing record of long-term environmental change.
Lichens are composite organisms, a symbiosis between a fungus and a photosynthetic partner, usually an alga or cyanobacterium. They absorb all their nutrients directly from the atmosphere, making them exceptionally sensitive to air quality. Because they are slow-growing and long-lived, their presence, absence, and diversity on a substrate of a known age—like a dated tombstone—record the history of atmospheric conditions. This methodology, using lichens to date surfaces and track environmental history, is called lichenometry.
From industrialization to acid rain
A survey of the tombstones in Okunoin cemetery found 87 species of lichen. The diversity and distribution of these species record Japan's industrial history. Different lichen species possess different sensitivities to airborne pollutants, particularly sulfur dioxide (SO2) and nitrogen oxides (NOx), the primary components of acid rain.
Japan's industrialization began in the Meiji Restoration of 1868 and accelerated dramatically after World War II. This period saw a massive increase in the burning of fossil fuels, releasing high concentrations of sulfur dioxide into the atmosphere and causing severe acid rain. On tombstones from this era, researchers find a decline in acid-sensitive lichen species and a proliferation of acid-tolerant ones. For example, species of Parmotrema are known to be sensitive to pollutants like sulfur.
Following the establishment of stringent air pollution controls in the 1970s and 80s, Japan's air quality began to improve. This change appears in the lichen communities on more recent tombstones. Acid-sensitive species began to reappear and recolonize surfaces where they had previously vanished. This historical archive, written in the slow growth of lichens on stone, gives scientists a detailed, 400-year chronology of regional air quality that predates modern monitoring equipment.
💡Fun Facts
Lichens have no roots, getting all their water and nutrients from the atmosphere, which is why they are such good indicators of air quality.
Okunoin cemetery contains the mausoleum of Kobo Daishi, the founder of Shingon Buddhism, who is believed to be in eternal meditation, not dead.
Some modern monuments in Okunoin cemetery are dedicated by corporations, including a pest control company's memorial to the termites their products have killed.
The slow growth of lichens makes them useful for dating geological events like rockfalls and glacial retreats over thousands of years.
The cemetery grounds are open 24 hours. The main offering hall, Torodo Hall, is open from 6:00 AM to 5:30 PM.
Admission
Free to enter the cemetery grounds.
Accessibility
The main path is about two kilometers long and paved with stone, but can be uneven in places. There are some stairs. The area is forested and dimly lit, especially at night.