The Industrial Revolution in London caked the city in a dense smog. People wheezed and coughed as they walked through the streets, squinting to see the sun through the haze. This was a time before many environmental regulations on pollution and before we understood the consequences of air pollution for both humans and the surrounding ecosystems. Recently however, a team of researchers from the United Kingdom found evidence of the effects of the Industrial Revolution in an unexpected place– in lichens growing on the bark of trees.
Lichens are made of fungus and algae living together in harmony. The fungus needs the algae to make sugar from light (photosynthesis) and the algae needs the fungus to harvest other nutrients like nitrogen and phosphorus. Lichens have long been used as indicators of air pollution. Because they don’t have roots, the fungus inside the lichen snatches all of its nutrients from the air, including hazardous air pollutants. By analyzing the nutrients inside of lichens, scientists can understand what pollutants are in the air during the lifetime of the organism. Theo Llewelyn and his team wanted to understand how the history of London’s air pollution impacted tree dwelling lichens today. Generally, the larger the tree, the more lichen diversity. However, the researchers wanted to test if the historic pollution within London disrupted this pattern.
In total, they sampled 124 trees within London that spanned a pollution gradient. Historically, there was less pollution in the west than in the east. For each tree, they measured the circumference which is an estimate of how old the tree is and measured the distance between the trees to gauge tree isolation. Then they identified every lichen on the lower branches of the trees.
Industrial era pollution still impacts ecosystems
Across all the trees, they found 24 lichen species and the researchers determined that lichen diversity was primarily determined by the tree size, a proxy for tree age. Larger trees supported less lichen species. This is likely a result of the sulfur dioxide from coal burning during the Industrial Revolution in the early 1800s. Trees that grow in areas with high levels of pollution may be more acidic and the bark could disintegrate which makes it difficult for lichens to establish.
The larger (and therefore older) trees were growing during the Industrial Revolution and lichens were colonizing their bark. Lichens could not tolerate the high pollution levels and were unable to get the nutrients they needed to live, leading to less lichen diversity on the trees over a hundred years later!
Another explanation for this trend is light intensity. Older trees have denser canopies which means less light makes it to the lichens on the bark. This selects for lichen species that are comfortable growing in low-light conditions and may lower the overall diversity. The researchers of this study refer to these as “specialists.” Lichens that can tolerate the low-light environments like Peltigera and Lobaria species were most commonly found on the older trees with dense canopies. Generalist species like Xanthorion were better at tolerating the pollution and prefered high light and exposed surfaces.
Since most of the species found in this study are classified as generalists, it makes sense that there was greater diversity on smaller trees with more light. Additionally, the researchers found that when there were a lot of trees in a small area, called local crowding, there were less lichen species most likely due to the inability of light to penetrate through the canopies of many trees. In this study, they only measured the tree density within 10 meters of the target trees but suggest that since lichen spores (the baby lichens) can travel long distances through the wind, future research should expand their diversity study to include this dispersal factor.
Pollution pauses lichen succession
Based on the 24 lichen species in this study, the researchers determined the communities are mostly dominated by pollution-tolerant species that are considered early successional. After a disturbance like high levels of pollution, early successional species are the first to return to the disturbed environment. Interestingly, most of the trees had an early successional community that was pollution tolerant. The pollution during the Industrial Revolution, and the subsequent pollution in the modern era, stopped succession in its tracks, maintaining the same early successional species for decades. Without the legacy effects of pollution, larger and older trees support later successional communities.
Although lichens are often overlooked, they are important indicators of ecosystem health. This study, and subsequent studies on lichen diversity, can help us understand how our urban environments are changing. The legacy effects of sulfur emissions and current nitrogen pollution show that our actions are impeding the natural succession of lichen communities and may have broader impacts on the ecosystem. The recovery of these sensitive organisms relies on the reduction of pollutants and subsequent dispersal from lichens in the surrounding rural areas less impacted by the historical pollution.
Source: Llewellyn, T., G. Ester, and D.J. Murrell. Are Urban Communities in Successional Stasis? A Case Study on Epiphytic Lichen Communities. 2020. Diversity 12(9)L 330. https://www.mdpi.com/1424-2818/12/9/330/htm