Boomzorg Scholar: how lichens can be used as an indicator for urban heat islands

ARTICLE

SOIL & SOIL BIOLOGY

Manon Botterblom, Monday 16 March 2026

106 sec

It has long been known that lichens can be used as an indicator of air pollution, but new research from Leiden University shows that lichens can also be used to map urban heat.

The researchers collected lichen samples from trees in twelve different urban heat islands in Amsterdam and Leiden. For each sample, the mass and water retention capacity were calculated. The researchers focused on eighteen common lichen species.

Lichens respond quickly to their environment

Lichens absorb water and nutrients directly from the air. Because they have no roots, they depend strongly on temperature and humidity and respond quickly to changes in their environment. Lichens have long been used as indicators of air pollution. Their sensitivity also makes them suitable for monitoring the effects of urban heat. In the study, the scientists examined two measurable properties of lichens: total lichen mass and water retention capacity. The mass reflects the thickness of the lichen tissue per unit area. Water retention capacity indicates how much water the lichen can hold.

Thicker lichens in warmer urban areas

The measurements show that lichen mass increases as temperatures rise. In warmer parts of the city, lichens are on average thicker. This pattern was observed across all studied species. No such relationship was found for water retention capacity. This suggests an adaptation to stressful conditions. In warm and dry urban areas, water evaporates faster and dew formation is more limited. A thicker lichen layer may help lichens cope better with these conditions, for example by improving resistance to heat.

What does this mean for practice?

The study shows that a thicker lichen layer can be a useful property for monitoring urban climate effects. This is relevant for professionals working with urban green spaces, trees and biodiversity. Lichens grow slowly, but they are sensitive to their environment. This makes them a useful additional tool alongside temperature measurements and mapping.
For tree managers and arborists, this can provide insight into microclimatic conditions on and around trees. Especially in locations with limited measurement data, lichens can help reveal differences within the city. The focus is not on individual observations, but on patterns across multiple locations and species.

Limitations and further research

The authors emphasize that urban heat is not the only factor affecting lichens. Humidity, precipitation, wind, tree species and local pollution also play a role. In the study, these factors were controlled as much as possible by sampling only on maple and elm trees with a similar bark pH.

Conclusion

In hot inner-city areas, lichens adapt by becoming thicker and more rounded. This helps them retain water better and improves their resistance to heat and drought.

Boomzorg scholar

Boomzorg Scholar is a new initiative by the trade journal Boomzorg, in which articles from scientific journals are made accessible to a broader professional audience. For each issue, one article is translated and adapted into a more accessible format. This edition features an article by Tim Claerhout, Michael Stech, Paul J.A. Keéler and Laurens B. Sparrius: Urban heat island effect as a driver for Specific Thallus Mass (STM) in lichens, published in The Lichenologist (2025, Volume 57, Issue 3-4).

This article was previously published on 16 March 2026 on the Boomzorg
website.

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