Lichens are everywhere in cities: on tree bark, old walls, and even between paving stones. Although they may look unchanging, they are surprisingly flexible. Research in Dutch cities shows that the so-called specific thallus mass – the weight of lichen tissue relative to its surface area – is clearly higher in urban areas. The culprit? The urban heat island: buildings, traffic, and heat emission make cities warmer than their surroundings. Lichens adjust by becoming thicker and more rounded. This helps them retain more water and withstand heat and drought.
Lichens as climate indicators
Traditionally, lichens are used as indicators of air quality, but this research shows they also respond to temperature. The thickening thallus – the body of the lichen – acts as a natural thermometer, reflecting how cities heat up. Large cities like Amsterdam, Rotterdam, and Utrecht are increasingly experiencing hot summers and prolonged heatwaves, especially in densely built-up neighbourhoods. The fact that even lichens adapt their structure to this microclimate highlights the profound impact of heat stress on urban nature – often unnoticed in daily life.
Lessons for the future
The discovery of thicker lichens in city centers underlines that urban nature is deeply intertwined with our living environment. As municipalities invest in measures against heat stress – such as green roofs and more city parks – lichens can serve as silent witnesses of progress. They remind us that climate change and urbanization affect not only people, but also the smallest organisms we share the city with. Perhaps next time, we will look at that humble lichen on a tree near the bus stop with fresh eyes.
More information
- Article: Urban heat island effect as a driver for Specific Thallus Mass (STM) in lichens.
- This study was conducted as part of the NWO-funded research project Hidden Biodiversity.
Text: Tim Claerhout, Hortus botanicus Leiden and Naturalis Biodiversity Center; Laurens Sparrius, Bryologische en Lichenologische Werkgroep (BLWG)
Images: Arne van Wingerden (lead photo: lemon lichen); Michiel Langeveld, Waarneming.nl