3. Cinquantenaire Park 🍁

The importance of lichens

Below you will find the audio and text of the walk !
Have a good adventure !

What do you think ? How important are lichens ?

We pass by lichens every day without realising their presence. But why should these small organisms matter to us ?

Lichens are pioneer species – they are a link between the animate and the inanimate.  

Every environment is constantly changing, this is due in part to climatic conditions, but also to the communities of organisms that make up the environment. Each living being is active in its habitat, it changes it and is changed by it. In environments devoid of vegetation, such as ancient volcanoes or mountain peaks, pioneer species such as lichens, mosses or grasses create space for other species to colonise.

But how?

Lichens’ penchant for rocks has literally changed our planet. Lichen erosion of rocks occurs in two ways. First, lichens, along with the roots of mosses and grasses, create crevices in rocks as they grow and develop. Second, lichens use their arsenal of chemicals to digest the rock. The lichen’s ability to erode rock puts it in the category of physical forces. But more than that, lichens, as they die and decompose, create nitrogen-rich humus and shape the first soils for other plants to grow. Lichens transform the inanimate, like rocks, by including them in the metabolic cycle of life. These symbiotic organisms are the links between what is animate and what is not, what lives and what does not live.

Lichens allow environments to form and prepare a habitat for other plants to colonize. This can be observed in any environment in a dynamic succession phase (see diagram below).  

Ecological succession. Lichens and mosses create habitat for a forest to grow over time. Photo by LucasMartinFrey on Wikimedia, CC BY 3.0. 

Lichens, inhabitants of extreme environments. They change the way we see space and the earth.

June 18, 2016, a module from a spacecraft lands in the steppes of Kazakhstan. The astronauts get out, but not alone. Below their seats are a hundred organisms in a box. Among the samples sent into space for a year and a half, there are several species tolerant to extreme conditions including bacterial spores, algae and tardigrades (Box 3) and some species of lichens. This experiment was part of the international astrobiology consortium named “Biology and Mars Experiment” (BIOMEX). The astronauts incubated the organisms outside the International Space Station under extra-terrestrial conditions. A few of these organisms can survive if they are protected from solar radiation, but few – apart from a few species of lichen – can live in space conditions, exposed to cosmic ray radiation. Indeed, when the lichens returned from space, they showed no sign of physiological changes nor in their photosynthetic activity, which they resumed 24 hours after being rehydrated.

Credit: Micrograph of an adult Tardigrade from Goldstein Lab on Wikimedia. CC BY-SA 2.0.

Lichens live in a wide variety of habitats. They live in the tropics, on our streets, on trees, on pavement, on beetles, on any surface really ! Lichens even exist in our mouths. These lichens are called lichen planus. Their presence is due to the immune system that turns against itself by attacking its own cells. Researchers have also found lichens 400 km from the South Pole and they can therefore live in the Arctic and Antarctic.

But how can these small organisms live in such inhospitable environments?

Lichens living in these extreme regions have a number of adaptations that allow them to survive. They can continue to use photosynthesis as an energy source while enduring temperatures near  -20°C. Lichens can absorb water from a saturated, snow-covered habitat. The trick of the lichens is the dormant, inactive state they enter when conditions are not good. They stop photosynthesis and can start again when conditions begin to be favourable.

The impressive tolerance of lichens to sunlight and other stars, to drought, to heat and to extreme changes between freezing, thawing and heating is due to their protection and repair mechanisms at the cellular level. These adaptation methods are not yet well known. Moreover, their symbiosis allows the organisms inhabiting the lichen to be more resistant.

Another experiment on a species known to be very tolerant – Circinaria gyrosa – showed that exposure to a radiation dose 12,000 times lethal to humans (6 kilograys of gamma radiation) has no effect on the species. When the dose is doubled (12 kilograys of gamma radiation – 2.5 times the lethal dose for tardigrades), the ability of the lichens to reproduce is impaired, but the species can continue to photosynthesise and survive. 

The thallus of Circinaria gyrosa, a vagrant lichen collected in central Spain (photo by Sanchez et al., 2012)

Lichens at the bottom of the food web 

In the extreme environments they inhabit, lichens are the primary producers of the food chain and are a source of food for many herbivores like reindeer. Their absence or disappearance could cause the extinction of many individuals. 

In our regions, lichens are inhabited by many species such as ladybugs and springtails. If you observe carefully you may see some insects… Even birds use the thallus of lichens to form their nests. Gastropods also feed on the lichen thallus (see photo below). All the photo in the diaporama below were taken by the author (2020) and are under a CC BY-SA 4.0 license.

Lichens store carbon 

Lichens cover 8% of the earth’s surface (Ahmadjian, 1995). Through the process of photosynthesis, lichens store carbon and release oxygen changing the composition of the atmosphere. Lichens do not represent a carbon capture as great as trees, which are larger and more massive, but they do store a significant proportion of carbon in cities. New research indicates that these organisms (along with mosses) take up about 3.9 petagrams Pg (corresponding to 3900,000,000,000 kg) of carbon per year, which is similar in magnitude to the annual carbon releases from biomass-forest burning (3.6 Pg/yr) and fossil fuel burning (7 Pg/yr) (Elbert et al., 2012).

In addition, lichens play a major role in the biogeochemical cycles of carbon and nitrogen. Climate change and changing land uses will influence the geographic distribution and metabolic activity of these organisms. A feedback effect could have unknown unintended consequences. 

Lichens have an intrinsic value; they matter just because they exist.

In total, there are about 20,000 identified species of lichens (Lücking et al., 2017). And there are still many species to be discovered. For example, in the tropics on the island of Sri Lanka, more than 3,000 different species of a single genus, Graphidacea, have been recorded (information from a conference of the British Lichen Society). In Texas, 7000 species have been identified while 200 are recorded at the Mexican border. This is certainly due to the number of lichenologists – people who study lichens – in the country, which leaves an incredible space for new discoveries (information from a British Lichen Society conference).

We can find lichens of all shapes and colours and on – almost – all substrates. Some lichens live on the east side of the substrate, others on the west side, some are competitive, others live on the ruins of dead lichens. Their diversity is impressive and at the same time frustrating. The probability that the identified species has a twin, but with a different chemistry is very high …. There is thus always something to learn from !

Lichens have an intrinsic value. We regularly try to put a value on living things and on the organisms surrounding us. These methods giving a value (monetary or other) to living beings is important for acknowledging them and for managing natural resources. Nevertheless, these methods consider organisms as passive beings. Lichens deserve their place in the ecosystem as much as humans do.


Think about the environment in which the lichens you see are. What are the important factors ? Access to sunlight, humidity, air quality. 

What type of habitat are you in ? Is there any infrastructure that could shade the substrate on which the lichens are found ? Are there roads ? What is the source of pollution emissions ? Is this source close or far away ?

We will talk about the effects of air pollution on lichens later on in this walk, here

How high up is the lichen you are trying to identify ?

This is important because, for example, at the height of a dog’s leg you will find lichens that live in a nutrient rich environment. Dogs which urinate at this height change the chemical composition of the bark and therefore affect the lichen communities. 

What type of tree are you looking at ?

The tree species can have an effect on lichen communities. For example, ash tend to be basic (in their chemistry) while other trees such as birches or oaks are more acidic. With increased atmospheric pollution in cities, the bark is becoming more basic and rich in nitrogen, so we only see species that tolerate these conditions.

Let’s look at the species on the tree we have in front of us !

Introduction to the lichen of the genus Physcia, species adscendens

We now know the lichen Xanthoria. Can you spot the species Xanthoria parietina on this tree ?

Xanthoria parietina, photo taken by Christian Thirion, used with permission. CC BY-SA 4.0

I would like to introduce you to lichens of the genus Physcia. It is a blue-green lichen that is very present in the city. The plane tree you see is covered with it. The plane tree has its bark peeling off, which creates a dynamic environment for the lichens. However, when the lichens fall and are on the ground, there is a high probability that the individuals will die. 

The characteristics of Physcia adscendens are quite simple: 

  • The tip of the thallus (called a lobe) is shaped like a small hat or helmet. You can see some of them on the tree. Some have their lobes turning outwards, lip-like. These lichens are Physcia tenella (another species) or young Physcia adscendens.
  • If you have the opportunity to look at the lichen with a magnifying glass, you will see small hairs coming out of the tip of the thallus, these are called cilia. The surface is ciliated. We do not know exactly the function of these hairs, but they represent one of the characteristics of this genus Physcia.
Image found on the website of Science Infuse, document accessible here. Shared and used with permission.
  • If you get even closer with the magnifying glass, you can see the macules. These are the white coloured parts on the thallus. This colour change is created by a heterogeneous distribution of algae. The white parts are the medulla (the fungal part) that we can see because in some places the algal layer is absent. All the pictures below are taken by the author and under a CC BY-SA 4.0 license.

Note: The monument just to your left when you look at the tree (and the arch of the Cinquantenaire is at your back) is a “Memorial to the Belgian Pioneers in the Congo”. The lack of indication on the history of this monument – discriminatory – and the lack of any means of awareness of its presence disturbed me. This walk is mainly dedicated to lichens and in a sense to the decolonization of cities through the inclusion of more-than-human organisms. Nevertheless, I encourage you to do more research and look at what the Collectif des Mémoires Coloniales et Lutte contre les Discriminations (CMCLD) does here

Check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark.


  • Ahmadjian, V. (1995). Lichens are more important than you think. BioScience45(3), 124.
  • Elbert, W., Weber, B., Burrows, S., Steinkamp, J., Büdel, B., Andreae, M. O., & Pöschl, U. (2012). Contribution of cryptogamic covers to the global cycles of carbon and nitrogen. Nature Geoscience5(7), 459-462.
  • Sánchez, F. J., Mateo-Martí, E., Raggio, J., Meeßen, J., Martínez-Frías, J., Sancho, L. G., … & De la Torre, R. (2012). The resistance of the lichen Circinaria gyrosa (nom. provis.) towards simulated Mars conditions—a model test for the survival capacity of an eukaryotic extremophile. Planetary and Space Science72(1), 102-110.
  • Lücking, R., Hodkinson, B. P., & Leavitt, S. D. (2017). The 2016 classification of lichenized fungi in the Ascomycota and Basidiomycota–Approaching one thousand genera. The Bryologist119(4), 361-416.

In our next step we will talk about symbiosis and the questions this phenomenon makes us reflect on ! Follow me to Leopold Park 👇🏾👇🏾

If you don’t continue the ride, can you give us feedback on your experience here ? It will help us improve!

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