Lichens versus Urban Traffic

In my previous blog post I wrote about the beauty and uses of lichens in the city, and I mentioned their use as indicators for pollution. In this post I want to go a little deeper into their interactions with pollution especially in urban habitats.

Ammonia Pollution

Urban traffic causes a great deal of pollution such as particulate matter and sulphur oxides which I covered last time, as well as large amounts of ammonia (NH₃). Traffic emits large ammounts of ammonia, which deposits on roadside trees. This abundance of ammonia on trees increases the pH of their bark (Manninen et al., 2023). Ammonia can absorb a hydrogen ion to form toxic ammonium (NH₄⁺), which reduces the hydrogen levels of the bark (measured as an increase in pH), in other words: reducing acidity. This shift in acidity has repercussions for the lichens living on the bark of these trees.

Many lichens are sensitive to ammonia pollution and the shift in pH of tree bark. They often disappear in high traffic urban areas. Others however, are more tolerant to high ammonium and can oxidize it to nitrate (NO₃^–), a non-toxic nitrogen compound (Manninen et al., 2023). Under the pressure of urban pollution, a community of lichens develops dominated by ammonia tolerant species (see top image). The species in such communities often overlap with the drought resistant ones I mentioned in my previous post. Thus, by arranging lichens by their tolerance for ammonia, functional groups can be described which can serve as indicators of ammonia pollution levels (Bryologische en Lichenologische Werkgroep (BLWG), 2005; Llop et al., 2017).

Urban Parks

Urban green spaces are often lauded as a means to reduce the impact of air pollution. In the case of traffic pollution, the ammonia deposits over relatively short ranges (up to about 500 meters from the source) (Matos et al., 2019). This deposition is accelerated by trees in urban green spaces, as tree leaves intercept particles. However, lower vegetation and grass fields are much less capable of buffering ammonia pollution. Matos et al. (2019) suggest that increase in tree density in urban green spaces can increase ammonia deposition, reducing the ammonia levels in the centre of the green spaces. This could be useful especially in dense cities where space for green development is rare and small.

Botanical Gardens

One unique urban space is a botanical garden. The diversity of plants and trees in such gardens allows for a higher diversity of lichen substrates and thus lichens. In 2021, the Leiden Hortus Botanicus was examined and the lichen community there inventoried (Timans, 2022). Among the 111 different species found were several endangered Red List species, despite the Hortus Botanicus being a relatively small patch of green in the city.

My local park

Armed with this knowledge, I took the time to look into my local park to take the pictures in this post. At a cursory glance, I could not find any lichens that should be suppressed by the ammonia other than a few Flavoparmelia caperata (right), and even this one is only moderately aversive to ammonia. The rest of the lichens I could find were the usual suspects for ammonia rich environments as noted in Llop et al. (2017) and the BLWG Zoekkaart (Bryologische en Lichenologische Werkgroep (BLWG), 2005). This only goes to show that the urban environment in the Netherlands remains very eutrophic and polluted, even in tree rich parks. As far as lichens go, we have a lot of work remaining in pollution reduction and healthier urban green spaces.

---

References

Bryologische en Lichenologische Werkgroep (BLWG). (2005). Zoekkaart korstmossen en ammoniak. BLWG. https://shop.blwg.nl/products/zoekkaart-korstmossen-en-ammoniak-10-stuks

Llop, E., Pinho, P., Ribeiro, M. C., Pereira, M. J., & Branquinho, C. (2017). Traffic represents the main source of pollution in small Mediterranean urban areas as seen by lichen functional groups. Environmental Science and Pollution Research, 24(13), 12016–12025. https://doi.org/10.1007/s11356-017-8598-0

Manninen, S., Jääskeläinen, K., Stephens, A., Iwanicka, A., Tang, S., & van Dijk, N. (2023). NH3 concentrations below the current critical level affect the epiphytic macrolichen communities – Evidence from a Northern European City. Science of The Total Environment, 877, 162877. https://doi.org/10.1016/j.scitotenv.2023.162877

Matos, P., Vieira, J., Rocha, B., Branquinho, C., & Pinho, P. (2019). Modeling the provision of air-quality regulation ecosystem service provided by urban green spaces using lichens as ecological indicators. Science of The Total Environment, 665, 521–530. https://doi.org/10.1016/j.scitotenv.2019.02.023

Timans, H. W. (2022). Grote diversiteit korstmossen in de Hortus Botanicus te Leiden. Buxbaumiella, 124, 1–6. http://www.buxbaumiella.nl/pdf/BUX2022124001.pdf

 

Featured image: Photo by the author