Urban environments consist mostly of bricks, concrete, asphalt, metal and glass: not very alluring substrates for many terrestrial organisms. Certain mosses and lichens can be seen sometimes on walls in the city, but even they will not easily grow in direct sunlight during dry periods, especially taking the ‘urban heat island effect’ into account (Deilami et al., 2018). However, on more shaded and humid places in cities, they, and other organisms, stand a better chance. In Leiden, the Netherlands, many canals run through the city, forming an extensive system of waterways that provide a habitat for numerous organisms. This is not only the case for organisms inside the water, but also for those near it. In the picture below, you can see Asplenium ruta-muraria (‘muurvaren’ in Dutch and ‘wall-rue’ in English), a fern that naturally grows on calcareous rocks on slopes (i.e. epilithic: growing on rocks).
The Netherlands is known for its low-lying land, a substantial part of which is situated below sea level. The ground surface in the west of the Netherlands is not rocky at all, with peat and clay soils being very common. Therefore, non-urban environments in this region are not naturally suitable for this species. Remarkably, specific urban places, such as the one on the photo above and below, are much more suitable. Suddenly, the ‘bare walls’ are being put in a different light: to the right inhabitant, they are in fact vertical rock faces that contain a calcium-rich substrate. In turn, other organisms can take shelter there, such as the spider that built its web on the fern in the picture above. Canal walls are more often sheltered from sunlight than the walls of buildings surrounding them and the evaporation of water from the canals helps to prevent these ferns from drying out.
This is not the only species of fern that thrives here: a fern that shares the same genus, Asplenium scolopendrium (‘tongvaren’ in Dutch and ‘Hart’s-tongue fern’ in English), can also be spotted on canal walls (see the picture to the right). However, the small size and yellow leaf colour of the ferns suggests that they are struggling to grow at that location. Sun exposure is most likely high there (facing the southwest), while this species of fern prefers moist and shaded conditions. In addition, the small crevices in the masonry put a limit on the amount of stable growth that is possible.
During my quest through the city, I found one more plant species that occurred relatively frequently: Sedum acre (‘muurpeper’ in Dutch and ‘goldmoss stonecrop’/’wallpepper’ in English), see the picture below. This small species with succulent leaves only needs a very thin layer of substrate to grow well and is less sensitive to high sun exposure and occasional moisture deficits.
The three species that are highlighted show that life is able to sustain itself on unexpected places. If more plants would succeed to take root there, imagine the lush ‘rock gardens’ that might form, in the middle of the city! This imagination can become reality in humid climates: Chen et al. (2020), found that this spontaneous colonisation of urban walls took place by a very diverse range of plant species, the majority being native species, in the city Chongqing, China. The findings of this study could be an inspiration for urban greening projects in other urban areas. Especially since colonisation takes place naturally and, under the right climate, could lead to lush green walls in a relatively short period.
Initially, I only planned to write about plant species growing on canal walls. However, when I recently cycled through The Hague, I noticed one Elm (Ulmus) tree that was completely covered with ferns on the north/northeast facing side of its trunk.
It could be said that the lush vertical garden we imagined earlier now became reality. Underneath the ferns there was a thick layer of moss, and on uncovered spots it could be seen that the bark of the tree was thick and had many crevices. This species of fern is most likely Polypodium vulgare or a hybrid of related species: they can be found as terrestrial plants or as epiphytes (growing on trees), such as on the pictures above. Also, they can grow on walls, just as the species mentioned earlier.
In Amsterdam, these ferns were also found on trees, in particular on old elm trees (Hoopen, 2019). The observation made in The Hague thus aligns with these findings. The number of such ferns on trees was found to have increased substantially in Amsterdam in less than a decade and coincided with an increase of plant species growing on canal walls in the city (Hoopen, 2019). Regarding the epiphytes, these observations show the value of old trees in urban areas, as they can provide a substrate for many more plants and help to create a natural ‘vertical’ ecosystem. Because of the limited space available in urban areas, this ability is especially valuable.
References
Chen, C., Mao, L., Qiu, Y., Cui, J. & Wang, Y. (2020). Walls offer potential to improve urban biodiversity. Scientific Reports, 10(1), 9905. doi: 10.1038/s41598-020-66527-3.
Deilami, K., Kamruzzaman, Md. & Liu, Y. (2018). Urban heat island effect: A systematic review of spatio-temporal factors, data, methods, and mitigation measures. International Journal of Applied Earth Observation and Geoinformation, 67, 30-42. https://doi.org/10.1016/j.jag.2017.12.009.
Hoopen, V. ten. (2019). Amsterdam varenstad: Eikvarens de boom in! Bomen: Vakblad Voor de Boomverzorging, 46, 4-7. https://edepot.wur.nl/519233.
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