Clarissa Siegfried

Trees can contribute to the adaptation of cities to anthropogenic climate change and therefore contribute to the SDG number 11. They do so by reducing rising land surface temperatures, surface runoff after rainfall events, air pollution and by improving the health of urban populations.

Figure 1: Trees in cities

With each passing year climate change is becoming more noticeable in the world. Cities are also struggling with the consequences of climate change. They must adapt to new climatic situations and prepare themselves against an increasing number of extreme events. Possible adaptation strategies range from urban gardening over solar-covered high-rise buildings to more efficient, climate-neutral transport systems. Trees are also being discussed as an adaptation strategy. In this blog post, we will take a closer look at how trees can protect cities from the consequences of climate change.

Cooling potential and reduction of surface runoff

Climate change leads to an overall temperature rise on our planet. Cities are especially challenged by this temperature rise as most of the surface cover in cities warms up easily and captures the heat which aggravates the consequences of extreme heat events and heat islands even more  (Taha, 1997). Heat islands are areas with consistently higher temperatures than surrounding areas because of lack of vegetation and low albedo of roofing materials and asphalt. Researchers found that vegetation can have a significant cooling effect in cities (Schwaab et al., 2021). Regarding the cooling effect of trees in particular, it has been shown  that both the land surface temperature can be significantly reduced, and heat islands can be mitigated (Schwaab et al., 2021). The main causes for the cooling effect are the albedo and the evapotranspiration of the vegetation (Konarska et al., 2016). They found  that the greatest effect of urban vegetation was measured during the day at high temperatures while simultaneously the humidity was low (Perini & Magliocco, 2014). There are large variations in the effectiveness of the cooling if we compare different locations, shapes and species of the trees. The following findings  can help policy makers and city planners to choose the most efficient tree species at each location to maximize their effectiveness:

• Trees should be planted in locations where sunlight is not blocked by buildings to maximise their potential as they have a stronger cooling effect than the shade of tall buildings (Konarska et al., 2016).
• The height of the vegetation influences the effectiveness and tall vegetation proved to be particularly effective (Cascone et al., 2019).
• If we compare vegetation on the ground with the same vegetation on roofs, vegetation on the ground is generally more effective (Cascone et al., 2019).
• The denser the structure of the tree canopy, the higher the cooling effect (Chen et al., 2020).
• The effectiveness depends significantly on the tree species and conifers have been found to be particularly effective in European cities (Speak et al., 2020).
• Cooling by wind can also be generated by planting trees in wind corridors of the city and in cities located next to a lake or the sea, trees were able to use the breeze to cool the city (Tan et al., 2016).

A research group looked  at the potential of trees as cooling systems in various European cities and they found that during hot extremes, trees behave differently, depending on their geographical location.  They showed that while the cooling effect of trees increases in Central Europe during hot extremes, it decreases in Mediterranean regions. This illustrates the necessity to evaluate the potential cooling effect of trees at a given location before planting trees as an adaption strategy. (Schwaab et al., 2021)

In cities, surface cover is mainly composed of asphalt which does not allow water to infiltrate into the ground (Berland et al., 2017). With climate change, extreme events get more frequent and thus we will observe an increase in extreme rainfall events which challenges the sewage system in cities (Selbig et al., 2022). Researchers analysed the potential of trees to reduce surface runoff after rainfall events and showed that there are several ways in which tress can do so, namely through interception, evapotranspiration or facilitated infiltration (Berland et al., 2017). It has been shown that for heavy rainfall events, facilitated infiltration through the root system is particularly important. In laboratory experiments, an improved infiltration of 63% was found on average (Bartens et al., 2008) and in a field experiment, a runoff reduction from 60% to 25% was measured for asphalted surfaces with trees compared to pure asphalt (Armson et al., 2013). Furthermore, the infiltration rate depends on the root system and recommendations vary with the depth of the soil (D. Zhang et al., 2019). Up to approximately one meter depth, fibrous roots are better whereas after that taproots have a greater effect (D. Zhang et al., 2019).

Additional Health benefits

Many cities around the world are known to be highly polluted. The main polluters which are responsible for bad air quality in inner cities are Nitrogen Dioxide (NO2) and particulate matter (Sicard et al., 2018). They usually occur in areas of high vehicle traffic (Sicard et al., 2018). Particulate matter includes dust, soot, smoke and liquid droplets of a certain diameter. Both polluters are associated with health risks including organ development during pregnancy and early childhood and lung function decline (Sicard et al., 2018). Studies show that  trees can remove these particles from the air, but the potential is highly dependant on the tree species (J. Zhang et al., 2020). Additionally, urban forests can play a role in the reduction of ozone but there are numerous other factors that influence ozone reduction (Av & Wa, 2010).  However, trees do not only absorb substances, but they also emit volatile organic compounds, from which ozone can be formed (Av & Wa, 2010). Also, computer simulations have shown that trees can even worsen air quality at the local level by reducing ventilation, therefore the benefit of trees as air purifier should be looked at critically (Vos et al., 2013).

Studies have found that trees can have a positive influence on human health (Twohig-Bennett & Jones, 2018). The effectiveness again varies with the type, location and density of trees (Salmond et al., 2016). As cooling mechanism, they can reduce morbidity and mortality from heat in cities and improve thermal comfort (Wolf et al., 2020). Furthermore, urban forests can intercept and reduce air pollution, which has an influence on numerous diseases (Turner-Skoff & Cavender, 2019). But not only air quality and temperature are health issues that citizens face nowadays, also our mental health is at risk. With increasing urbanisation, we observe a constantly decreasing contact with nature. Green spaces in cities are therefore not only important as cooling mechanisms and air purifier but also for our mental health (Twohig-Bennett & Jones, 2018). Spending time in the forest can also lead to a decrease in anxiety and influence depression and anger (Kotera et al., 2022).

Conclusion

Trees have the potential to reduce climate change impacts in cities. They can have a cooling effect, reduce surface runoff, work as an air purifier and also strengthen the mental health of the urban population. However, a few trees planted cannot stop or reverse climate change and should therefore not be glorified as the perfect adaptation strategy. Cities, as well as the urban population, must do more than plant trees and must agree to greater adjustments in their behavior, otherwise the consequences of climate change and climate change itself cannot be stopped.

References

Armson, D., Stringer, P., & Ennos, A. R. (2013). The effect of street trees and amenity grass on urban surface water runoff in Manchester, UK. Urban Forestry & Urban Greening, 12(3), 282–286. https://doi.org/10.1016/J.UFUG.2013.04.001

Av, D., & Wa, D. J. N. O. (2010). Air Quality Effects of Urban Trees and Parks. www.NRPA.org

Bartens, J., Day, S. D., Roger Harris, J., Dove, J. E., & Wynn Virginia Tech, T. M. (2008). Can Urban Tree Roots Improve Infiltration through Compacted Subsoils for Stormwater Management? Journal of Environmental Quality, 37(6), 2048–2057. https://doi.org/10.2134/JEQ2008.0117

Berland, A., Shiflett, S. A., Shuster, W. D., Garmestani, A. S., Goddard, H. C., Herrmann, D. L., & Hopton, M. E. (2017). The role of trees in urban stormwater management. Landscape and Urban Planning, 162, 167–177. https://doi.org/10.1016/J.LANDURBPLAN.2017.02.017

Cascone, S., Gagliano, A., Poli, T., & Sciuto, G. (2019). Thermal performance assessment of extensive green roofs investigating realistic vegetation-substrate configurations. Building Simulation, 12(3), 379–393. https://doi.org/10.1007/s12273-018-0488-y

Chen, J., Jin, S., & Du, P. (2020). Roles of horizontal and vertical tree canopy structure in mitigating daytime and nighttime urban heat island effects. International Journal of Applied Earth Observation and Geoinformation, 89. https://doi.org/10.1016/J.JAG.2020.102060

Konarska, J., Holmer, B., Lindberg, F., & Thorsson, S. (2016). Influence of vegetation and building geometry on the spatial variations of air temperature and cooling rates in a high-latitude city. International Journal of Climatology, 36(5), 2379–2395. https://doi.org/10.1002/joc.4502

Kotera, Y., Richardson, M., & Sheffield, D. (2022). Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: a Systematic Review and Meta-analysis. International Journal of Mental Health and Addiction, 20(1), 337–361. https://doi.org/10.1007/s11469-020-00363-4

Perini, K., & Magliocco, A. (2014). Effects of vegetation, urban density, building height, and atmospheric conditions on local temperatures and thermal comfort. Urban Forestry & Urban Greening, 13(3), 495–506. https://doi.org/10.1016/J.UFUG.2014.03.003

Salmond, J. A., Tadaki, M., Vardoulakis, S., Arbuthnott, K., Coutts, A., Demuzere, M., Dirks, K. N., Heaviside, C., Lim, S., MacIntyre, H., McInnes, R. N., & Wheeler, B. W. (2016). Health and climate related ecosystem services provided by street trees in the urban environment. In Environmental Health: A Global Access Science Source (Vol. 15). BioMed Central Ltd. https://doi.org/10.1186/s12940-016-0103-6

Schwaab, J., Meier, R., Mussetti, G., Seneviratne, S., Bürgi, C., & Davin, E. L. (2021). The role of urban trees in reducing land surface temperatures in European cities. Nature Communications 2021 12:1, 12(1), 1–11. https://doi.org/10.1038/s41467-021-26768-w

Selbig, W. R., Loheide, S. P., Shuster, W., Scharenbroch, B. C., Coville, R. C., Kruegler, J., Avery, W., Haefner, R., & Nowak, D. (2022). Quantifying the stormwater runoff volume reduction benefits of urban street tree canopy. Science of The Total Environment, 806, 151296. https://doi.org/10.1016/J.SCITOTENV.2021.151296

Sicard, P., Agathokleous, E., Araminiene, V., Carrari, E., Hoshika, Y., de Marco, A., & Paoletti, E. (2018). Should we see urban trees as effective solutions to reduce increasing ozone levels in cities? Environmental Pollution, 243, 163–176. https://doi.org/10.1016/J.ENVPOL.2018.08.049

Speak, A., Montagnani, L., Wellstein, C., & Zerbe, S. (2020). The influence of tree traits on urban ground surface shade cooling. Landscape and Urban Planning, 197, 103748. https://doi.org/10.1016/J.LANDURBPLAN.2020.103748

Taha, H. (1997). Urban climates and heat islands: albedo, evapotranspiration, and anthropogenic heat. Energy and Buildings, 25, 99–103.

Tan, Z., Lau, K. K. L., & Ng, E. (2016). Urban tree design approaches for mitigating daytime urban heat island effects in a high-density urban environment. Energy and Buildings, 114, 265–274. https://doi.org/10.1016/J.ENBUILD.2015.06.031

Turner-Skoff, J. B., & Cavender, N. (2019). The benefits of trees for livable and sustainable communities. In Plants People Planet (Vol. 1, Issue 4, pp. 323–335). Blackwell Publishing Ltd. https://doi.org/10.1002/ppp3.39

Twohig-Bennett, C., & Jones, A. (2018). The health benefits of the great outdoors: A systematic review and meta-analysis of greenspace exposure and health outcomes. Environmental Research, 166, 628–637. https://doi.org/10.1016/j.envres.2018.06.030

Vos, P. E. J., Maiheu, B., Vankerkom, J., & Janssen, S. (2013). Improving local air quality in cities: To tree or not to tree? Environmental Pollution, 183, 113–122. https://doi.org/10.1016/J.ENVPOL.2012.10.021

Wolf, K. L., Lam, S. T., McKeen, J. K., Richardson, G. R. A., Bosch, M. van den, & Bardekjian, A. C. (2020). Urban trees and human health: A scoping review. In International Journal of Environmental Research and Public Health (Vol. 17, Issue 12, pp. 1–30). MDPI AG. https://doi.org/10.3390/ijerph17124371

Zhang, D., Wang, Z., Guo, Q., Lian, J., & Chen, L. (2019). Increase and Spatial Variation in Soil Infiltration Rates Associated with Fibrous and Tap Tree Roots. Water 2019, Vol. 11, Page 1700, 11(8), 1700. https://doi.org/10.3390/W11081700

Zhang, J., Ghirardo, A., Gori, A., Albert, A., Buegger, F., Pace, R., Georgii, E., Grote, R., Schnitzler, J. P., Durner, J., & Lindermayr, C. (2020). Improving Air Quality by Nitric Oxide Consumption of Climate-Resilient Trees Suitable for Urban Greening. Frontiers in Plant Science, 11, 1491. https://doi.org/10.3389/FPLS.2020.549913/BIBTEX

 

Figure Reference:

Figure 1: Photo by Robert Bye on Unsplash (https://unsplash.com/s/photos/tree-city); accessed: 17.05.2022