Seeking shelter from the sun: The importance of shade provision by trees
Jess Quinton
2020-11-16
Anyone who has walked down a city street on a sunny summer day knows how quickly the experience can go from pleasant to excruciating. I love a sunny day as much as the next person, but I hate feeling like I’m melting further into the pavement with each step. Cities feel warmer than their surrounding rural areas due to the urban-heat-island effect. Due to an abundance of built infrastructure (such as buildings, sidewalks, and roads) and people, along with a lack of vegetation, heat is more easily absorbed from the sun and generated as energy from people, buildings, cars, etc., in these environments. This is an issue that is further exacerbated by climate change and increasing global temperatures. The past several summers have seen heat waves around the world, which have caused discomfort and even resulted in the death of individuals particularly vulnerable to extreme heat.
So how do we reduce the urban-heat-island effect and make cities safer and more comfortable places to live? Given that this issue is in part caused by a lack of vegetation, it makes sense that we can help address it by putting vegetation back into cities. Trees in particular are beneficial for reducing city temperatures. One of the ways in which they do this is through shade provision (Spoiler alert: shade is not the only way in which trees can help cool their surrounding environment—but you’ll have to wait for our future blog post on urban cooling to learn more about this!).
You may notice that well-treed streets, parks, cemeteries, botanical gardens, etc., feel much cooler than streets with few or only small trees. Compare the experience of walking down a poorly treed street (such as some of those surrounding the Halifax Citadel) and a well-treed one (e.g. many of the residential streets surrounding Dalhousie University) in the summer. You’ll notice a difference in temperature, I’m sure. This cooler temperature makes physical activity such as biking, running, and walking more attractive (and safer) on these streets in the warm summer months. Shade not only provides the benefit of cooling the surrounding environment, it can also reduce human exposure to harmful solar UV radiation (Sivarajah, Thomas, & Smith, 2020), which also makes it safer for people to spend increased time outdoors.
Shade not only directly benefits humans, but also the infrastructure they depend on. For example, two of our recent blog posts discussed conserving fuel and prolonging the life of infrastructure, both of which can be achieved in part through the shade provided by trees. Strategically placed shade trees can also reduce the cooling costs and energy requirements of buildings (e.g. Hwang, Wiseman, & Thomas, 2015).
Not all trees provide the same amount of shade, however. Depending on the species, crown size and shape, leaf size and shape, height, planting location, etc., trees can provide a different number of hours of shade provision, area of shade, and temperature reduction. In the two images below, you can see that the height and shape of the tree canopy differs between these two areas. Due to differences in their crown and leaf shape and their branching structure, deciduous trees tend to provide more shade than coniferous trees.
Of course, it is worth considering that not all cities will value shade to the same degree. For example, cities in cooler climates (e.g. Scandinavian countries) may value shade less than those in warmer climates (Tyrväinen, Mäkinen, & Schipperijn, 2007)—although this may change with the impact of climate change and increasingly warm summer temperatures. In the meantime, cities in cooler climates can still benefit from shade through the protection it affords to people and infrastructure from harmful UV rays.
Multiple Canadian studies in different cities have found that city residents highly value shade, whether it be due to cooling the urban environment or reduced cooling costs of buildings (Ordóñez, Duinker, Sinclair, Beckley, & Diduck, 2016; Peckham, Duinker, & Ordóñez, 2013; Quinton et al., 2019; Sinclair, Diduck, & Duinker, 2014). However, it should also be noted that shade has sometimes been indicated as problematic for urban residents, particularly in relation to trying to garden and grow non-shade-loving plants (Fraser & Kenney, 2000; Nowak & Dwyer, 2007). This concern (and many others about shade) can be addressed through strategic planting with the input of residents in the planning stages of future tree-planting efforts.
Beyond some considerations of geography and gardening, it is evident that shade provision by trees is highly valued by urban residents and not only contributes to cooling the surrounding environment but also protecting humans and infrastructure from the damage caused by UV rays. The cooling effect of shade from trees makes it safer and more enjoyable to spend increased time outside, which has been shown to have a positive impact on physical and mental health and well-being. Overall, shade provision is beneficial for both humans and buildings alike, making it just one of many reasons to encourage tree planting in our cities.
References
Fraser, E. D. G., & Kenney, W. A. (2000). Cultural background and landscape history as factors affecting perceptions of the urban forest. Journal of Arboriculture, 26(2), 106–113.
Hwang, W. H., Wiseman, P. E., & Thomas, V. A. (2015). Tree planting configuration influences shade on residential structures in four U.S. cities. Arboriculture & Urban Forestry, 41(4), 208–222.
Nowak, D. J., & Dwyer, J. F. (2007). Understanding the benefits and costs of urban forest ecosystems. In J. E. Kuser (Ed.), Handbook of Urban and Community Forestry in the North East (2nd Edition, pp. 25–46). https://doi.org/10.1007/978-1-4020-4289-8{_}2
Ordóñez, C., Duinker, P. N., Sinclair, A. J., Beckley, T., & Diduck, J. (2016). Determining public values of urban forests using a sidewalk interception survey in Fredericton, Halifax, and Winnipeg, Canada. Arboriculture and Urban Forestry, 42(1), 46–57.
Peckham, S. C., Duinker, P. N., & Ordóñez, C. (2013). Urban forest values in Canada: Views of citizens in Calgary and Halifax. Urban Forestry & Urban Greening, 12, 154–162. https://doi.org/10.1016/j.ufug.2013.01.001
Quinton, J. M., Duinker, P. N., Gallant, K. A., Steenberg, J. W. N., Charles, J. D. (2019). To tree or not to tree: User and management perspectives of cemetery trees. Urban Forestry & Urban Greening. https://doi.org/https://doi.org/10.1016/j.ufug.2019.126385
Sinclair, A. J., Diduck, J., & Duinker, P. N. (2014). Elicitation of urban forest values from residents of Winnipeg, Canada. Canadian Journal of Forest Research, 44, 922–930. https://doi.org/10.1139/cjfr-2014-0016
Sivarajah, S., Thomas, S. C., & Smith, S. M. (2020). Evaluating the ultraviolet protection factors of urban broadleaf and conifer trees in public spaces. Urban Forestry and Urban Greening, 51(February), 126679. https://doi.org/10.1016/j.ufug.2020.126679
Tyrväinen, L., Mäkinen, K., & Schipperijn, J. (2007). Tools for mapping social values of urban woodlands and other green areas. Landscape and Urban Planning, 79, 5–19. https://doi.org/10.1016/j.landurbplan.2006.03.003