Last updated on September 22, 2022
Blog post by Loïc Gillerot.
Heatwaves have been ubiquitous this year, popping up all around the globe. They started already in January in southern countries of South America, then India and Pakistan before appearing in some US states around June. Little later, heatwaves started to affect multiple areas in Europe, with the UK recording a temperature above 40°C for the first time, as well as a 7% increase in mortality compared to baseline levels during three heat periods of July. At the time of writing, major parts of China and the US are still facing heat waves.
Forests help to reduce the frequency and severity of such heatwaves in the long-term by mitigating climate change, but they also reduce heat stress in people in the very short term through shading and evapotranspiration (i.e. heat is absorbed to convert water from liquid to gas in the leaves, thereby reducing ambient temperature). However, little is known about which forests are most efficient at reducing heat stress in humans.
To study this, we installed microclimate stations in forests of France, Belgium, Germany, and Poland, while ensuring we covered very different forest types, from 10-year-old plantations to primary forests. Thermal stress (which also includes cold stress) was measured from the end of summer 2020 until the winter of 2021-2022. Unfortunately, the summer of 2021 was exceptionally cold in these forests! Some very hot conditions were luckily still captured at the local level, though it means that our results would likely be even more pronounced this year.
In this first paper of my Ph.D., forests were found to be very potent thermal stress reducers, especially on hot days. We found that, on average, the temperature you would perceive in a mature forest is 12.1°C lower than in an open field right next to it under strong heat stress conditions. When a person in the open field is subjected to extreme heat stress (perceived temperature > 41°C), the forest cooling effect is even higher, reaching 14.5°C. Even young plantations, with canopies barely 10m high, were 10°C cooler under the same conditions. On average, that means that one is 84.1% less likely to experience dangerous levels of heat under forest canopies!
The substantial difference between young and mature forests clearly suggests that the type of forest matters. Indeed, we found that the forest structure is crucial, and it becomes incrementally more important the hotter it gets. Densely packed trees with thick and tall canopies are best to foster cool conditions. A secondary yet significant role is played by the tree species composing the forest. That is, the cooling capacity is further enhanced by evergreen species with small leaves (e.g. needles) that cast a deep shade. The diversity of the stand, which is actually the focus of Dr. Forest, was not shown to have a direct impact. This doesn’t mean tree diversity is not important. Diverse forest stands have likely grown more through time, and their interlocking canopies efficiently take up the available crown space – thereby thickening the tree layer. Since we measured the potential consequences of tree diversity (i.e. the denser structure), indirect diversity effects are already included in our other results.
In sum, forests are a great option to mitigate risks on heat-prone sites in general, but their cooling effect can still be greatly enhanced through smart forest management that mainly targets the forest structure. Even though heat stress is especially prevalent in cities and not in the rural areas we studied, we are confident that our results can be translated to urban forests because we compared our forests to open fields, while the dark infrastructure of a city will heat up much more in the sun and thus increase the contrast with the urban forest even more. Compared to other cooling strategies such as reflective pavements, white roofs, and large shading structures, forests have the huge added advantage of simultaneously providing many other ecosystem services such as the filtering of air pollution, supporting biodiversity and regulating water quality, and, of course, the many health-related benefits that we study in the Dr. Forest project. This is why the ideal healthy city is a green city with the highest quality greenery that is omnipresent and easily accessible for each and every resident, during a heatwave or not.
Gillerot, L., Landuyt, D., Oh, R., Chow, W., Haluza, D., Ponette, Q., Jactel, H., Bruelheide, H., Jaroszewicz, B., Scherer-Lorenzen, M., De Frenne, P., Muys, B. and Verheyen, K. (2022), Forest structure and composition alleviate human thermal stress. Glob Change Biol. Accepted Author Manuscript. https://doi.org/10.1111/gcb.16419