Hot take – we need diverse and rich tree species, not just more trees to reduce the urban heat island effect
Wang, X., M. Dallimer, C.E. Scott, W. Shi, J. Gao. (2020). Tree species richness and diversity predicts the magnitude of urban heat island mitigation effects of greenspaces. Science of the Total Environment 770(20):145211
In the heat of the summer, walking under the shade of a nearby tree can almost feel like jumping into a pool, especially if you are in a city and can feel the heat radiating off of your concrete surroundings. On a hot summer day, being in a city can feel almost unbearable. The excessive heat we associate with walking around a city on a hot day is caused by a phenomenon called the urban heat island effect (UHIE). The UHIE is defined as the experience of hotter temperatures in urban areas, as compared to nearby rural areas, due to the lack of vegetation, extensive concrete surfaces, and hub of activity and energy consumption in cities. Unfortunately, the UHIE has become a big concern for urban planners across the world as temperatures continue to rise and the occurrence of heat-related morbidity and mortality in urban areas have become a major public health concern. Urbanization is expected only to increase in coming decades, and with that, so will the intensity of the UHIE. Therefore, researchers across the world are looking for solutions to help keep temperatures down in cities and promote public health as more and more people flock to urban centers.
Vegetation as a solution
One well-documented solution for helping to reduce the intensity of the UHIE is the planting of trees to increase vegetation in cities. Green spaces, such as parks, sports fields, street trees, and nature conservation areas, have multiple benefits to urban populations including improved urban sustainability and livability, improved air quality and stormwater management, and helping to reduce the intensity of the UHIE. Trees and vegetation help to cool the temperature in urban areas in two ways: 1) providing shade to reduce the amount of heat that reaches the surface, and 2) through a process called evapotranspiration that occurs when a plant uses heat to absorb water through their roots and release it as vapor into the air. Both characteristics make vegetation an effective tool for reducing the UHIE in cities.
While there is ample evidence that vegetation helps to alleviate some of the UHIE, there is little research exploring what characteristics of greenspace are important in determining the amount of cooling greenspace can provide. A team of researchers lead by Dr. Xinjun Wang undertook field research to better understand what characteristics of plant community structure improves the cooling effect of green spaces without increasing their size. To do this, the research team looked at the cooling effect of a variety of tree community structures across Changzhou, China. The team compared the temperature drop, using temperature data from Landsat 7 ETM+ and Landsat 8 TIRS, of 15 different greenspaces during each of the 4 seasons. Specifically, the authors examined 156 sample plots with different tree community structures within the 15 greenspaces.
Greenspace characteristics are crucial
To understand the impacts of greenspace characteristics on the ability of the greenspace to cool, the authors utilized land surface temperature satellite data and compared the temperature within the green space and the temperature of the surrounding built area. The authors performed these measurements for 156 plots that were each 400m2. Within each plot, the research team counted the number of trees and recorded their diameter, species, height, crown height and width, and the health of the crown. From these measurements, the authors were able to assign two metrics of tree diversity (or the number of species) and the Shannon-Wiener diversity index (combines species richness and the relative frequency of these species).
Results and moving forward
From their analyses, the authors found that the tree community structure of urban greenspaces indicate the strength of the cooling effect. The authors discovered that the Shannon-Wiener diversity index, tree species richness, and tree canopy coverage were all positively correlated with the cooling effect of the greenspace meaning that the higher value for these three characteristics means more cooling effect can be achieved from that greenspace. However, it is important to note that the strength of the cooling effect changes with the seasons. The results also showed mean crown width to be positively correlated with cooling during the summer and fall. Lastly, greenspace tree density was negatively correlated with cooling in the winter.
The authors hope that results from this paper can help to inform urban planners and city officials when it comes to the design and species diversity of their greenspaces. Greenspaces are crucial for helping to alleviate some of the public health and environmental stressors of urbanization but ensuring that these resources are used to their full potential is even more critical.