Amanda Sheepherd and Dr. Ryan Jensen, Geography Department
Introduction
It is well known that urban areas are warmer than the surrounding rural areas, with various negative (though sometimes not immediately perceived) consequences for urban residents and the environment. As the world’s human population becomes increasingly urban, methods of mitigating urban heat become more imperative to explore. Though not nearly as abundant as in many natural environments, trees are fairly common in urban areas and can be utilized to offset urban heat. Studying the cooling effects of trees on the surrounding areas can further our understanding of the relationship between trees and temperature. Leaf Area Index (LAI) is a specific biophysical metric of trees that may mitigate urban heat. Our research focused on measuring urban tree LAI and comparing it to thermal characteristics of the given tree to determine the cooling effects of trees on permeable urban surfaces (such as grass or loose dirt).
Methods
Over 200 trees in four urban parks throughout Provo and Orem, Utah, were selected and measured during August 2016. A ceptometer was utilized to measure the LAI of each individual tree. A ceptometer measures LAI by comparing photosynthetically active radiation (PAR) above and below tree canopies. We used a FLIR (Forward Looking Infrared) camera to measure the thermal characteristics of both (1) the tree canopy and (2) the area of cooling beneath the tree in each of the four cardinal directions (resulting in eight thermal images per tree- four of the tree canopy and four of the area of cooling). FLIR cameras measure emitted energy of objects based on Wein’s law, which states that an object’s temperature determines the peak wavelength of energy it will emit. We also used a GPS to take the geographic coordinates of each tree for future tree location reference, and recorded each tree’s species. We analyzed the thermal characteristics of the trees using FLIR Tools, a program designed for analyzing FLIR images. Using the information contained in these images (Figures 1 and 2), we extracted temperature values at thirty random points under the tree, in each one of the four photographs of the areas of cooling beneath each tree. We then transferred these values to an Excel spreadsheet, where averages for each one of the four pictures and the average of those averages was computed. This resulted in one average temperature value for each tree. Each tree’s average temperature was then compared with each tree’s LAI and the correlation between these variables were determined.
Results
We found that tree LAI appears to be inversely correlated with the temperature beneath the canopy. In other words, as LAI increases, temperature beneath the tree decreases. Trees with lower LAI values included species such as honeylocusts, some oaks, and some elm species. These tree types were less effective at cooling, and exhibited higher temperature values (on average) under their canopies. Conversely, trees that averaged higher LAIs, such as certain maple species, pines, and spruces, had lower temperatures under their canopies and therefore seemed to be more effective at cooling in the given conditions.
Discussion
This research has valuable applications for city planners and developers. Results from this research, coupled with results from future research, will allow us to determine which trees are best for cooling the urban environment, and therefore assess which trees will best assist us in mitigating the urban heat island effect. Additionally, it will permit developers and planners to create a more sustainable urban environment, with a smarter layout of future cities and developments. Knowing where to place trees (and which type of tree to plant) can result in increased comfort for urban inhabitants by using trees to help moderate temperatures. Also, the strategic placement specific types of trees could lead to less energy consumption in homes, reducing energy bills and demands on the environment. Results from this research can allow urban residents to move towards working harmoniously with the environment, not against it.
Conclusion
Our research focused on evaluating a specific biophysical variable, LAI, to determine the thermal effects of trees on permeable urban surfaces (such as grass in a city park setting). This, however, is a gateway question to other questions we may explore after this, since there are many variables besides LAI that affect which types of trees are most effective at cooling. Understanding the relationship between these variables and the dynamics associated with them can grant greater insight to how trees can be used to strategically reduce urban heat.