Urban forests have long been identified as providing essential ecosystem services to human populations. The emerging effects of climate change and variability creates an urgent need to examine the role of urban forests in mitigating heat waves, which all climate models indicate will increase in severity, duration, and frequency. Earlier work on the role of the urban forest relies on coarse scale descriptions (e.g. parks, green/ space, etc.) for reducing urban heat stress. Moreover, since previous studies often describe urban heat islands as isolated, static, monolithic areas of cities, many managers and decision makers emphasize a need to expand into specific areas without considering the hindrances posed by the built environment. We challenged this contention by hypothesizing that diurnal temperature cycles and diverse forest features create variation in places that amplify heat. In this study, we ask two research questions: (1) to what extent does functional diversity in the urban forest help to ameliorate ambient temperatures throughout the day and during a heat wave? And (2) how can alternative physical designs of urban neighborhoods affect ambient temperatures, and concomitantly reduce impacts to vulnerable populations? With the use of higher resolution landscape data for cities (i.e. LiDaR), and the emergence of high performance computing and analysis, we addressed these questions at previously unattainable scales. Following an established protocol, we collected hundreds of thousands of temperature readings through ground-based vehicle traverses in five U.S. cities during an extreme heat event. We used machine learning and spatial statistical techniques to identify those features that amplify urban heat, and develop a complex fluid dynamics model (ENVI-met) to evaluate how alternative physical designs may reduce disproportionate heat impacts on vulnerable populations.
Our findings suggest that four characteristics of the urban forest help to explain over 60% of the variation in ambient temperatures – functional type, biomass, tree height, and presence of ground-level vegetation. In addition, the introduction of trees and other forms of green infrastructure can reduce ambient temperatures in urban heat islands by over 12oC in all of our study areas. These findings corroborate earlier findings about the importance of the urban canopy, and go further identify how the location and distribution of the urban forest can have help to mitigate the imminent impacts from a warming planet. These findings support current greening programs and offer specific guidance for the placement and expansion of the urban forest.