Effects of the spatial configuration of trees on urban heat mitigation: A comparative study

Urban greenspace has significant cooling effects on urban heat. Recent studies investigating the effects of spatial configuration of greenspace show significant, but inconsistent results, including both positive and negative effects. To investigate the causes of this inconsistency, we compared Baltimore, MD and Sacramento, CA, USA, two cities with very different climatic conditions. We quantified and compared the relationships between the spatial configuration of trees and land surface temperature (LST) using different statistical approaches, and conducted the analyses using spatial units of different sizes, based on trees mapped from 1 m high resolution imagery. We found: (1) trees' cooling efficiency was higher in Baltimore than in hotter and drier Sacramento. Additionally, percent cover of trees was more important than their spatial configuration in predicting LST in Baltimore, but the opposite was found in Sacramento. (2) Spatial configuration of trees affects LST more in Sacramento than in Baltimore, and the effects of spatial configuration of trees on LST varied greatly in terms of magnitude, significance, and even direction, between the two cities. Notably, mean patch size had significantly positive effects on LST in Baltimore, but negative effects in Sacramento. In contrast, edge density had negative effects on LST in Baltimore, but positive effects in Sacramento. (3) Different statistical approaches resulted in dramatic changes in the relationships between LST and configuration metrics. Our results underscore the necessity of controlling the effects of percent cover of trees, when quantifying the effects of spatial configuration of trees on LST. (4) Spatial autocorrelation may influence relationships between landscape metrics and LST, particularly when the unit of analysis is relatively small. (5) The relationships between spatial configuration metrics and LST are stronger with an increase of the size of the analytical unit. This study can enhance our understanding of the effects of spatial configuration of greenspace on urban heat island (UHI). It also provides important insights to urban planners and natural resource managers on how to mitigate the impact of urbanization on UHI through urban design and vegetation management.