Abstract
Urban trees’ cooling effect would change over time with different extents of warming. We conducted a comparative study of the greater Boston region (BOS) and Baltimore-Washington (BW) metropolitan area, two agglomerations with similar geographic and climatic conditions, as the study area. Cooling rate of urban trees (the changes in land surface temperature (LST) per fraction of tree cover) has been estimated based on the correlation between tree cover and LST on all extreme heat days over five years (2013–2017). We found that (1) the cooling rate of urban trees exhibited a nonlinear correlation with average LST across the two selected metropolitan areas, and the slope of the cooling rate increased rapidly when the average LST increased; (2) the average cooling rate was higher (0.12 °C/%) and more stable in BOS, and lower (0.10 °C/%) while fluctuated in BW; and (3) spatially, for promoting cooling effect, urban trees need to be more dispersed in Boston while more assembled in BW. More importantly, this difference tends to be more pronounced when the background LST increased. We think this different is caused by almost doubled wind speed in Boston than BW, even though they located in the same climate zone. This study further confirmed that not only the total cooling rate of urban trees, but the effects of spatial configuration will also be changed with different extents of outdoor warming in the same city.
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