Different explanations for same result

Text 1 Urban ecologist Jon Bell compared pavement beneath tree canopies with pavement beneath fabric structures that cast equally dense shade. At midday, the structures lowered pavement temperature by 5 degrees Celsius, while the canopies lowered it by 6 degrees. Because the reductions were similar, Jon Bell concludes that shade accounts for most of trees’ daytime surface-cooling effect. Text 2 Urban ecologist Mei Tan agrees that tree-lined streets are generally cooler. However, Mei Tan compared equally leafy, well-watered and drought-stressed trees: air temperature fell by 4 degrees near the watered trees but by only 1 degree near the stressed trees. Mei Tan argues that water released by leaves also contributes substantially and that shadow comparisons alone cannot isolate every cooling mechanism.