Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer

As cities warm and the need for climate adaptation strategies increases, a more
detailed understanding of the cooling effects of land-cover across a continuum of spatial
scales will be necessary to guide management decisions. We asked how tree canopy cover and
impervious surface cover interact to influence daytime and nighttime summer air temperature,
and how effects vary with the spatial scale at which land-cover data are analyzed (10, 30,
60 and 90-m radii). A bicycle-mounted measurement system was used to sample air temperature
every 5 m along 10 transects (about 7 km length, sampled 3-12 times each) spanning a range
of impervious and tree canopy cover (0 to 100%, each) in a mid-sized city in the Upper
Midwest, USA. Variability in daytime air temperature within the urban landscape averaged 3.5
degreeC (range 1.1 to 5.7 degreeC). Temperature decreased nonlinearly with increasing canopy
cover, with the greatest cooling when canopy cover exceeded 40%. The magnitude of daytime
cooling also increased with spatial scale, and was greatest at the size of a typical city
block (60-90 m). Daytime air temperature increased linearly with increasing impervious
cover, but the magnitude of warming was less than the cooling associated with increased
canopy cover. Variation in nighttime air temperature averaged 2.1C (range 1.2 to 3.0
degreeC), and temperature increased with impervious surface. Effects of canopy were limited
at night; thus, reduction of impervious surfaces remains critical for reducing nighttime
urban heat. Results suggest strategies for managing urban land-cover patterns to enhance
resilience of cities to climate warming.