Concepedia

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

Carly D. Ziter, Eric J. Pedersen, Christopher J. Kucharik, Monica G. Turner

Proceedings of the National Academy of Sciences · 2019 · 787 citations · 38 references

Open access

TL;DR

Urban warming demands detailed knowledge of how land cover cools cities across spatial scales to inform adaptation. The study examined how tree canopy and impervious surface interact to affect summer daytime and nighttime temperatures across multiple spatial scales. Researchers collected 5‑m interval temperature data along 10 transects covering a full range of canopy and impervious cover in a midsized Upper Midwest city using a bicycle‑mounted sensor. Daytime temperatures varied up to 5.7 °C, cooling increased nonlinearly with canopy cover—most pronounced beyond 40 %—and grew stronger at larger scales (60–90 m), while warming rose linearly with impervious cover but was less than the canopy cooling; nighttime temperatures were less affected by canopy, underscoring the need to reduce impervious surfaces to mitigate urban heat.

Abstract

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 (∼7 km length, sampled 3-12 times each) spanning a range of impervious and tree canopy cover (0-100%, each) in a midsized city in the Upper Midwest United States. Variability in daytime air temperature within the urban landscape averaged 3.5 °C (range, 1.1-5.7 °C). 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, 1.2-3.0 °C), 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.