Heat-Tolerant Urban Trees Assure Cooler Hot Sites During Heatwaves

Summary: Heat-tolerant urban trees in a Virginia Cooperative Extension publication alleviate hot sites analyzed by a Macquarie University researcher in Australia.

Daytime thermal imaging of land surface temperature, obtained by aircraft (lower left insert) Feb. 9, 2017, during peak of three-day heat wave, reveals cooler areas (right; blue shades) in Walkley Heights, Adelaide, neighborhoods with more tree cover and hotter areas (left; red shades) in a residential development built with smaller yards and less canopy cover in 2003; AdaptWest and Airborne Research Australia: 木lessandro Ossola @lessandrOssola, via Twitter March 10, 2020

Published research, Urban Trees and People's Yards Mitigate Extreme Heat in Western Adelaide, by Alessandro Ossola of Macquarie University, Sydney, Australia, in March 2020, associates private grassy, woody gardens with ameliorated heatwaves.

A five-year-old, prescient publication, Trees for Hot Sites, by Virginia Cooperative Extension Specialist Bonnie Appleton broaches backyard heat-tolerant urban trees that battle globally warmed climate change. High temperatures cause heat-intolerant and heat-tolerant trees to cool down by releasing as much as 100 gallons (377.54 liters) of water vapor out through leaf-surface pores. Organic-matter compost and 2 to 4-inch (5.08 to 10.16-centimeter) layered mulch develop air and water spaces that defend against evaporation, reflected heat and light, and runoff.

Heat-tolerant urban trees endure in such hot sites as built-up environments, containers and raised beds, and open fields and parks even without hot-site maintenance and mitigation.

Heat-tolerant urban trees flourish on hot sites near asphalt and concrete parking lots and roads; concrete buildings and pavements; and underground utilities such as steam lines.

Heat-tolerant urban trees gather among Callery pear (Pyrus calleryana), cherry plum (Prunus cerasifera) and Freeman (Acer x freemanii), Norway (A. platanoides) and sugar (A. saccharum) maples. They herd together maidenhair (Ginkgo biloba) trees in their male-flowering manifestations; and red horse-chestnut (Aesculus x carnea), small-leaved linden (Tilia cordata) and sweetgum (Liquidambar styraciflua) trees. Evergreen inclusions integrate Chinese (Juniperus chinensis) and Rocky Mountain (J. scopulorum) juniper and eastern red (J. virginiana), eastern white (Thuja occidentalis) and Japanese (Cryptomeria japonica) cedars.

Heat-tolerant urban trees juxtapose adaptable shagbark hickory (Carya ovata), drought-intolerant river birch (Betula nigra) and drought-tolerant cockspur hawthorn (Crataegus crus-galli) and Leyland cypress (x Cupressocyparis leylandii).

Heat-tolerant urban trees knit together drought-tolerant English oak (Quercus robur), hedge maple (Acer campestre), London plane (Platanus x acerifolia) and silver linden (Tilia tomentosa).

Heat-tolerant urban trees line up among drought-tolerant white ash (Fraxinus americana) and pest-tolerant Buddhist pine (Podocarpus macrophyllus), Chinese pistache (Pistacia chinensis) and trident maple (Acer buergerianum). They materialize among poor soil-tolerant honey locust (Gleditsia triacanthos), Japanese elm (Zelkova serrata) and Oriental cedar (Thuja orientalis) and poor soil-tolerant, urban-tolerant lacebark elm (Ulmus parvifolia). They number among sand-tolerant wax myrtle (Myrica cerifera) trees and summer-tough chaste (Vitex agnus-castus), crape myrtle (Lagerstroemia) and goldenrain (Koelreuteria paniculata) trees that nurture hot-weather blooms.

Heat-tolerant urban trees occur as thorny, tough-tolerant Washington hawthorn (Crataegus phaenopyrum) and urban-tolerant hardy rubber (Eucommia ulmoides), pagoda (Sophora japonica) and red maple (Acer rubrum) trees.

Heat-tolerant urban trees pull together urban-tolerant red (Quercus rubra) and willow (Q. phellos) oak trees and wind-tolerant common hackberry (Celtis occidentalis) and green ash (Fraxinus pennsylvanica).

Heat-tolerant urban trees queue up, for hot sites, borer-resistant Japanese white and Siberian silver birch (Betula platyphylla) trees and borer-intolerant Yoshino cherry (Prunus x yedoensis) trees. They reunite coast-loving, inland-intolerant live (Quercus virginiana) and sand laurel (Q. hemisphaerica) oaks; drought-intolerant river birch (Betula nigra); and spider mite-intolerant Colorado blue spruce (Picea pungens). Pre-existing black locust (Robinia pseudoacacia), box elder (Acer negundo ) and tree-of-heaven (Ailanthus altissima) serve as heat-tolerant urban trees unless pest-intolerant invasiveness surpasses heat-tolerant site takeovers.

The Virginia Cooperative Extension-tallied heat-tolerant urban trees annually tackle 14 to 120-day-long temperatures above 80 degrees Fahrenheit (26.66 degrees Celsius), threshold for the Adelaide 2017 heatwave.

The American Horticultural Society's Plant Heat-Zone Map presents 12 heat zones, ranging from Zone 1, with less than one heat day annually, to Zone 12, with more than 210 heat days per year: UF/IFAS Miami U.C.U. @MiamiUCU, via Twitter Jul 2, 2015

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Daytime thermal imaging of land surface temperature, obtained by aircraft (lower left insert) Feb. 9, 2017, during peak of three-day heat wave, reveals cooler areas (right; blue shades) in Walkley Heights, Adelaide, neighborhoods with more tree cover and hotter areas (left; red shades) in a residential development built with smaller yards and less canopy cover in 2003; AdaptWest and Airborne Research Australia: 木lessandro Ossola @lessandrOssola, via Twitter March 10, 2020, @ https://twitter.com/lessandrOssola/status/1237480493441900544

The American Horticultural Society's Plant Heat-Zone Map presents 12 heat zones, ranging from Zone 1, with less than one heat day annually, to Zone 12, with more than 210 heat days per year: UF/IFAS Miami U.C.U. @MiamiUCU, via Twitter Jul 2, 2015, @ https://twitter.com/MiamiUCU/status/616595289474998277

For further information:

"Adapting the Western Region of Adelaide for Climate Change." AdaptWest. Copyrighted 2020.
Available @ https://www.adaptwest.com.au/

"Adapting to Heat." United States Environmental Protection Agency > Heat Islands > Reduce the Risks > Cooling Strategies. Last updated 21 November 2019.
Available @ https://www.epa.gov/heat-islands/adapting-heat

木lessandro Ossola @lessandrOssola. 10 March 2020. "Backyard #trees can cool #cities up to 6 degree C off during an extreme 40 degree C summer #heatwave. Here our new article in The Conversation https://bit.ly/39FX1pb and final report https://bit.ly/33bUGQf." Twitter.
Available @ https://twitter.com/lessandrOssola/status/1237480493441900544

American Horticultural Society. "American Horticultural Society Plant Heat-Zone Map." Copyrighted 1997.
Available @ http://solanomg.ucanr.edu/files/245158.pdf

The American Horticultural Society @americanhorticulturalsociety. 11 January 2016. "New article about the AHS Heat Zone Map at the Hobby Farms website! Get even more info (and order a map) here http://www.ahs.org/gardening-r…/gardening-maps/heat-zone-map." Facebook.
Available @ https://www.facebook.com/americanhorticulturalsociety/posts/1129758883703930

Anderson, Meg. 4 September 2019. "Trees Are Key to Fighting Urban Heat - But Cities Keep Losing Them." National Public Radio > All Things Considered > Special Series > Heat and Health in American Cities.
Available @ https://www.npr.org/2019/09/04/755349748/trees-are-key-to-fighting-urban-heat-but-cities-keep-losing-them

Appleton, Bonnie; Eva Lynn Trump Rudiger; Roger Harris; Kathy Sevebeck; Dawn Alleman; and Lynnette Swanson. 2015. "Trees for Hot Sites." Virginia Polytechnic Institute and State University > Virginia Cooperative Extension > Publications > Publication 430-024.
Available @ https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-024/430-024_pdf.pdf

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International Society of Arboriculture @InternationalSocietyofArboriculture. 11 March 2020. "The Centre for Smart Green Cities measured how much trees and other home vegetation reduced day and night heat in residential neighborhoods. The density of greenery in household yards kept land surface temperatures up to 6 degrees Celsius cooler vs non-vegetated areas." Facebook.
Available @ https://www.facebook.com/InternationalSocietyofArboriculture/posts/10158267601122557

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Available @ https://www.nature.org/content/dam/tnc/nature/en/documents/20160825_PHA_Report_Final.pdf

Ossola, A.; L. Staas; & M. Leishman. 2020. "Urban Trees and People's Yards Mitigate Extreme Heat in Western Adelaide: Final Summary Report. North Ryde, Sydney, New South Wales, Australia: Macquarie University. https://doi.org/10.25949/5df2ef1637124
Available @ https://researchers.mq.edu.au/en/publications/urban-trees-and-peoples-yards-mitigate-extreme-heat-in-western-ad

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Available @ https://www.sciencedirect.com/science/article/abs/pii/S0034425719301154