Ed Mulrean, Ph.D., Justin Morrow, and Kevin Salamandra, Arid Zone Trees
Commercially grown, desert-adapted landscape trees became widely available in the late 1970s and early 1980s. Communities throughout the desert southwest embraced these trees as they offered a landscape aesthetic that reflected the natural qualities and unique beauty of the Sonoran Desert. These species held the promise of lower water use and greater adaptation and durability in high temperatures and desert alkaline soils. Since then, mesquites, palo verdes, acacias, and ironwoods have become an integral part of residential, commercial, and municipal landscape tree palettes.
While region-wide concerns about water conservation in the landscape remain critical, trees (and specifically tree shade) are increasingly seen as an essential element in the potential mitigation of urban heat islands. Their heightened significance has led to community tree planting campaigns, municipal development guidelines changes, and public education programs to preserve, expand, and properly maintain the urban forest. Absent from these efforts has been limited empirical data to substantiate how much shade individual desert-adapted trees species generate, or that quantifies the influence this shade has on reducing surrounding surface temperatures.
The Phoenix Metro Freeway system presented a unique opportunity for gathering baseline data on several popular desert-adapted tree species in a xeric, highly uniform landscape environment. Freeway construction and easement landscaping are funded with both federal and state dollars. All the tree species in these setting share the same tree planting practices, site development guidelines, irrigation systems, and surface mulch. Further, the details of landscape construction follow requirements prescribed by the US Department of Transportation, in coordination with the Arizona Department of Transportation. All trees planted within the Phoenix Metro Freeway system share uniform planting practices, were planted in large numbers within months or weeks of each other (large groupings are essentially the same age and stage of maturity) and represent an assortment of widely used desert-adapted tree species. Trees are irrigated similarly, have had limited pruning, maintenance, or fertilization. They are surrounded by the same grade, color, and screen size of decomposed granite through the entire landscaped easement. Collectively they constitute the urban forest equivalent of an agricultural research field plot and are ideal for experimentation.
A total of 11 desert-adapted landscape tree species were included in a study to assess the impact of leaf canopy generated shade on the surface temperature of the soil beneath the trees. Both field and nursery/container grown trees were examined. Temperatures were collected from July to September 2018. In these studies, the soil surface (field soil in the nursery study and decomposed granite on the highway easement) served as surrogates for any non-biological surfaces in the built environment. A detailed discussion of the data collection instruments and methodologies are available in the article listed below (a).
Reducing the effects of the urban heat island, in part, involves reducing the thermal impact of the sun’s radiant energy on the built environment. Driven by the movement of the sun across the sky, tree shade is a dynamic (constantly moving), rather than a static, means of moderating the urban heat island. The studies described here showed that shade from desert-adapted landscape trees significantly lowered soil surface temperatures and suggest that this shade would have similar impacts on other surfaces within the built environment.
The studies showed that substantial soil surface temperature reductions were provided by both container-grown and landscape-planted trees when compared with unshaded surfaces. Surface temperatures were reduced by an average of 21% by container grown, desert-adapted tree shade (Table 1). The response by individual species appeared equivalent to their canopy density and structure. Greater soil surface temperature reductions were seen with field-planted trees, with an average 26% temperature reduction when compared with unshaded soil surfaces (Table 2).
The movement of the shade canopy has both an immediate and residual effect on soil surface temperatures with the center of the shade oftencooling the soil surface to near or slightly below ambient air temperatures.As the leaf shade canopy overtakes exposed soil surfaces there is an immediate reduction in soil surface temperature, while surfaces newly exposed (as the leaf shade canopy migrates to the east) briefly maintains a temperature that is significantly cooler than continuously exposed soil. These studies suggest that maximizing the temperature-reducing potential of desert-adapted tree shade will require an understanding of the extent, rate, and duration of the residual surface temperature cooling over time.
Shade from desert-adapted landscape trees makes a significant contribution to reducing surface temperatures within and around the trees’ leaf shade canopy and helps mitigate urban heat island temperatures. Several questions about desert-adapted trees merit additional investigation: the extent and duration of surface temperature reduction by the leaf shade canopy; the role of canopy and branch geometry, leaf distribution and density in temperature mitigation of the soil surface, how late afternoon temperature differences compare with those observed in the morning, and how leaf shade canopies of desert-adapted tree species compare with those generated by traditional, non-desert landscape trees.
Table 1
Table 2
a) “Quantitative Assessment of Desert-Adapted Tree Shade.” Western Arborist, Summer 2019. 36-41
Acknowledgements: The authors wish to thank the Arizona Department of Agriculture/USDA Specialty Crops Block Grant Program, Enumeral Research and Consulting, LLC, the Arizona Department of Transportation and Arid Zone Trees for material and financial support of this research. Portions of the results shown here have appeared in other publications by the authors.