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The Scorching Effect of Heat Islands

By Kristin Withrow posted 05-08-2023 12:12 PM


The amazing cooling power of trees is being explored in the Inland Empire to bring relief.

By Kristin Withrow, CSDA Communications Specialist

Anyone who has felt the difference between standing on a natural surface in the shade of a tree or standing in the blazing sun on an asphalt parking lot can understand the concept of a “heat island.” What you may not realize is the large-scale, lasting effect of these significantly elevated temperatures in areas dominated by both impervious (impenetrable by water) surfaces including asphalt and concrete, and porous but still heat-capturing surfaces like black mulch and artificial lawns.

In general, unshaded pavement can be more than 60 degrees Fahrenheit hotter than shaded pavement. However, in desert areas already subjected to elevated temperatures, these growing heat islands have been shown to cause extreme increases in temperatures when compared with vegetated areas wholly or partially covered with tree canopy. Research conducted in the Redlands area and the Coachella Valley in the summers of 2021 and 2022 by University of California Agriculture and Natural Resources (UC ANR) Environmental Horticulture Advisor Janet Hartin demonstrated the astounding significance of this range in temperature. Data taken during the late afternoon in late Spring and Summer with an infrared thermometer indicate that unshaded black asphalt, dyed mulches, and artificial lawns attain surface temperatures of up to 160 degrees in Redlands and up to 170 degrees in Palm Springs. Temperatures of living groundcover and asphalt shaded by trees reached a “relatively cool” 95 and 104 degrees in these two cities, respectively. The staggering difference in temperature was not only shocking given proximity of shaded and unshaded testing sites, but concerning due to their contribution to concentrated heat and impact on human health and the surrounding environment.

man with tree in pot

The ramifications for increased heat go beyond personal discomfort. For sensitive populations, including seniors and young children, those who have limited access to air conditioning and people who work outside, adding even a few degrees to the natural heat of summer can increase risk of heat-related illnesses. Elevated heat increases the drain on the energy grid as people lean on their air conditioners for relief, which can result in rolling power outages as utilities attempt to manage overall grid demand.

Inland Empire Resource Conservation District (IERCD) is working with partners to bring some relief to the heat island conditions in their focus area that include large areas of San Bernardino and part of Riverside Counties. IERCD is focused on open space preservation, wildland rehabilitation and education and outreach to the residents within the nearly 1,300 square miles they cover. Heat islands pose a new angle for conservation education – how to include natural materials in urban zones to mitigate the heat bubbles forming over population centers. One solution: Trees.

The district’s interest and success in enhancing tree canopy cover in low-shade, often underserved communities, was spawned through a partnership with UC ANR and Hartin, co-lead on a Climate-Ready Tree project at UC Riverside between UC ANR and the USDA Forest Service. The study is assessing performance of multiple species of under-planted, but promising, climate-resilient shade trees currently growing well in warmer climates than the study site. CalAdapt climate change models were used to predict weather-related changes the trees must withstand over the next several decades due to climate change. The purpose of the project is to identify a range of trees that can thrive in elevated temperatures, drought, and pests and diseases. These conditions will all be heightened unless actions aren’t taken to turn things around in an already hot Inland Empire. Fortunately, all but one of the
twelve tree species is thriving with no irrigation since March 2020.

two rows of trees in planter pots

“The partnership enabled by IERCD with our UC ANR team has resulted in opportunities not otherwise available to us that ensure future generations have cooler living green spaces. IERCD has been a great partner to provide education and outreach to communities that benefit from increased tree canopy for heat island mitigation,” said Hartin. “This is one of the few research projects that has resulted in a quick ‘shovel ready’ turn- around, with over 700 trees already in the ground. The UC Cooperative Extension Master Gardener volunteers in San Bernardino County have helped ensure that education on tree care in English and Spanish is provided to everyone receiving a free tree, which helps trees reach their maximum potential to provide these valuable ecosystem and societal benefits.”

Together, the UC ANR and IERCD teams have worked to transition the results of the Climate-Ready Tree Study into positive community action. Beginning in the Spring of 2021, the “Trees for Tomorrow” program sought community members and municipal partners interested in receiving and pledging to care for one or more climate-ready trees. Installation was completed by Climate Action Corps fellows hosted by IERCD and supported by a state-provided stipend to support their work on elevating local urban forest function and size. Trees planted on community member properties, and in a park in north Redlands, received direct technical assistance from University of California Cooperative Extension (UCCE) Master Gardener volunteers to empower city employees and community members to provide effective long-term tree care.

Historically, tree distribution and planting programs have struggled due to lack of available support for recipients in ensuring effective initial establishment and long-term health of trees. Often, tree care-related errors are entirely preventable as they are connected to problematic planting location, over or under-watering, or drastic or ill-timed pruning, all of which can be remedied with information and resources. Trees for Tomorrow sought to address this deficiency through connecting program participants to Master Gardener mentors, individuals who have completed an 18-week multi-topic training regimen followed by 50 hours of post-training volunteer work. This statewide program, run on a county basis by UC Cooperative Extension, results in well-informed Master Gardener volunteers able to provide free, bilingual community support on a variety of topics including tree planting and care. Master Gardener mentors are available via email, phone, and where allowed, site visit, to troubleshoot issues and provide recommendations. Their presence in the program has been effective in everything from initial species selection to follow up care instructions and tree health issue resolution.

woman with shovel teaching kids how to plant tree

“IERCD has partnered for more than eight years with Hartin and her team of professionals and volunteers working to increase urban tree canopy and community engagement throughout the Inland Empire,” said Parkes. “What started as a partnership on small workshops has turned into a high-functioning multi-partner collaboration connecting community members to information and resources on climate-ready fruit and shade trees. What truly works about Trees for Tomorrow is the addition of highly effective free Master Gardener technical assistance to help each tree recipient provide the most effective care to result in long-term tree health and benefits.”

Implementation of Trees for Tomorrow was done on a smaller scale in its first year and a much larger scale in its second year. In 2021, 71 trees were planted; in the following year, the program was expanded and altered to include both planting and large-scale climate-ready tree giveaways alongside resources and direct assistance from Master Gardeners. In the second year of facilitation, Trees for Tomorrow resulted in 178 trees planted on properties of community-serving organizations in San Bernardino, as well as 536 trees distributed at 16 community events in Redlands, San Bernardino, and Highland.

Increasing the tree population, and, therefore the tree canopy cover in urban areas, also filters pollutants from air and water, reduces water runoff and soil erosion, decreases energy use and related costs, supports and expands habitat, provides wind breaks, and has even been found to improve mental health. “It’s a win-win for everyone resulting in healthier communities and ecosystems," said Hartin.

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