Johnny Appleseed was a visionary, not for feeding the masses with apple trees, but for creating shady retreats on hot days. Although his main aim was promoting cider production, his legacy of shade is increasingly appreciated today. Over two centuries later, U.S. cities are realizing the importance of Appleseed’s approach as they face rising temperatures and inadequate tree cover, making urban environments more oppressive.
Recent studies highlight how planting more trees can significantly reduce urban temperatures, while also enhancing biodiversity and improving mental health. The first study reveals that tree cover can mitigate half of the urban heat island effect, where cities become significantly hotter than their rural surroundings. The second study compares 65 American cities, showing that neighborhoods with fewer trees experience up to 40 percent more excess heat than those with ample greenery.
Cities like New York, Atlanta, and Los Angeles must focus not only on maintaining their “gray” infrastructure, such as roads and sidewalks, but also on nurturing their living infrastructure. “Heat is already a major public health threat, claiming 350,000 lives annually by some estimates, and it’s worse in cities,” noted Robert McDonald, the Nature Conservancy’s lead scientist for nature-based solutions and Europe, who led the first study. He added that without trees, the urban heat island effect would be double its current intensity.
Expanding tree canopies allows cities to mimic their cooler rural counterparts. Vegetated areas cool down by releasing moisture from leaves and providing shade, unlike concrete, which absorbs heat during the day and releases it at night, preventing the usual evening cooling. This poses a danger to vulnerable populations like the elderly, and is a reason why extreme heat claims more American lives annually than all other extreme weather events combined.
Lower-income neighborhoods, in particular, face greater risks due to their lack of tree canopy compared to wealthier areas. Industrialized zones often consist of concrete expanses that absorb and emit heat. Urban policymakers may have prioritized dense housing over tree planting, unlike suburban areas with ample parks and curbside trees that help to lower temperatures.
The gap in greenery between neighborhoods results in noticeable temperature differences. The second study calculated this “cooling dividend,” finding temperature differences of nearly 4 degrees Fahrenheit between areas with low and high tree cover. Residents in heavily treed areas may experience 20 to 40 percent less excess heat. This phenomenon is consistent across the U.S., according to Steve Whitesell, executive editor at the Healthy Green Spaces Coalition, which conducted the study. “The impact was consistently evident,” he said.
The key lies in planting not just enough trees, but the right kinds. Larger species offer the most shade, but some trees provide superior evaporative cooling. Drought-resistant trees, for example, conserve water more effectively. Communities might prioritize food production with trees that offer both shade and fruit, while native varieties support local wildlife like birds and pollinators.
Climate change further complicates these considerations. Even rural areas, without the urban heat island effect, are seeing native plants migrate north for cooler conditions. Urban plants face even greater heat challenges, and temperatures are expected to rise. Urban arborists are thus selecting species that will not only thrive now but endure future climates. “Using trees as living infrastructure to combat rising temperatures is crucial,” stated Edith de Guzman, a researcher at the University of California, Los Angeles.
However, trees alone are insufficient for saving urban populations. McDonald’s research suggests that even with maximum tree planting, only 20 percent of potential temperature increases from climate change can be offset. Additional strategies, like reflective rooftops, are necessary. This is especially critical in developing nations with rapidly growing cities that lack sufficient tree cover. “Planting more trees helps, but it’s not a standalone solution to climate change,” McDonald emphasized.
Historically, urban areas have faced similar challenges. During the Industrial Revolution, urban dwellers traveled to rural areas for greenery, with London’s abundant green spaces serving as an exception. Paris later drew inspiration from London when it redeveloped in the 1800s, creating large parks. Today, planners are similarly integrating nature into cities, blurring the lines between urban and rural. “We know how to enhance tree cover if we commit to it,” McDonald noted. “But it requires dedication and time.”
