A single tree along a city street or in a backyard can provide measurable
cooling benefits, according to a new study from American University. The
research shows that “distributed” trees, those that are stand-alone and
scattered throughout urban neighborhoods, can help to reduce evening heat.
The research suggests that planting individual trees can be a strategy to
mitigate urban heat, particularly in areas where land for parks can be
scarce.
“There are plenty of good reasons to plant trees, but our study shows we
shouldn’t underestimate the role that individual trees can play in
mitigating heat in urban areas,” said Michael Alonzo, assistant professor of
environmental science and lead author of the new study. “City planners can
take advantage of the small spaces that abound in urban areas to plant
individual trees.” The study is published in Environmental Research Letters.
While urban parks provide important mid-day cooling for residents and
visitors, the key to cooling from individual trees happens in the evening.
In the new study, which was conducted in Washington, D.C., cooling benefits
from distributed trees were found to occur around 6 or 7 p.m. and after
sunset. The study revealed lower temperatures in neighborhoods where at
least half the area was covered by canopy from distributed trees.
Temperatures were 1.4 degrees Celsius cooler in the evening compared with
areas with few trees. Even in the predawn hour, areas with only modest
distributed canopy cover (about 20 percent of the area) were cooler than
those with no trees, showing that on average, afternoon and evening cooling
effects last well into the night, Alonzo added.
To arrive at the findings, Alonzo and his colleagues examined air
temperature readings. The data was collected over one hot summer day in
2018, across different areas in Washington, D.C. and at multiple times
throughout the day, resulting in more than 70,000 air temperature readings.
In their analysis, Alonzo and his colleagues examined tree canopy over paved
surfaces, over unpaved surfaces, and both patches such as parks, and
distributed trees, such as those one might plant in their back or front
yards.
The new study confirms that planting individual trees should be considered
as part of a strategy to combat rising temperatures in urban areas. In hot
summer months many cities across the United States turn into “heat islands.”
Due to the urban heat island effect, urban areas, with fewer green spaces
and higher amounts of impervious surface, get hotter compared to their rural
surroundings.
In urban areas, people are more likely to live adjacent to distributed trees
rather than parks. In D.C., there are many places to plant individual trees
where canopy will shade paved or unpaved surfaces: on streets with single
family homes, streets with rowhouses, backyard or small park plantings,
Alonzo said. This opens up avenues for increasing the racial and
socioeconomic equity of tree planting, but more effort is required to first
reduce impervious surface cover in the most built-up residential and
commercial districts, Alonzo added. The top five trees along D.C.’s streets
include several species of maples, oaks and elms, all of which provide
plentiful shade.
Climate studies show that urban temperatures are warming at all times of day
including evenings. Yet studying the cooling benefits from individual trees,
as well as their benefits during evening hours, has not been widely
researched, Alonzo said, and this is an area scientists should continue to
explore. More research will be needed in other locations in the United
States and under different weather conditions. Alonzo also plans to conduct
more research and has collected air temperature readings by bicycle around
D.C. during the pandemic.
Though the study was conducted in D.C., Alonzo said the findings are likely
applicable along the East Coast or in other cities with a similar climate.
“Evenings are not quite the respite from heat that we once had,” Alonzo
said. “These distributed trees do help the city cool off in the evening and
that’s important for human health.”
Reference:
Spatial configuration and time of day impacts the magnitude of urban tree
canopy cooling by Michael Alonzo, Matthew E Baker, Yuemeng Gao and Vivek
Shandas, 9 July 2021, Environmental Research Letters.
DOI: 10.1088/1748-9326/ac12f2