We all know that shade from a tree is a welcome relief from the summer sun. Trees are one solution to the problem of urban heat islands.
A group called Tree San Diego wants to better understand trees, how well they do their job and the challenges to expanding the urban forest.
Veronica Ferman explained the measurements they made in Chula Vista's Memorial Park and elsewhere to understand the cooling effects of trees.
“For us the tree canopy is more the size of the canopy since we’re studying shading,” Ferman said, who is leading the study for Tree San Diego.
They want to understand how trees can reduce high temperatures in urban areas. In cities, it’s not just about the shade trees cast. It’s a matter of how trees interact with the built environment.
“Each material, such as asphalt, concrete have different thermal properties compared to grass and dirt. So when you’re measuring surface temperature under a tree canopy you’re also dealing with what kind of surface material you’re studying,” Ferman said.
Asphalt, they have found, gets the hottest when it’s unshaded. It also cools down the most dramatically when shaded. The goal of Tree San Diego and the city of San Diego is to increase tree canopy coverage to 35%.
The latest estimate shows San Diego is only 13% covered. It's 9% shaded in the area surrounding Chula Vista’s Memorial Park.
Canopy density is a good indicator of the wealth of a community. Treeequityscore.org rates the quality of tree canopies on a scale from 1 to 100. In San Diego County, National City is at 63. Encinitas scores 89.
“These are things we can see from space. We can measure them from space. The urban areas we can see are much hotter than the corresponding areas around them that are not heavily built up,” said Morgan Levy, who studies urban heat islands at the “Heat Hub” at UC San Diego.
Levy pointed out that shade wasn’t a tree’s only cooling tool. Evapotranspiration, the production of water vapor by plants, also can work to reduce urban heat.
Levy sometimes partners with Tree San Diego. She said it’s tempting to say “just plant trees” but those trees need to be watered and maintained.
“Water infrastructure is key to our capacity to go around planting trees everywhere. In particular planting trees in some of the more heavily built up urban environments that house some of the region’s more disadvantaged populations,” Levy said.
Organizations have to plan for the real costs of planting a tree and keeping it healthy.
“We always get asked, 'How much does it cost to plant a tree?' And it’s way more than people think,” said Elektra Fike-Data, the executive director of Tree San Diego. “It’s labor. It’s mulch. It’s delivery of the tree itself. It’s long-term care. Most of our grants require a two-to-three year maintenance plan.”
Ferman said that’s why Tree San Diego is always looking for partners, like school districts.
“In this area of Chula Vista, many schools need trees. They are the perfect place to plant trees because they usually have someone taking care of the trees already in the school” Ferman said.
And it’s another step toward creating a healthy urban forest.
