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Urban Tree Diversity and Global Warming

Cal Poly student in Plant ConservatoryMarlon Gomez-Rubalcava handling Harpephyllum caffrum seedlings grown in the Cal Poly Plant Conservatory as part of the Urban Tree Diversity Project. Gomez-Rubalcava is an environmental management and protection student minoring in ethnic studies and indigenous studies in natural resources and the environment. PHOTO BY JOE JOHNSTON


by nick wilson

Matt Ritter

As global temperatures become increasingly hotter, and communities throughout California ramp up efforts to add shade and sequester carbon, the Cal Poly Plant Conservatory is playing an important role in taking climate action.

"What we need are trees that are going to survive and do well in the climate future of California.

Matt Ritter

Cal Poly Plant Conservatory director and biological sciences professor

Through the Plant Conservatory’s Urban Tree Diversity Project, Cal Poly faculty and students are partnering with arborists and city governments to experiment with growing different types of trees, often unfamiliar in California, that can withstand warmer climate conditions and remain sustainable.

“All the models show that California is getting hotter and drier and that’s harsh on trees,” said Matt Ritter, Plant Conservatory director and biological sciences professor. “So, what we need are trees that are going to survive and do well in the climate future of California.”

Ritter said that there are an estimated 60,000 tree species globally, with an about 1,400 of those found in California.

Unlike many government agencies and private industries, which are far more cautious about trying new trees, the Conservatory’s urban planting project can experiment with those that have potential to thrive in California, Ritter said.

Gage Willey

Plant Conservatory botanists partner with Devil Mountain Wholesale Nursery to grow the trees. The Cal Poly team cultivates seeds into small plants and hands them off to Devil Mountain to grow the trees taller before distribution to buyers.

"We're trying to put trees that perform really well into cities so that in 15 years or 20 years, you’re not going to have a bunch of dead trees on every street.

Gage Willey

Cal Poly Plant Conservatory curator

One tree example is the Chilean soapbark tree, a drought tolerant, non-invasive, evergreen plant.

“It does incredibly well in California, and we’re thinking ‘Why aren’t we growing this here?’” Ritter said. “Let’s put it on the streets of Fresno and it’ll turn into a nice big, beautiful tree where others might suffer and die. The answer is we just don't have very many trees that are native to Chile growing in California.”

The Island Oak, a drought-tolerant tree native to the Channel Islands, has also been successful in California’s coastal areas.

“We have just grown a ton of these and part of the city of San Luis Obispo’s campaign to plant 10,000 new trees includes planting a bunch of island oaks,” Ritter said.

Partnering with the city of Goleta, the Plant Conservatory has replaced a suffering eucalyptus with Australian species more capable of surviving, in an area where endangered Monarch butterflies congregate.

“We’re growing 1,800 different trees with five or six different species in the city of Goleta,” Ritter said. “Students are stoked on it, and we get to support an endangered species.”

Cami Pawlak

And through a Circle 4 grant from Cal Fire, the project is adding trees to 75 disadvantaged communities all over California, including hot valley communities such as Bakersfield and Fresno, involving 50 to 60 trees “of really interesting species,” Ritter said.

There's a big push to plant native trees in cities. But tree diversity is also important.

Cami Pawlak

M.S., Biological Sciences, ’23

“A lot of the trees selected are from those drier regions where they don't have many nutrients in the soil,” said Gage Willey, the Plant Conservatory’s curator. “We're trying to put trees that perform really well into cities so that in 15 years or 20 years, you’re not going to have a bunch of dead trees on every street.”

With climate action planning taking place across the state. The city of San Luis Obispo is seeking to add 10,000 more by 2035; San Francisco envisions 50,000 more trees and Sacramento is targeting 1,000 trees each planting season.

The average tree absorbs an average of 22 pounds of carbon dioxide per year for the first 20 years, according to estimates by the nonprofit One Planted Tree.

To document tree numbers and species, Cal Poly’s Urban Forest Ecosystem Institute has compiled a suite of public online tools focusing on California’s urban forest, which allow residents to analyze species and numbers of trees in individual communities and regions.

“My research has been really focused on California's native trees,” said Cami Pawlak, a recent graduate of the biological sciences master’s degree program. “There's a big push to plant native trees in cities. But tree diversity is also important.”

According to Pawlak, a diverse urban forest is needed to provide adequate flora, but research shows that California towns and cities often grow limited numbers of native trees. “On average, California cities only have seven species of native trees,” Pawlak said.

Pawlak said the benefit of planting trees, native or non-native, includes cooling effects, lower energy bills and positive mental health. “Property values are higher in places with high canopy cover. People spend more money on goods and services in places with high canopy cover.”

Ritter said that the Cal Poly role is multi-faceted, bringing “diversity in tree species to urban forests, while providing opportunities for Cal Poly students.

“Students learn a ton in the process of doing something that people in the world care about, so it's a great college experience,” Ritter said.

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