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Report: Future climate could affect street trees
street trees
A new report says cities in the Central Valley — and all of inland California — should consider climate change when choosing varieties of street trees. - photo by KRISTINA HACKER/The Journal

Eighty-one years from now, Turlock’s climate could resemble more of southeast California’s high desert areas, according to a new report that says inland California municipalities should consider increasing temperatures due to climate change when planting street trees.

Many common street trees now growing in the interior of California are unlikely to persist in the warmer climate expected in 2099, according to research published in the July 2018 issue of the journal Urban Forestry & Urban Greening.

"Urban foresters in inland cities of California should begin reconsidering their palettes of common street trees to prepare for warmer conditions expected in 2099 due to climate change," said the study’s co-author, Igor Lacan, UC Cooperative Extension environmental horticulture advisor in the Bay Area.

"Our research shows that some trees now lining the streets of cities like Fresno, Stockton and Ukiah are likely to perform poorly in 2099," Lacan said. "Those cities need to look at the conditions – and trees – now found in El Centro, Barstow and Fresno respectively."

The City of Turlock recently made changes to its street tree program.

In 2015, following Governor Jerry Brown declaring a drought state of emergency, the City of Turlock established an ad hoc subcommittee to review the City’s street tree ordinances and policies.  As a result of Council’s ad-hoc subcommittee’s review of the City’s street tree policies, the Parks, Recreation and Public Facilities Department requested to adopt a revised Subdivision Street Tree Theme list and map. 

Through the subcommittees review, it was found that many of the species of trees on the original list should no longer be planted for a variety of reasons, including not being drought tolerant, high root systems, susceptibility to mistletoe or other diseases (often caused by drought) and insufficient parkway strip space.  This determination was confirmed by an outside review by a local nursery owner and a landscape architect contracted with the City, said Parks, Recreation and Public Facilities Manager Erik Schulze.

“A few of the more popular drought resistant street trees you will see around town include Keith Davey Chinese Pistache, Red Maple, October Glory, Red Oak and California Sycamore,” said Schulze.

Report authors Lacan and professor Joe McBride of UC Berkeley used space-for-time substitution to come to their conclusions.  They compared the most common street tree species in cities representing each of the 16 California climate zones with trees in cities that currently have climates that approximate the expected warmer conditions in the 16 cities 80 years from now.

For example, Eureka can expect a climate like Berkeley’s today; Fresno’s climate will resemble the climate of El Centro today. The corresponding cities were determined with climate predictions from Cal-Adapt, which synthesizes California climate change scenarios to reach a consensus view of the magnitude of climatic warming.

"We used the mid-range models," Lacan said. "It’s very reasonable to say the warming predicted by the model we used is already ‘baked in,’ regardless of any mitigation efforts. While we should take measures to prevent even greater warming – mostly by reducing the use of fossil fuels – this study aims to help adapt California urban forests to the warming that can be reasonably expected to occur."

Lacan said he and McBride were surprised to find that coastal cities and their warm equivalents contain most of the same common urban tree species, while the warm equivalents of inland cities seemed to lack most and, in some cases, all of the common trees there today.

"It’s really a sharp distinction," Lacan said. "Perhaps they were lucky, but coastal cities are better positioned for the climate of 2099 than the inland cities."