San Diego’s urban forest: 4.9 million trees hard at work

Say the word “city” and images associated with tall buildings, traffic jams, of crowds of people, likely come to mind. Forests are likely a bit further down the list of urban descriptors. 

Photo Credit

John Mills, Forest Service

Urban Forest Inventory and Analysis staff taking inventory of San Diego’s trees on the city waterfront.

Cities and forests, however, are intrinsically interwoven. Urban forests provide critical services that directly affect human health and well-being. Trees moderate air and water pollution in neighborhoods, reduce storm runoff, and provide shade. For a city property owner, the presence of trees reduces heating and cooling costs and improves real estate values. Urban green spaces help to foster active living by getting people outside, they also provide important wildlife habitat. 

In short, urban trees contribute to urban residents’ physical, mental, and economic well-being. In San Diego, California, for example, the urban forest reduces the annual energy costs of residential buildings by an estimated $3.5 million each year, according to a recent Forest Service publication. 

Fast facts about San Diego’s trees 

• 4.9 million trees (at least 1-inch in diameter ) are estimated to be growing in the city. 
• Chinese banyan (Ficus microcarpa), arroyo willow (Salix lasiolepis), and Japanese cheesewood (Pittosporum tobira) are the most common species. 
• Trees in San Diego store about 289,000 tons of carbon, valued at $49.3 million. 
• These trees remove about 34,600 tons of carbon from the air per year (or 127,000 tons of carbon dioxide per year), valued at $5.9 million per year. This is roughly equivalent to the annual emissions associated with 24,891 passenger vehicles or energy usage by 13,295 homes, as calculated using the Greenhouse Gas Equivalencies Calculator. 
• The replacement value of San Diego’s trees, as structural assets, is estimated at $4.7 billion.  

Why San Diego? 

The USDA Forest Service has maintained the National Forest Inventory program since 1928 which now includes an Urban Forest Inventory and Analysis (FIA) program. 

“FIA’s goal is to include some of the nation’s largest cities while also ensuring each state has at least one city in the program. We are currently operational in 44 cities,” said Mark Majewsky, a supervisory forester with the Forest Service’s Northern Research Station.  

“Some cities have their own inventories of trees, or estimates of tree and shrub canopy cover, but Urban FIA adds data from privately-owned lands, collects data annually, and uses nationally consistent methods and tools for timely and accurate reporting, critical for urban forest management,” explained Majewsky. 

Effective urban forest monitoring allows for the formulation of data-driven management strategies to maximize the benefits of San Diego’s urban forest while minimizing the cost associated with its maintenance. 

“The City of San Diego’s Urban Forestry Department has been a very engaged partner in this inventory effort,” said Kathryn Baer, a research ecologist with the Forest Service’s Pacific Northwest Research Station and lead author on the recent analysis of San Diego’s urban forest. 

“San Diego’s climate means that most of the trees in its urban forest are planted rather than naturally occurring, so active management is essential for the forest to thrive and grow,” Baer continued.   

“This report offers urban forest managers a snapshot of the city’s urban forest that can be compared to future inventories, discusses the ecosystem services provided by the urban forest as a whole and, by individual species in particular, helps identify the potential challenges facing the urban forest, and offers insight into which tree species may be more or less likely to thrive under those challenges,” Baer explained.   

What’s next? 

The work isn’t done after one round of inventory and data analysis. 

A single snapshot of a forest’s status and health is useful, but even greater value comes from repeated measurements over time that can be analyzed to reveal trends. Urban FIA field crews return to a city every year, completing the next round of measurements every 5 to 10 years. Each consecutive inventory leads to a better understanding of the urban forest landscape. 

This summer, urban field crews on the West Coast are collecting data from urban forests in Portland, Oregon, and in urban areas throughout Washington state.   

Resources 

Curious about your city’s tree census? Find more information about your city’s forest inventory with the My City’s Trees application.

Interested in creating estimates or reports with the most current data for your city? Try this user-friendly tool to create customized summaries of current Urban FIA estimates: Urban Forest Stats.

Learn more about the Urban Forest Inventory and Analysis Program. 

Read the San Diego report: 

Baer, Kathryn C.; Nowak, David J.; Brandeis, Thomas J.; Dooley, Kerry; Hellman, Kurt; Henning, Jason; Herrick, Christina; Hoehn, Robert E., III; Jennings, Katie; Lepine, Lucie, C.; Lister, Tonya W.; Majewsky, Mark; Owen, Suzanne; Sonti, Nancy F.; Schanning, Sjana; Zehnder, Rebekah J. 2024. San Diego’s urban forest, 2017. Resource Bulletin NRS-134. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station. 95 p. https://doi.org/10.2737/NRS-RB-134.