USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Urban Forestry

Urban Ecosystems and Processes

Since 1992 we have provided our customers with reliable scientific evidence that urban forests add real value to communities. Among their many benefits, trees reduce energy costs, intercept air pollutants, store carbon, and reduce stormwater runoff.

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What's New

Demographic Trends in Claremont California's Street Tree Population Reveal Ways to Improve Urban Forest Resiliency
Mature camphor trees along a street in Claremont, California.
Mature camphor trees (Cinnamomum camphora) line a street in Claremont, California. (U.S. Forest Service/Natalie van Doorn)

Urban forests can provide many important ecological functions and economic benefits, but continuous delivery of those services depends on the long-term health and resilience of the population.

The demographic analysis conducted in this study (i.e., quantifying mortality, growth, and replacement rates) can provide insights into current and future vulnerabilities of the population and help focus management efforts.

By sampling individual tree sites in 2000 and 2014, research urban ecologist Natalie van Doorn and research forester Greg McPherson quantified all three demographic components in Claremont, CA to gain a complete perspective of the state of the city’s street tree population and assess drivers of change.

Science in Practice

Street trees provide shade in front of businesses along the road.
California’s city trees producing $8.3 billion in nature’s services, but canopy per capita lowest in U.S.
Trees in California communities are working overtime. According to this study, the state’s 173.2 million urban trees are producing $8.3 billion in nature’s services annually, including removing carbon dioxide from the air equivalent to the amount released by 1.8 million vehicles. While these benefits are considerable, they could be greater if the amount of tree canopy cover per capita wasn’t the lowest in the U.S. With approximately 236 million vacant tree sites, Californian’s have ample opportunity for new tree planting. Results of this study are being used to identify priority areas for planting and tree conservation, including in disadvantaged communities.
17-year old Texas red oak (Quercus buckleyi) in Davis, CA (photo courtesy, USFS PSW).
Studies quantify urban trees’ ability to intercept rainfall across 40 years
In urban settings, a tree’s ability to intercept or slow rainfall reaching the ground can reduce the amount overflowing onto paved surfaces and lost as stormwater runoff. Researchers measured the crown storage capacity of 20 common California urban tree species, and then calculated each species’ potential interception amount at five year intervals for 40 years. The findings reveal that species selection makes a difference, and the difference tends to increase as trees age. Landscape architects and arborists can use this research to select trees to maximize rainfall interception at selected periods after planting.
Two workers in plant a tree near a body of water.
Study evaluates the performance of climate-ready trees
California’s recent drought illustrated that many urban tree species are sensitive to stressors associated with changing climates. In anticipation of warmer, drier climates predicted for California, researchers are introducing new or under-represented tree species into urban forests that are expected to better weather these conditions. The study intends to gradually shift the tree palette within test cities to ones that are more tolerant of heat, drought, salinity and pests. Researchers will be evaluating the performance of promising, but underused, species for the next 20 years. A five-step process has been implemented to test trees in three California climate zones, with regular monitoring updates posted to the project website.
17-year old Texas red oak (Quercus buckleyi) in Davis, CA (photo courtesy, USFS PSW).
Study evaluates the performance of two bioswales on urban runoff management
Bioswales are engineered depressions in the ground – the swale -- designed to capture precipitation in order to reduce runoff, while also trapping and aiding the breakdown of certain pollutants. This study evaluated the effectiveness of a treatment bioswale constructed with an engineered soil mix consisting of 75% local lava rock and 25% loam soil by volume. An identically sized control bioswale consisting of non-disturbed native soil also was studied. When compared to the control, the treatment bioswale absorbed water more quickly, which reduced surface runoff by 99.4%, while also reducing nitrogen, phosphate, and total organic carbon loading by 99.1%, 99.5%, and 99.4%, respectively. After eight years, tree growth was similar across both sites. A bioswale using an engineered soil mix can be a highly effective option in the suite of green infrastructure strategies.
17-year old Texas red oak (Quercus buckleyi) in Davis, CA (photo courtesy, USFS PSW).
Seventeen years after planting, drought tolerant tree species are evaluated
In 1999, seven species of drought-tolerant trees were planted in Sacramento, Modesto and Davis, California. Growth and mortality data were surveyed annually from 2000 to 2009, and in 2013 and 2016. By 2014, all individuals of one species, Parkinsonia hybrid 'Desert Museum,' had died. In a recent article, PSW researchers share their findings on the growth and mortality of the remaining six species, and assess their vulnerability to climate change stressors. The adaptive capacity of each species was scored in three areas: habitat suitability, physiological tolerances, and biological interactions.The researchers also share information about the potential of each species to produce selected ecosystem services, such as cooling urban heat islands, reducing stormwater runoff, improving air quality and storing carbon. This information can assist in selecting trees that will increase the resilience of urban forests.
Cover image psw-gtr-253
Urban Tree Database
City planners and urban foresters have a resource to more precisely select tree species.

General Technical Report 253: Urban Tree Database and Allometric Equations and the corresponding database catalogs projected tree growth tailored to specific geographic regions. The products are a culmination of 14 years of work that analyzed more than 14,000 trees across the United States. Whereas prior growth models typically featured only a few species specific to a given city or region, the newly released database features 171 distinct species across 16 U.S. climate zones. The trees spanned a range of ages with data collected from a consistent set of measurements. Advances in statistical modeling have given the projected growth dimensions a level of accuracy never before seen. Moving beyond calculating a tree’s diameter or age to determine expected growth, the research incorporates 365 sets of tree growth equations. Also, the manual provides species-specific data on foliar biomass that is critical to projecting uptake of air pollutants.

Trees with browns leaves in autumn line both sides of a city street with parked cars and no traffic
The state of California's street trees

Although the number of street trees in California has increased from 5.9 million in 1988 to 9.1 million in 2014, street tree density has declined by 30% as cities added more streets than trees. The total annual benefits provided by street trees are $1 billion and $5.82 in benefits is returned for every $1 spent on tree care. Research Forester Greg McPherson and colleagues describe the structure, function and value of California's street trees. The City of Claremont Municipal Forest Assessment shows the value of annual benefits of street trees in the vibrant college town.

Workers planting trees in the city
Million Trees Los Angeles: carbon sink or source?

Are urban Tree Planting Initiatives (TPIs) likely to be effective means for reaching local carbon dioxide reduction targets? Research forester Greg McPherson and partners determined that a large-scale TPI, Million Trees Los Angeles (now called City Plants), is achieving success in terms of survival, growth and performance.

A photo of a tree in the city
New research evaluating tree species for climate adaptation

Climate change poses challenges for California, where an already parched region is expected to get hotter and, in its southern half, significantly drier. Research forester Greg McPherson and partners are identifying the resilience of different tree species to future climate exposure and threats from pests and disease in the Central Valley.

CO2 graphic
California's climate change policies and programs

Station research and partner California ReLeaf’s advocacy efforts have resulted in $17.8 million in cap-and-trade auction revenues for California Department of Forestry and Fire Protection’s (CAL FIRE) Urban and Community Forestry Program in 2014-15, the largest one-year single-state allocation for urban forestry. Research forester Greg McPherson and partners published an article that traces the history of this effort.

Text:Carbon storage in urban trees. Background: Trees on a city street
A New Approach to Quantifying Carbon

California is at the forefront of efforts in the U.S. to develop policies that address climate change, including the first set of protocols that allow urban forestry projects to participate in the state's cap-and-trade program. In his blog for American Forests, Greg McPherson describes hurdles that make it difficult to "cash-in" by selling carbon credits. The Climate Action Reserve is streamlining their Urban Forest Protocol to make it more attractive by reducing costs and expanding the scope of projects. Read more on the American Forests' blog or visit the American Forests Carbon Offsets and Urban Forests website for more information.

Graphic of city with hot spots
Urban Tree Canopy (UTC) Assessments

A new generation of remote sensing and GIS technologies have spurred UTC assessments for urban forest planning and management. This top-down approach was applied in San Jose, CA, where the council proposes to plant 100,000 trees by 2022. The San Jose Urban Forest Inventory and Assessment found that the annual ecosystem services and property values for the current urban forest provide $239.3 million in benefits. The city contains 2.1 million potential tree planting sites and by estimating the benefits of planting 100,000 trees, it was found that the benefits would increase almost 7% to $255.8 million annually. The city is using the report as a baseline for a proposed study of climate change impacts on the urban forest. This knowledge is especially important in the San Francisco Bay area, where increasing temperatures and fluctuations in precipitation might cause salt intrusion from rising sea levels. The FS study has helped the city of San Jose see the big picture and what they need to do to prepare for the changes to come.

City trees with large buildings in the background
Growing and Governing Green Infrastructure

Urban green infrastructure, including urban forests, is an important strategy for providing public goods and increasing resiliency while reducing ecological footprints and social inequity in metropolitan areas. In a recent paper, Drs. Robert Young and Greg McPherson found that visioning, planning and management of six large-scale tree planting initiatives was largely dominated by the public sector, unlike more transdisciplinary strategies in environmental governance. Many of these initiatives have had little success becoming institutionalized. In a previous paper, they described strategies for sustaining such initiatives. Also, Dr. Young described the role of planning in advancing these initiatives, and identified best practices that can inform future efforts to expand tree planting on a metropolitan scale.

Recent Publications

cover imageWhat is the value of a tree? What environmental services do they provide and at what cost? Our new Trees Pay Us Back brochures answer these and other questions. Produced in partnership with CAL FIRE Urban and Community Forestry, these brochures present information on trees in the 16 U.S. climate zones where research was conducted for our Community Tree Guide series and i-Tree Streets. From the Southern and Northern California Coast to the Northeast, Coastal Plain, and Central Florida regions, click to view and download the PDF for your region.

New approach to quantify and map urban forest carbon. By incorporating age-related differences among census block groups that influence tree species composition and stand structure, this novel approach improves carbon estimates and increases the resolution at which carbon can be mapped across a region.

Special Issue on Water Scarcity and Urban Forest Management. The articles in this special issue of Arboriculture & Urban Forestry offer "lessons learned" from a serious drought in Australia to inform future policy, research, design and management.

Trees Give Roads a Breath of Fresh Air May was Clean Air Month and the role of roadside trees in cleaning the air and helping us feel better was described in a recent blog. A multidisciplinary group of researchers, planners and policymakers from the U.S. Environmental Protection Agency, U.S. Forest Service and other organizations found that strategically planting trees near busy roadways may significantly enhance air quality. Their findings were published recently in the Transportation Research Board magazine.

All Trees Not Equal. Greg McPherson's blog supports the California Urban Forest Council's Invest From the Ground Up campaign by providing guidelines for selecting and locating trees to maximize climate, energy and environmental benefits.

Greenhouse Gas Inventory of an Ornamental Tree Production System. This article reports the results of a study to determine the greenhouse gas (GHG) emissions from nursery production of ornamental trees for urban forestry.

Comparison of Methods for Estimating Carbon Dioxide Storage by Sacramento's Urban Forest. This chapter by Drs. Elena Aguaron and Greg McPherson in Carbon Sequestration in Urban Ecosystems determines and examines variability among CO2 estimation approaches.