Pacific Southwest Research Station

Demographic Trends in Claremont California's Street Tree Population Reveal Ways to Improve Urban Forest Resiliency

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.

California’s city trees producing $8.3 billion in nature’s services, but canopy per capita lowest in U.S.

Studies quantify urban trees’ ability to intercept rainfall across 40 years

Study evaluates the performance of climate-ready trees

Study evaluates the performance of two bioswales on urban runoff management

Seventeen years after planting, drought tolerant tree species are evaluated

Urban Tree Database

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.

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.

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.

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.

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.

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.

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.

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.