Green Rx

Trees serve a variety of functions that improve health in our communities.

Urban Forests for Clean Air


In 2006, the Sacramento Tree Foundation and the Sacramento Metropolitan Air Quality Management District (SMAQMD) entered into a partnership to evaluate the effects of the urban forest on regional air quality, with the goal of including a voluntary control strategy based on trees in the Sacramento Regional 8-Hour Ozone Attainment and Reasonable Further Progress Plan (the Plan). Once approved, the Plan will become an update to the California State Implementation Plan for Air Quality.


The Tree Foundation’s role includes project management, liaison to the jurisdictions, and community education. The scientific partners collaborating on this project are the USDA Forest Service Urban Ecosystems and Social Dynamics Program, UC Davis, and Altostratus, Inc., an atmospheric research and modeling company. Other state and federal agencies involved with this project are the California Air Resources Board and the Environmental Protection Agency.

Preliminary Modeling

Through preliminary modeling based on the current regional tree population and species data as well as projected potential changes in the tree population, it has been shown that a small, gradual change in the species mix of the tree population can help reduce ground-level ozone. Ground-level ozone is a strong irritant that can cause constriction of the airways, forcing the respiratory system to work harder in order to provide oxygen. It can also cause other health problems. Changing species mix can reduce biogenic emissions of trees, which are precursors to ozone formation.

The gradual change in species mix pursued by this control strategy will be predicated on the urban forest philosophy of “the right tree in the right place”. The Plan, which included the Urban Forests for Clean Air control strategy, was approved by the five local air quality management districts in early 2009.

Species Shift

Currently, there are 6.9 million trees in the urban areas of the Sacramento region. To maintain a stable population by replacing each tree that dies, approximately 3 million trees will be planted over the 9 years of the control strategy.

Through community education and planting programs with jurisdictions and nonprofits, this control strategy calls for a minimum of 86,000 lower emitting trees to be planted over 9 years that would otherwise have been high biogenic emitting trees. Overall, this represents a shift of approximately 1.3% in the total urban tree population.

Meteorological and Photochemical Modeling

More detailed modeling was recently completed. Satellite images of the study area were analyzed to establish land use, permeable and impermeable surfaces, vegetation, and tree canopy cover. These data were entered into sophisticated meteorology and photochemical models, which established a baseline for the potential impact of the urban forest on air quality. Once the baseline was determined, various tree species scenarios were run to determine the impact on air quality. The analysis shows that the effects of the control strategy are relatively smaller than those from more traditional emission control strategies, but still significant.