Value of Urban Forests

Urban forests help reduce air pollution in direct and indirect ways.

Emissions of carbon dioxide from power plants and other sources contribute to climate change. Motor vehicles are a primary source of other air pollutants that can threaten human health and environmental quality. Urban forests improve air quality by reducing atmospheric carbon dioxide levels and absorbing air pollutants. Trees can directly sequester carbon dioxide as woody and foliar biomass while they grow.

By shading buildings and streets, trees reduce energy used for heating and air conditioning, thereby lowering emissions from power plants. Properly planted and managed trees can also reduce the level of particulates, ozone and other pollutants in the atmosphere.

A study of urban forests in Modesto, CA, showed that for each $1 invested in urban forest management, $1.89 in benefits is returned to residents. City trees removed 154 tons of air pollutants, increased property values by over $1.5 million, and provided shade to save more than $1 million in energy costs.

This information convinced city officials to increase the tree budget and an electric utility company to invest $20,000 in developing the Modesto Tree Foundation. More examples like these are available in our published research about the costs and benefits of urban forests.

Trees of the same species planted at the same time may have different growth trajectories depending on many factors including climatic conditions.

The benefits provided by an urban tree, such as energy conservation and improved air quality, are dependent on the size of the tree. The total value of the benefits a tree can provide are calculated and then projected over a 40-year period, making it important to know how large a tree is likely to grow.

Growth equations had previously only been developed for a limited set number of species, usually pines and firs on forested lands. To lend the values generated by our benefit-cost analyses greater accuracy and applicability, we developed growth equations for tree species that exist in urban forests across the United States.

Our General Technical Report 253: Urban Tree Database and Allometric Equations and the corresponding database catalogs projected tree growth tailored to specific geographic regions.

Strategically placed urban trees reduce energy costs and atmospheric emissions from power plants.

Healthy urban forests have the ability to cut heating and air conditioning use, resulting in reduced costs and atmospheric emissions from power plants. Tree shade reduces air temperature and the amount of radiant energy absorbed and stored by built surfaces.

Additionally, trees reduce the velocity of wind, slowing the infiltration of outside air. Research shows that properly selected, located, and managed trees can drastically reduce city and residential energy costs and lessen our reliance on new power plants.

Urban trees provide people with a variety of ecosystem services but can also be the source of disservices, such as damage to infrastructure.

Trees need space to grow. With cities struggling to accommodate the needs of increasing populations, tree space is a precious commodity. In California alone, well over $70 million is spent each year to repair damages caused by tree roots to urban infrastructure, such as sidewalks, curbs, sewer lines, driveways, and foundations. In many cases, repairs are deferred due to budget limitations, which results in increased liability risks associated with 'trip and fall' incidents.

In some cities the cost of sidewalk repair is being shifted to residents. This shift impacts residents in older areas, where trees are larger, infrastructure has deteriorated, and tree-sidewalk conflicts are most severe. We study how cities can best alter tree planting and maintenance to minimize damage to the urban infrastructure, while maintaining the health of the urban forest.

Urban forests reduce stormwater runoff, thereby slowing the flow of pollutants into streams, lakes and oceans. As a result, water runoff from storms can be more readily absorbed by soil. In most cases, models of hydrological processes in forests cannot be applied to cities.

Forest stands typically consist of fewer species than found in urban forests and trees in cities rarely occur in large groups of trees like those in large tracts of forested land. For this reason, scientists have developed rainfall interception models for single open-grown trees and for trees in urban watersheds.