Urban forests can be useful both in mitigating climate change and in helping cities adapt to higher temperatures and other impacts of climate change. Urban trees reduce the amount of greenhouse gases in the air by sequestering carbon dioxide and by reducing the amount of energy needed to heat and cool buildings. These roles can be quantified at the scale of individual trees or entire cities (McPherson et al. 2005) and also for states (McPherson and Simpson 2003).
The overall effect of urban trees is to cool the local environment during summer. Trees provide shade that keeps some sunlight from reaching the surface below their canopies. When trees shade buildings, this interception can reduce summer demand for air conditioning, which in many cities is powered by greenhouse-gas-emitting fossil fuels, such as natural gas or coal. Shade around air-conditioning units can reduce energy use by partially pre-cooling air before it enters the building.
Trees also cool the air through evapotranspiration (i.e., the evaporation of water through their leaves). Reduced emissions from shading and evapotranspiration can be substantial, especially in cities with long cooling seasons and many air-conditioned buildings drawing their electricity from coal-powered plants.
For cooling purposes, it’s important to plant trees near buildings and strategically locate them so they shade the building, especially from the setting sun in summer. Planting trees where they do little to provide shade, such as on the north side of buildings, can be counterproductive because they can prevent breezes from getting through.
On the other hand, planting to help shield the building from prevailing winds in winter can help reduce heating costs in some cities. Winter shade, however, can increase the use of fossil fuels for heating, potentially surpassing savings from wind protection. Deciduous trees near buildings allowed more winter warming than evergreens, but they still blocked an estimated 30 percent of incoming sunlight after shedding their leaves (McPherson 1984). This compares to blocking 85 percent of incoming sunlight in summer.
Trees can also help mitigate global warming by sequestering the greenhouse gas carbon dioxide. As trees photosynthesize, they collect carbon dioxide from the air and convert it into carbon-based products, such as sugar and cellulose. Carbon dioxide enters through the stomata on their leaves, combines with water in a chemical reaction powered by sunlight, and transforms these into carbohydrates, releasing oxygen in the process. Much of the carbon-based materials become fixed as wood, although some are respired back as carbon dioxide or are used to make leaves that are eventually shed by the tree.
The decomposition of leaves and branch trimmings must be taken into account when assessing a tree’s carbon sequestration value. Once trees die or are cut down, they begin to decompose, returning some of the stored carbon to the atmosphere. The rate of decomposition differs greatly based on species and the fate of the wood. Wood that is chipped and applied as mulch decomposes relatively quickly, while wood salvaged for use in wood products, such as furniture, can survive intact for 50 years or more before gradually decomposing.
In addition, the maintenance of urban trees can produce greenhouse gas emissions through the use of gasoline and diesel fuels by vehicle fleets, and by gas-powered equipment such as chainsaws, chippers, stump removers, and leaf blowers. Typically, carbon dioxide released due to tree planting, maintenance, and other related activities amounts to about 2 to 5 percent of annual reductions obtained through carbon sequestration and reduced power plant emissions.
Urban forests can help cities adapt to rising temperatures and other climate change effects, including an increase in heat waves. In addition, an ongoing research effort at the U.S. Forest Service Pacific Southwest Research Station suggests planting trees can help mitigate carbon dioxide levels.
Preparers: Greg McPherson, Jim Simpson, Dan Marconett, Paula Peper, Elena Aguaron, Center for Urban Forest Research, Pacific Southwest Research Station.
See http://www.fs.fed.us/ccrc/topics/urban-forests/ for a variety of resources on this topic and other related topics.
Adapted by: Melanie Lenart, University of Arizona
McPherson, G., 1984. Solar control planting design. Pages 141-164 in E.G. McPherson, ed., Energy-conserving site design. Washington, DC: American Society of Landscape Architects.
McPherson, G., and J.R. Simpson, 2003. Potential energy savings in buildings by an urban tree planting programme in California. Urban Forestry and Urban Greening 2: 73-86.
McPherson, G., J.R. Simpson, P.J. Peper, S.E. Maco, and Q. Xiao, 2005. Municipal forest benefits and costs in five U.S. cities. Journal of Forestry 103: 411-416.