Urban tree programs have expenses for planting, maintenance, even sidewalk repair. Yet the benefits of urban trees in five western cities analyzed outweighed the costs by ratios of 1.37 to 3.09.
For the analysis, samples of 30 to 70 randomly selected trees from each of the most abundant species were surveyed in five cities: Fort Collins, Colorado; Cheyenne, Wyoming; Bismarck, North Dakota; Berkeley, California; and Glendale, Arizona. All of these cities were surveyed by the U.S. Forest Service during the development of the computer program STRATUM (Street Tree Resource Analysis Tool), which is now available online as iTree Streets.
Costs assessed in the five-city analysis included tree planting and maintenance expenses. The latter included costs of pruning, removing and disposing of damaged trees, infrastructure damage (e.g., broken sidewalks or sewage pipes), inspection costs, litter cleanup, and trip-and-fall damage claims.
Benefits assessed in the five-city analysis included:
- Energy savings based on computer modeling of the effect of shading on buildings’ heating and cooling costs;
- Atmospheric carbon dioxide reduction from both the sequestration of carbon in wood and the reduction in greenhouse gas emissions related to energy savings;
- Air quality improvements based on the pollutants collected on leaves (but not counting the effect of reduced emissions);
- Improvements in aesthetics, as measured by relative increases in property value; and
- Reduced storm water runoff, based on average precipitation amounts.
Both the decomposition of leaves and pruned materials and the respiration of the trees were considered when accounting for carbon sequestration. In addition, the impact on air quality of some species’ emissions of volatile organic compounds was included in the analysis.
A small fraction of the carbon in leaves and wood transforms into soil organic matter. In warm, dry urban areas with irrigated plantings, the density of soil organic matter can exceed that of nearby, undisturbed soils (Pouyat et al. 2006). However, it is difficult to quantify this transfer, so soil carbon changes are not included in these assessments.
As well as providing a cost-benefit analysis, the comparison of urban forest programs in the five cities yields some general concepts that can help guide tree selection and program maintenance.
Adapted from: McPherson, G., J.R. Simpson, P.J. Peper, S.E. Maco, and Q. Xiao, 2005. Municipal forest benefits and costs in five US cities. Journal of Forestry. 103:411-416. Adapted by Melanie Lenart, University of Arizona
Pouyat, R.V., I.D. Yesilonis, and D.J. Nowak, 2006. Carbon storage by urban soils in the United States. Journal of Environmental Quality. 35: 1566-1575.
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