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Valuing an Urban Forest: Clemson University Campus Forest Ecosystem Services Case Study

Urban forests are often overlooked as they are “hidden in plain sight.” We provide a broad overview of the urban forest, detailing its functional and structural values and ecosystem services. Urban forests and ecosystem services can be vague concepts. Using the Clemson University campus as an example, we give clear descriptions of both terms and the benefits produced from urban forests in terms of ecosystem services and provide a dollars and cents valuation of the benefits. Quantitative information on these benefits helps land-use planners, local governments, and decision-makers see the contributions often overlooked in urban forests.

Introduction

A simple definition of an urban forest is the trees or forest just outside our front doors. It is also an urban ecosystem, a complex human-environment system, that requires understanding to ensure sustainable urban planning. Urban forests come in various shapes and sizes, including urban parks, street trees, greenways, river corridors, gardens, and wetlands.1 They are interconnected networks of open, green spaces and green infrastructure that provide multiple ecosystem services in urban environments.2 Green infrastructure, with urban trees as an integral component, can serve as a natural solution to drainage, heat, air, and water quality issues common in urban landscapes.3

Escalating urbanization has led to increasing demands for the ecosystem benefits provided by urban forests. Four-fifths of Americans and two-thirds of South Carolinians live in urban areas (on approximately 6% of the land area).4 National and state urbanization trends make urban trees even more important to sustaining and improving the quality of life in these urban areas.

Understanding the function and value of urban forests can promote management decisions that improve human health, environmental quality, and even local economies by increasing property values and aesthetics in urban communities.5 Urban trees provide physical, mental, emotional, and psychological benefits to urban residents and add to the beauty and aesthetics of their local environment. Recent studies have quantified the benefits of urban trees on wildlife diversity, human health, reduced crime rate, and a myriad of other social issues.6

Urban trees also help control stormwater, reduce air pollution and energy costs, and offset carbon dioxide emissions.7 Urban vegetation provides direct and indirect benefits including heat regulation, air pollution reduction, and air quality improvement. Trees cool air through evapotranspiration, provide shade, reduce energy use in warm months, and allow warmth from sunlight in winter while protecting from the wind. Leaves also filter air pollution particles from the air.8 Another benefit is carbon storage, which has attracted significant attention due to climate change.9

Urban forests also have a structural value – the cost of having to replace a tree with a similar tree.10 Nevertheless, it is also important to recognize that the urban environment is a dynamic and complex landscape, where socio-ecological processes interact to provide multiple ecosystem services at multiple scales. The structure and composition of urban forests will influence the supply of those valuable ecosystem services.11

Valuation Model

The i-Tree Eco Model12 merges tree inventory data with local air pollution and weather data, permitting valuation of the structural and functional benefits of urban trees. Structural benefits represent the cost to replace a tree with one of similar value (considering factors like tree species, diameter, condition, and location) and correspond to replacement value, serving as an indication of forest health. The structural value of urban forests depends upon the extent of canopy cover over an urban area. Functional benefits represent the contribution to ecosystem functions, such as air pollution removal, climate management, carbon sequestration, avoided runoff, and energy savings.13

Valuing Clemson University’s Urban Forest

We used the Eco 6 version of the i-Tree Eco Model12 to estimate the value of Clemson University’s urban forest. The annual functional benefits of Clemson University’s urban trees include sequestered carbon ($13,200/yr), avoided runoff ($6,630/yr), and pollution removal ($1,590/ yr). The structural value of the urban trees is $13.2 million. This estimate represents the overall compensatory value, based on the Council of Tree and Landscape Appraisers method. The structural and functional value estimates for Clemson University were similar to values obtained on other campus areas around the country using the same model (Auburn University in Alabama,14 University of Pennsylvania,15 and Ohio State University16). Both the annual functional and structural values tend to increase with the increase in the number and size of healthy trees.17 Clemson University campus trees provide multiple ecosystem services.

Map of trees on Clemson University campus

Figure 1. Distribution of trees across the Clemson University campus (N=6,361 trees). Image Source: Clemson University Landscape Services.

Air Pollution Removal

The urban forest helped improve air quality by reducing air temperature, directly removing pollutants from the air, and reducing energy consumption in buildings, consequently reducing air pollutant emissions from the power sources. It is estimated that Clemson campus trees remove 1.459 tons of air pollution [ozone (O3), carbon monoxide (CO), nitrogen oxide (NO2), particulate matter less than 2.5 microns (PM2.5), and sulfur dioxide (SO2)] per year with an associated value of $1,590.

Carbon Storage

Trees reduce the amount of carbon in the atmosphere by sequestering carbon in new growth every year. The amount of carbon annually sequestered increases with the size, age, and health of the trees. Clemson University campus trees sequester about 77.44 tons of carbon per year with an associated value of $13,200.

Oxygen Production

Oxygen production is one of the most commonly cited benefits of urban trees. Clemson University campus trees are estimated to produce 206.5 tons of oxygen per year. However, this tree benefit is somewhat insignificant because of the large and relatively stable amount of oxygen in the atmosphere and extensive production by aquatic systems. Our atmosphere has an enormous reserve of oxygen for free to use, so we estimated no dollar value for this benefit.

Avoided Runoff

Surface runoff can cause concern in many urban areas as it can contribute pollution to streams, wetlands, rivers, lakes, and oceans. Clemson University campus trees and shrubs can help reduce runoff by an estimated 99.2 thousand cubic feet per year with an associated value of $6,600.

Summary

The Clemson University campus urban forest provides valuable ecosystem services, such as air pollution removal, carbon storage, oxygen production, and runoff prevention. Species distribution and size of trees are significant factors controlling the present and future supply of these ecosystem services. Considerable attention in selecting forest establishment, protection, and species management is needed to meet the current and future demand of ecosystem services. The demand for these ecosystem services is increasing in urban population centers. Yet development strategies often encompass the use of natural spaces rather than renovation of existing (underutilized) spaces as new development is less expensive than redevelopment. Urban forests need management and protection strategies to meet the challenge this presents. Quantification and valuation of ecosystem services are one tool urban forest managers can use to protect mature, high-value trees during development planning. Other benefits of urban forests, including improved health, emotional well-being, and energy savings, should also be included in any urban forest valuation.

References Cited

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  2. McPherson EG. A benefit-cost analysis of ten street tree species in Modesto, California, U.S. Journal of Arboriculture. 2003 Jan;29(1):1–8.
  3. Nowak DJ, Crane DE, Dwyer JF. Compensatory value of urban trees in the United States. Journal of Arboriculture. 2002 July; 28(4):104–199.
  4. Iowa Community Indicators Program. Urban percentage of the population for states, historical. Ames (IA): Iowa State University. 2020 [accessed 2020 Sep 4]. https://www.icip.iastate.edu/tables/population/urban-pct-states.
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  10. Maco SE, McPherson EG. A practical approach to assessing structure, function, and value of street tree populations in small communities. Journal of Arboriculture. 2003 Mar;29(2):84–97.
  11. Escobedo FJ, Kroeger, T, Wagner JE. Urban forests and pollution mitigation: Analyzing ecosystem services and disservices. Environmental Pollution. 2011 Sep;159(8-9):2078–2087.
  12. i-Tree. Tools. Washington (DC): USDA Forest Service, Climate Change Resource Center. 2020 [accessed 2020 Sep 4]. https://www.itreetools.org/tools
  13. Maco SE, McPherson EG. A practical approach to assessing structure, function, and value of street tree populations in small communities. Journal of Arboriculture. 2003 Mar;29(2):84–97.
  14. Martin NA, Chappelka AH, Keever GJ, Loewenstein EF. A 100% tree inventory using i-Tree Eco protocol: A case study at Auburn University, Alabama, U.S. Arboriculture & Urban Forestry. 2011 Sep;37(5):207–212.
  15. Bassett C. The environmental benefits of trees on an urban university campus. MES Thesis, University of Pennsylvania. December 2015.
  16. ARTrees. i-Tree ecosystem analysis, Ohio State University trees: Urban forest effects and values. ARTrees, Ohio State University. March 2019.
  17. Geiger J. The large tree argument: The case for large vs. small trees. Western Arborist 2004;30(1):14–15.

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