Measuring the Urban Forest: Benefits and Methods

With fall finally underway, our urban forest is heading into leaf season. And with increased interest in green infrastructure has come increased interest in measuring the benefits of the urban forest – defined as “all publicly and privately owned trees within an urban area” [1]  by the U.S. Department of Agriculture’s Forest Service. The urban forest provides numerous ecosystem services, ranging from decreased stormwater runoff, flooding, urban heat island conditions, and energy use to increased habitat and improved air quality[2].

So where do measurement and data come into this?

Part of it is fiscal. Many of the ecosystem benefits provided by the urban forest are among the goals of implementing green infrastructure – or “vegetation, soils, and other elements and practices [used] to restore some of the natural processes required to manage water and create healthier urban environments,” [3] as defined by the U.S. EPA. However, traditional stormwater infrastructure – the pipes, drains and culverts that make up much of how municipalities currently handle runoff – is a known quantity, and deviating from longstanding practice can be a hard, and risky-sounding, proposition to sell. Being able to measure the urban forest and quantify the monetary value of the ecosystem services it provides can be a strong argument in favor of green infrastructure techniques and practices.

In addition to the advantages of being able to communicate about the benefits of the urban forest in terms of monetary value, there are also operational advantages to surveying the urban forest. Municipal public works departments care for and maintain trees on public land and in public rights-of-way, and having detailed data on the age, size, and species of these trees can aid in planning and implementing maintenance tasks. Tree species can be an especially important data point with species-specific pests and problems, such as the emerald ash borer, a major concern.

So what kinds of tree data and other resources are available for the Champaign-Urbana area? On its Forestry Section webpage, the City of Champaign offers a percentage breakdown of tree species accounting for more than one percent of the total trees in the City rights-of-way. The City of Urbana, which has held the designation of Tree City USA since 1976[4], has a publicly accessible tree inventory that shows not only trees on public land in Urbana, but their calculated ecosystem benefits, both aggregated and categorized as greenhouse gas benefits, water benefits, energy benefits, air quality benefits, or property benefits. The University of Illinois was named a Tree Campus USA in 2015[5], and has published the plan 2015 Illinois Tree Campus: A Tree Care Plan for the University of Illinois at Urbana-Champaign. The University of Illinois Extension also offers numerous tree-related resources on its website.

Like any data resource, tree inventories don’t just grow on trees. (Sorry, not sorry.) As we noted in our July 1 post, all data collection and publication processes require staff time and (let’s all say it together, now) funding. So as useful as full tree inventories are (and we applaud the City of Urbana’s), there can be operational barriers to developing one. If you’re trying to quantify or assign a monetary value to ecosystem benefits, without a publicly accessible, comprehensive tree inventory to work from, we’ve got a couple suggestions on how to get started.

Use of a tree value calculator is the easiest way to go, and there are a few options available online that are free to use. One option is the i-Tree suite of applications, which includes a variety of tools appropriate for different scales, levels of detail, and topics. The i-Tree suite is developed and maintained by the U.S. Forest Service, Davey Tree Expert Co., the Arbor Day Foundation, the Society of Municipal Arborists, the International Society of Arboriculture, and Casey Trees. Another option is the Tree Benefit Calculator, also developed by Casey Trees and Davey Tree Expert Co., which is based on i-Tree data and provides data on the benefits of individual trees.

Once you choose a tool, the next step is to get your input data. The i-Tree tools allow the user to identify their area of interest on a map in the application, so minimal, if any, data collection on the user’s part is needed. The Tree Benefit Calculator requires the user’s zip code of interest, then the species and diameter of the tree of interest. If you’re only looking for the value of the trees in a small area that you have access to (the trees on your property, for example, or in the immediate area of your academic building), doing a manual survey of trees in the area by species and performing the basic diameter measurement for each one can be a viable option. If you’d only like to know the benefit you would get by planting one more tree in your yard, that’s easy – simply input one tree of the type or types you’re considering, and there you have it.

But if your area of study is larger (like your neighborhood or your entire zip code) and manual data collection is out of the question, it might be time to call your municipality’s public works department. They may be willing and able to share whatever data they have on trees on public land – as far as trees on private land go, you’re likely out of luck unless it’s private land that you happen to own.

The urban forest is a complex system, but methods of collecting data and learning about it are becoming more common and accessible. So enjoy using the available data resources, and then go out and enjoy the urban forest.

[1] David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, Sustaining America’s Urban Trees and Forests (United States Department of Agriculture Forest Service: General Technical Report NRS-62), 2010. PDF. http://www.fs.fed.us/openspace/fote/reports/nrs-62_sustaining_americas_urban.pdf

[2] The Value of Green Infrastructure (Center for Neighborhood Technology, American Rivers), 2010. PDF. http://www.cnt.org/sites/default/files/publications/CNT_Value-of-Green-Infrastructure.pdf

[3] “What is Green Infrastructure?”, US EPA, last modified September 23, 2016, https://www.epa.gov/green-infrastructure/what-green-infrastructure.

[4] “Urbana’s Tree City USA Designation,” City of Urbana, created November 4, 2009, http://www.urbanaillinois.us/visitors/tree-city-usa.

[5] “Tree Campus USA,” University of Illinois at Urbana-Champaign Facilities & Services, accessed September 26, 2016, http://fs.illinois.edu/services/more-services/grounds/tree-campus-usa.

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