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Economic benefits of the urban forest

i-Tree

The Streets module in the i-Tree software package can be used to estimate the economic benefits provided by street trees. Benefits are calculated in five areas: energy savings, air quality, carbon dioxide removal, stormwater run-off reductions, and property value increase.

To calculate the benefits of the urban forest, users must provide a database that includes species and diameter class for each tree. If a complete inventory is not available, data from a sample survey can be used. Data can be input directly into i-Tree using the inventory software that is part of i-Tree Streets. Users with a pre-existing inventory must format the species and diameter fields as a Microsoft Access™ database. If you have experience manipulating database files, the conversion is relatively easy. Even so, data conversion may take a day or more, depending on the data formats used in the source database.

Calculations in the program are based on one of 16 climate zone-based regional tree guides produced by the Center for Urban Forest Research, UFSF-PSW Research Station. A model city was chosen for each climate zone. Figures from this city are used as default values by the software.

Air quality benefits calculated by i-Tree need to be considered with some background information. Ground-level ozone (i.e., ozone in the lower atmosphere, not the stratspheric ozone layer) is a regulated air pollutant that has adverse impacts on human health. Ozone pollution is formed in still air in the presence of sunlight, nitrous oxides (NOx) and volatile organic compounds (VOC). About 90-95% of NOx are produced by burning fossil fuels, such as coal, gas, and oil.

VOC are carbon-containing gases and vapors such as gasoline fumes and solvents (excluding carbon dioxide, carbon monoxide, methane, and chlorofluorocarbons). The main human sources of VOCs are gasoline combustion and the evaporation of liquid fuels and solvents.

VOC can come from natural or biogenic sources (BVOC) as well as from human activities. The i-Tree program accounts for trees as sources of BVOC. Although trees can produce BVOC, the degree to which BVOC emissions contribute to ground-level ozone production is still unclear. However, the i-Tree program currently treats potential BVOC emissions from trees as being equivalent to those produced by human activities. Consequently, the costs assigned to BVOC emissions by the program may be higher than the calculated air quality benefits provided by trees. This can lead to a calculated result that trees have a net negative air quality benefit.

Field studies and other modeling that accounts for the cooling effects of trees and reduction of the urban heat island effect have generally shown that ozone pollution levels decrease as tree canopy increases (see Piety, 2007). Until better data can be obtained on this complex relationship, the i-Tree air pollution values should be considered with a rather large grain of salt.

The main take home lesson is that less desirable trees that are also high BVOC emitters should be avoided where possible. However, current data do not support discontinuing the use of well-adapted, good quality trees, such as many native oaks, that are high BVOC emitters.

CITYgreen

A GIS extension called CITYgreen is available from American Forests. CITYgreen is an extension of the popular GIS program ArcGIS available from ESRI, Inc. It can only be used on computers with ArcGIS installed. The version of CITYgreen that works with ArcGIS versions 9.2 or higher also requires the ESRI ArcGIS Spatial Analyst extension.

CITYgreen requires a landcover dataset for the area to be analyzed. The landcover classification is developed from user-supplied imagery using software other than CITYgreen. The user must classify elements in the imagery into land uses such as tree canopy, impervious area, etc., before the CITYgreen analysis can be run. The CITYgreen extension uses models developed by the USDA Forest Service, USDA Natural Resources Conservation Service, and other sources to calculate the benefit of the green infrastructure in a city. CITYgreen also includes a tool for modeling alternative scenarios for a given land area. This can be used to track changes over time. It can also be used to model the future effects of different development or urban forest management plans.