By Dan Buckler, DNR Urban Forest Assessment Specialist
Daniel.Buckler@wisconsin.gov or 608-445-4578

Trees work in mysterious ways, and, for some users, so does i-Tree. The suite of software, developed by the USDA Forest Service and Davey Tree Expert Company, can, among other things, calculate the societal benefits of trees. Below, I try to summarize and demystify some of these estimates of ecosystem services, a somewhat sterile term for some of the cool things trees do for society and the wider environment.

Because the benefits identified in i-Tree only scratch the surface of trees’ importance to one’s community, it could be problematic and limiting if estimates from i-Tree are solely relied on to justify the investment in tree preservation, maintenance or planting. Nonetheless, these estimates help tell the story of the good work trees do in our communities.

Sample ecosystem services exported from MyTree, an i-Tree program.

The i-Tree suite of software produces a broad range of estimates, with the most often seen being (1) avoided water runoff, (2) air pollution reduction, (3) carbon removal and storage, and (4) trees’ “compensatory” value.

In the background, i-Tree uses equations that estimate hard-to-measure things like height or canopy spread based on more readily identified things like species and stem diameter. All the tree data are then related to social or environmental factors to produce the estimates explored below.

Avoided Runoff
Avoided runoff is the amount of precipitation trees prevent from traveling to the ground and, in developed spaces, going into the local stormwater system. This is water that is either captured on the tree’s surface and eventually evaporated or taken up by the tree’s root system.

The tree’s total surface area is important for calculating avoided runoff, particularly its foliage. Generally, bigger trees with more leaf area prevent the most runoff. Perhaps counterintuitively, conifers often provide the most surface area with their small needles or scales. Evergreen conifers that keep their foliage year-round are hypothetically the best trees at this task. However, the tree’s crown condition or dieback is a critical input into the i-Tree model. If the tree is in poor shape, its leaf area and ability to decrease runoff are likely significantly reduced.

People may sometimes notice large discrepancies in the amount of avoided runoff provided by one or many trees over time. This may be due to significant fluctuations in precipitation between years. If there is less precipitation, the tree is not reducing as much stormwater.

The monetary benefit of avoided runoff is estimated by multiplying the estimated volume of water a tree affects by the dollar amount that volume would cost to treat by the local stormwater or wastewater facility. The default used for many i-Tree estimates is $0.008936 per gallon.

Air Pollution Reduction
Trees absorb some pollutants directly through the pores in their leaves and provide large, complex surfaces onto which pollutants can be deposited. i-Tree specifically measures the amount of nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), carbon monoxide (CO) and particulate matter less than 2.5 microns (PM2.5) removed from the air by trees. It does so by pairing weather data, deposition rates for those pollutants (derived from the Environmental Protection Agency) and leaf area. As with avoided runoff, leaf surface area is critically important. Trees with more leaf area, held for longer periods (e.g., evergreens), generally reduce more pollutants.

The monetary value for most pollutants is calculated by linking the removed pollutant to the impact of that pollution on healthcare costs, per the EPA’s Environmental Benefits Mapping and Analysis Program. This software produces estimates of the number and economic value of air pollution-related deaths and illnesses.

As with avoided runoff, users may note some discrepancies between time or place. In addition to varying weather data, locations will experience different pollution levels. Local human population numbers will also impact the “value” of an individual or group of trees. Trees in a more densely populated area will likely be more impactful than those in less dense spaces.

Carbon Removal and Storage
During photosynthesis, trees remove carbon dioxide from the atmosphere and store carbon in their fibers. This active removal of carbon dioxide is known as carbon sequestration, while the long-term residence of that carbon is known as carbon storage. Approximately half of the dry weight of trees is composed of carbon. Thus, bigger trees store more carbon than small trees.

Climate change has enormous impacts across many economic sectors, such as agricultural productivity, damage from rising sea levels and changes in energy consumption. The total cost across those sectors per ton of additional carbon dioxide in the atmosphere has been converted into a dollar amount known as the “social cost of carbon.” When you see the value of carbon sequestration, it is the weight of carbon removed by a tree over one year converted into a monetary value, while carbon storage is the weight of carbon in a tree’s entire body, converted into a monetary value.

Compensatory Value
A final benefit commonly encountered by i-Tree users is the compensatory value of trees. This is the hypothetical structural value of a tree on the landscape should it be injured or killed and the subject of litigation or insurance claims. The value is calculated based on a formula from the Council of Tree and Landscape Appraisers (1992).

Compensatory values are determined by an equation incorporating trunk area, species, condition, and location. In this equation, bigger, healthier trees used in certain landscape functions could be significantly more valuable than others. Additionally, there are multiplication factors for different species. For example, based solely on the tree type, each oak species listed in i-Tree’s Wisconsin database is more valuable than each poplar species.

Note that the estimates exported from i-Tree should *not* be used in any formal claim. A local appraiser would have to be used to calculate a value for litigation or insurance purposes.

i-Tree tools have built-in assumptions and uncertainties, but the underlying models are constantly being refined, and the background reference data are growing. See the Understanding i-Tree document for a lot more information about these and other ecosystem services. There’s a whole world under the i-Tree canopy!

So next time you’re using one of the i-Tree programs or something linked to them, like the DNR’s Wisconsin Community Tree Map, you know a little more about how we get those numbers. If you require or desire more customized, accurate estimates, the flagship program i-Tree Eco is recommended. While learning to use these tools is important, sometimes, you only want the data. In those cases, please contact Dan Buckler at Daniel.Buckler@wisconsin.gov to help you along.