A study published by the US Forest service values the State of Tennessee's urban forest at $80 billion thanks to its contributions to the environment. With an urban population of 284 million, that equates to a mean value of $282 per tree.

The total is based on a number of costs that are to some extent offset by the presence of Tennessee's urban forest (its urban tree population, in other words). These include $350 million-worth of carbon storage based on the current standing stock, over $204 million every year in pollution removal, $18.4 million per year in additional carbon sequestration, and $66 million per year in energy savings-"the most significant contribution" made by the urban forest, according to State Forester Steven G. Scott. But how are the environmental benefits of the trees evaluated?

Data was collected and analyzed using the Forest Service's own i-Tree Eco software. Using a mobile app providing strict protocols for data collection, researchers took information from 2418 trees and saplings across 255 field plots. Variables noted include species, diameter at breast height (or DBH—taken at 1.4 meters above ground), height, crown dimensions, foliage transparency, damage, and proximity to buildings. The pool of sample data is assumed to be representative of the total population, and from there the software crunches the numbers using "peer-reviewed equations" to paint a macro-scale picture of the urban forest, based on quantifiable characteristics that describe its structure, condition and function.

In the case of Tennessee, the software describes a distributed urban tree population with more Chinese privets, Virginia pines and eastern red cedars than other species. The red maple and yellow-poplar (Tennessee's State tree) are more common among Tennessee's 8-billion-strong rural tree population, so their absence from the top three of the State's 99 urban species should not necessarily come as a surprise. Canopy coverage was estimated to be 37.7 percent of the urban area. Only nine percent of urban trees were found to fall within maintained areas. Just under two percent of the population was dead. With these characteristics identified, the researchers were able to go about evaluating economic value.

Knowing a tree's size and chemical composition, carbon storage is a relatively straightforward thing to ascertain. Clearly, larger trees store more carbon, but they also sequester, or capture, carbon dioxide from the atmosphere at a greater rate—provided they're healthy. Calculating a monetary value of the carbon stored is simple, provided you can come up with an agreeable rate. The Forest Service uses Professor Samuel Frankhauser's figure of $20.7 per ton of carbon, which is based on "the estimated marginal social costs of carbon dioxide emissions" between 2000 and 2010.

Tennessee's urban forest contains an estimated 16.9 million tons of carbon, thus the estimated carbon storage value of $350 million. Sequestration rates are a little trickier to determine, relying on estimates of growth which are dependent on future conditions. And when it comes to carbon sequestration not all trees are created equal, with large, deciduous species, the chestnut oak, hackberry and yellow-poplar contributing more than other species to the $18.4 million per year contribution.

The urban forest's $204 million annual contribution to pollution removal is based upon hourly pollution data, and Nashville weather data from the year 2000. The report estimates that urban trees remove some 27,100 tons of pollution per year, of which 15,000 is ozone, 7,000 particulate matter, with sulfur dioxide, nitrogen dioxide and carbon monoxide making up the remainder. The overall value of this pollution removal is put at $203.9 million.

The trees' contribution to energy saving is a result of the shading they afford to nearby buildings. The shading provided in summer reduces the need for cooling, though the same effect can be detrimental in winter time when the solar heat gain is desirable. However, when suitably located, trees can also offer energy savings in winter by acting as a wind barrier. According to the study, the net effect of trees in winter is calculated to be a loss of $29 million, but this is more than made up for by the savings of $95 million during summer, giving an overall annual energy saving of $66 million.

The report is at pains to highlight the beneficial yet "unrecognized and unreported" role of urban trees, and identifies a number of threats to Tennessee's urban forest. The emerald ash borer, accidentally introduced in the US during the 1990s was recently identified in east Tennessee, while the Asian long-horned beetle—also introduced inadvertently—are a threat to the various hardwoods in which the larvae tunnel. Hemlock woolly adelgid and thousand cankers disease are noted as additional threats. The reports notes that no single cause of damage had affected more than 9 percent of the urban tree population, however.

The report concludes by calling for management plans targeted specifcally toward urban trees. "These plans also need to be dynamic due to the continuous forces of change that alter urban forest environments," the report says. "Data from urban forest monitoring programs should be incorporated within State and local urban forest planning and management regimes to allow local constituents to develop canopy goals and/or tree planting goals to sustain or enhance urban forest canopy across the State."