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Trees in the City:
A New Role for the "Ultimate Urban Multitaskers"

Urban trees, in the words of American Forests' Gary Moll, are the "ultimate urban multitaskers," providing a vast array of benefits to the people of cities fortunate enough to be graced by their presence.  High on the list of services provided by the trees is their absorption of large quantities of air pollutants, which phenomenon saves city dwellers significant amounts of money that might otherwise be needed to be spent on more traditional forms of pollution mitigation.  D.J. Nowak of the U.S. Forest Service's Urban Forest Ecosystem Research Unit, for example, estimates that trees in the city of New York save its taxpayers some ten million dollars annually, as a result of improvements they make in the city's air quality.  Urban trees also serve as homes for resident bird populations, as well as rest-stops for many migratory species.  In addition, they provide numerous recreational opportunities, not to mention good old shade from the sun for spectator-types, who only venture out to watch their kids or grandchildren's soccer games.  And, of course, there's the feeling of exhilaration that a touch of nature brings to country-starved city folks.

All that's well and good; but now there's even more to be added to the urban trees' list of virtues.  They are, as it turns out, front-line warriors in the worldwide battle to mitigate global warming, as they sequester large amounts of atmospheric carbon - and do it with a vengeance - right in the midst of where much of it originates ... in cities.

The first assessment of national carbon storage by urban trees in the United States was carried out by Nowak (1993).  Based on an extrapolation of carbon storage data from Oakland, CA, together with tree-cover data from a number of other cities, it yielded an estimated national carbon storage range of between 350 and 750 million tons.  A later assessment, which included carbon storage data from a second city (Chicago, IL), advanced these numbers to 600 and 900 million tons.  Now, Nowak and Crane (2002) provide an updated - and much more robust - assessment of this phenomenon, based on carbon storage data from ten cities: Atlanta, GA; Baltimore, MD; Boston, MA; Chicago, IL; Jersey City, NJ; New York, NY; Oakland, CA; Philadelphia, PA; Sacramento, CA; and Syracuse, NY.

What do the data show?  The updated and likely more accurate results of Nowak and Crane's most recent analysis puts total carbon storage by urban trees in the coterminous United States at approximately 700 million tons.  This number, though seemingly large, is only 4.4% of the estimated 15,900 million tons of carbon stored in the country's non-urban forests; and it is equivalent to the amount of carbon that is released to the air by the country's anthropogenic activities over a period of only 5.5 months.

In terms of annual impact, the numbers look even less impressive.  The total amount of carbon extracted from the air each year by all of the urban trees in the United States (22.8 million tons) is equivalent to the amount of carbon emitted by the country's human population over but a mere five days.

Although this amount may seem woefully small, it is still significant for a couple of different reasons.  First of all, individual urban trees typically sequester considerably more carbon than individual trees in non-urban forests, because (1) there are more large trees in cities and (2) the more open structure of the urban environment generally allows individual trees to intercept more light and, therefore, grow faster.  In fact, Nowak and Crane note that "individual urban trees, on average, contain approximately four times more carbon than individual trees in forest stands."  Also, and perhaps most important of all, planting trees in strategic locations near buildings can reduce building energy usage via enhanced shading and evaporative cooling in summer, and by wind speed reduction in winter, which phenomena lower the demand for electricity needed for cooling and heating and, in most cases, offset the burning of a certain amount of coal, gas or oil.

This latter effect is very significant.  In fact, Nowak and Crane note that the atmospheric CO2 "avoidance" provided by such strategically planted trees is approximately four times greater than the amount of CO2 they physically remove from the air.  Hence, it can be appreciated that the average urban tree, which is four times more effective in physically removing carbon from the atmosphere than the average non-urban tree, is fully sixteen times more effective than the average non-urban tree in mitigating global warming when planted in places designed to reduce the cooling and heating costs of buildings.

Although costs associated with the planting and upkeep of urban trees offset some of their CO2 avoidance benefits, they still come through smelling like a rose in the race to devise effective and efficient means to temper the greenhouse effect of anthropogenic CO2 emissions.  Surely, planting more of them is something we could all agree is a truly "no regrets" activity.  Indeed, it is a multi-benefits undertaking, which is well suited to the many talents of the world's "ultimate urban multitaskers."

Dr. Sherwood B. Idso Dr. Craig D. Idso

References
Nowak, D.J.  1993.  Atmospheric carbon reduction by urban trees.  Journal of Environmental Management 37: 207-217.

Nowak, D.J.  1994.  Atmospheric carbon dioxide reduction by Chicago's urban forest. In: McPherson, E.G., Nowak, D.J. and Rowntree, R.A. (Eds.), Chicago's Urban Forest Ecosystem: Results of the Chicago Urban Forest Climate Project.  USDA Forest Service General Technical Report NE-186, Radnor, PA, pp. 83-94.

Nowak, D.J. and Crane, D.E.  2002.  Carbon storage and sequestration by urban trees in the USA.  Environmental Pollution 116: 381-389.