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Urban Trees: Their Long List of Virtues
Canberra, the national capital of Australia, was established in 1911 on a largely treeless plain that was originally covered by native eucalypt forests, but which was depleted of most of its substantial woody plants by the activities of graziers in the early 1820s.  As urban development proceeded, therefore, the residents of the new capital were strongly encouraged to re-vegetate the area.  The townsfolk responded; and the city, with a current population of 300,000 inhabitants, now boasts many good-sized trees that surround people's residences, as well as some 400,000 other trees found in publicly managed parks and along roadways.

Because of this century-long history of urban arboriculture, the people of Canberra have maintained what could well be called an ongoing love affair with their local trees.  How do they love them?  Let us count the ways.

In a recent publication that extols the many virtues of Canberra's trees - or those of any urban forest, for that matter - Brack (2002) presents a table wherein are listed what he calls the "benefits of an urban forest," which we here repeat.

Benefits relating to pollution mitigation
          Amelioration of urban climate extremes
          Mitigation of urban heat islands
          Store and sequester carbon
          Reduce noise pollution
          Improve air quality
          Improve water quality
          Lower temperatures of parked cars
          Reduce volatilization of bitumen
          Reduce consumption of electricity for heating and cooling
          Reduce need to invest in new power utilities

Other benefits
          Aesthetic contribution, scenic beauty, visual amenity
          Architectural enhancement of buildings
          Improve property values
          Increase privacy, barrier against unpleasant/stressful scenes
          Control urban glare and reflection
          Improve general livability and quality of urban life
          Increase tourism
          Provide opportunities for outdoor recreation and enjoyment
          Contribute to human health and relaxation, reduce stress and anxiety levels
          Attract birds and other wildlife
          Act as a source of specialty timbers
          Act as a source of general timbers

Most of these benefits, according to Brack, are related to tree size, because "larger trees tend to extract and store more carbon dioxide from the atmosphere and have a greater leaf area to trap air borne pollutants, cast shade, and intercept or slow rainfall run-off."  Hence, it is encouraging to learn, as reported by Nowak and Crane (2002), that urban trees are generally larger than rural trees, making the average "city tree" considerably more effective in these several respects than each "country tree."  In the United States, in fact, individual urban trees store approximately four times more carbon than individual rural trees; and they render unnecessary an equally large amount of CO2 emissions to the atmosphere when they are planted so as to shade buildings and thereby reduce the need for fossil fuel-generated electricity that is required to run air refrigeration units.  Hence, as pointed out by Eric Beckers of the Texas Forest Service, city trees are "15 times more capable of reducing carbon in the atmosphere" than rural trees.

With such significant leverage in terms of atmospheric carbon sequestered and avoided per tree planted, the "foresting" of cities is an effective way for a large portion of the world's population to make a positive personal contribution to the effort to curtail what some people consider to be potentially catastrophic global warming.  At the same time, for those people who do not believe such efforts are needed - because they do not believe the warming of the past two centuries is human-induced, or that humanity's CO2 emissions will have any measurable impact on the future climate of the earth - the planting of urban trees has plenty of other virtues to recommend itself.  Indeed, the purposeful "greening" of the world's cities by their inhabitants is something all of us should be able to support with great enthusiasm.  We do.  How about you?

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

References
Brack, C.L.  2002.  Pollution mitigation and carbon sequestration by an urban forest.  Environmental Pollution 116: S195-S200.

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