Tree Fall

Tree small Tree FallUrban development may have thrown a kink into novelist Betty Smith’s central metaphor for life: It might not be so easy for a tree to grow in Brooklyn today. A new analysis of aerial photographs taken from above 20 cities shows an alarming and not-green trend. Trees seem to be dwindling across urban America.

Trees, from broad-leafed maples to tall oaks, give urbanites a little shade and eye candy. They can also help to freshen a bustling city’s air. But these leafy plants are expensive to sow, and are easily felled by fierce winds, old age and new development. Cities that don’t restock their walnut or cottonwoods may soon begin to run out, say David Nowak and Eric Greenfield at the U.S. Department of Agriculture’s Northern Research Station in Syracuse, New York.

To see just how well U.S. cities are keeping pace, the researchers turned to a series of wide panoramic shots taken from high above urban areas, including Chicago, New York and New Orleans. These towns differed wildly in their green cover, ranging from Atlanta–whose surface is more than half tree–to Denver–less than 10%. The team poured through the images like before and after weight-loss photos, gauging the growth or spread of shade during the mid-to-late 2000s.

On the whole, tree cover shrank in 17 out of the 20 cities. The losses amounted to about 7,900 hectares of greenery per year or 4 million trees, the researchers report in the current issue of Urban Forestry & Urban Greening. New Orleans had the most fallen timber, they add, for a very obvious reason–Hurricane Katrina. Between 2005 and 2009, close to 30% of the city’s existing tree cover was washed away.

But it wasn’t all bad news. Trees took root across Syracuse’s neighborhoods from 2003 to 2009, covering an extra 1% of the northern town’s area. Most of this bonus shrubbery was dominated by European buckthorn (Rhamnus cathartica), a hedge-like plant newly arrived in the United States. That suggests that the city’s gains may be accidental, not the result of concerted planting efforts, Nowak and Greenfield say.

For many towns, extensive planting may not be an option, the researchers say. Los Angeles, for instance, lacks the water to sustain hundreds of new trees. But a little prevention could go a long way, they add: More responsible grass mowing practices, for instance, can give trees a better shot at surviving their first years. With a little luck, new trees could grow in Brooklyn, after all. Daniel Strain | February 8, 2012

Source: Nowak DJ and Greenfield EJ. (2012). Tree and impervious cover change in U.S. cities. Urban Forestry & Urban Greening, 11(1). http://www.sciencedirect.com/science/article/pii/S1618866711000999. DOI: 10.1016/j.ufug.2011.11.005.

Image © Yuryz | Dreamstime.com

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3 Comments

  • tom February 8, 2012 at 11:21 am

    One thing not mentioned is the death of many species through disease, such as Dutch Elm disease which has decimated these iconic trees throughout the eastern U.S. The city that I grew up in was once beautifully shaded by huge Elms everywhere. All gone and not replaced. A mature Elm is now a rarity even in he countryside. I was born just as the last of the American Chestnuts vanished. A major loss. The list goes on.

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    • CarstenG March 11, 2013 at 7:30 am

      The Nowak and Greenfield study finds NYC as the nations 6th worst city of the 20 city study in terms of canopy loss and increase in impervious surface.
      Boots on the ground not only sees annual losses of large canopy trees by pests and disease pathogens but more by infrastructure projects inhabited by trees and the absence of enforcement of tree protection code and policy. Construction projects whether new private developments or highway projects sees business as usual with the blatant removal of irreplaceable trees irregardless of whether they are able to be preserved or not. And if not cut and removed then by way of a slow declining health with limb shedding over successive years from impacts and root loss by ripping and tearing from excavators. Enough to register some 2250 acres of canopy decline over the 5 year study- targeted areas likely to be the outer boroughs of Staten Island and Queens. Although it is 2013 tree removal trends and practices by City planners, engineers and architects have not skipped a bit.

      Reply

  • Tree Fall | evolve+sustain | global sustainability & local green innovation February 8, 2012 at 3:43 pm

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