September 14, 2000

Life without Trees

Paving Paradise for Parking Lots  

As fall approaches, much of the country will soon be treated to a grand display of orange, yellow, and red leaves. But if you are like an estimated 80% of Americans, you live in an urban or suburban environment. That means more buildings, more paved streets, and fewer trees. Why should you be concerned?

People around the world are cutting down trees to make way for homes, businesses, and parking lots. But as we grow our cities and cut down our urban trees, we lose far more than pillars of our landscape. We lose a valuable environmental resource. Did you know that trees scrub pollutants from the air? You could think of them as nature's vacuum cleaners. The particles of many pollutants cling to the leaves of trees, so less pollution reaches us. Trees also take the carbon dioxide CO 2 from the atmosphere and use it in the process of photosynthesis. Excess CO 2 , principally from the burning of fossil fuels, is considered the primary cause of global warming.

  • Find out more about how trees reduce air pollution, from the Maryland Department of Natural Resources.


    According to the conservation group, Eagle Eye Institute, trees help humans in the following ways:

    • Reduce air pollution and global warming through respiration and removing particulate matter

    • Moderate local climate by lowering air temperature

    • Provide shade and conserve energy

    • Reduce noise pollution

    • Provide food and shelter for generations of birds and wildlife

    • Reduce soil erosion

    • Increase property values

    • Make communities more beautiful and livable

    • Make you feel good

The View from Space  

Earlier this year, pictures from NASA's Earth-observation satellites told a dramatic story about this man-made disruption in the great circle of life. From the United States to China, rapid urbanization has meant rapid deforestation. It also has meant increased motor traffic, smog, and the type of unplanned growth we have come to call urban sprawl.

Scientists at the Goddard Space Flight Center in Maryland have concluded that urban sprawl makes it harder to remove CO 2 from the atmosphere. According to a story from the Environmental News Network, areas that are heavily built up can lose the equivalent of 20 days a year of photosynthetic activity. This loss means more CO 2 in the air and less food production from the ground.

Cutting down trees also increases land temperature. Cities with few trees become "heat islands." Without vegetation to block and absorb the sunlight, surface temperatures can rise as much as 12 °F as the pavement bakes. The National Arbor Day Foundation notes that an additional 100 million mature trees would save $4 billion dollars in energy costs for cooling. Planted on the eastern and western sides of buildings, trees shade the rooms inside from sunlight.

  • Based on the figures above, how much in energy savings would a single tree account for?

  • How much would a dozen trees save?

Tree Diseases

As if trees don't face enough already, blights sometimes wipe out or severely diminish tree populations. One of the most famous blights was Dutch Elm Disease, which killed off many of the most beautiful street trees in America. The graceful American elm was a tree that adapted particularly well to the harsh conditions of American cities where soil is compacted and roots are often starved for oxygen. Still the elm could not survive the voracious appetite of the elm bark beetle, which carried a fungus that attacked the tree's vascular system. The fungus was especially deadly because it could travel to other trees through interlocking root systems. In the midwestern United States alone, 42 million elms were lost.

A Gift to Cities  

Believe it or not, trees were once an afterthought in urban areas. When cities were evolving, little thought was given to urban or landscape planning. Cities were for commerce, and beauty was the furthest thought from anyone's mind. But over time, as more and more people moved into cities, an idea grew that cities should be pleasing and welcoming. About 150 years ago, "a beautify America" movement developed in this country led by the young landscape architect Frederick Law Olmstead. Olmstead designed Boston's Emerald Necklace, the belt of green parkland that surrounds the city; Central Park in New York City; and other green spaces in cities around the country. Without them, our cities would be less beautiful and less hospitable.



Hands on, Mind's On


What Olmstead understood is that we must be in the presence of trees to appreciate them. We asked some experts what they would do to increase our appreciation. Start by identifying all the trees and shrubs in your neighborhood, and when you don't know what something is, go home and look it up, suggests Peter Del Tredici of the Arnold Arboretum in Jamaica Plain, Massachusetts. Learn the common names and the Latin names, as well as what the tree needs in order to survive. Build on your understanding from there.


"Hands on, mind's on," suggests Abdu Salaam Moulta Ali, a naturalist with the Eagle Eye Institute in Somerville, Massachusetts. Eagle Eye works with urban and under-served youths to help them discover their own relationship with trees. Moulta Ali says he starts the "hands on" phase with a tree inventory, getting kids out looking at different trees, so that they get some understanding and respect for the tree as a living organism.

The "mind's on" part occurs when they make a relationship of their own between trees and people, Alit adds. "The kids will buy the fact that trees grow, they move, that they're alive. But they think they've got me when they say, 'But trees can't talk.' And, then I say, 'But trees communicate all the time, it's just up to us to find out. When we learn their language, we can understand them. There are signs all the time: when the leaves rustle, when they turn color, when they shrivel up. We just have to learn to watch the signs. Trees are talking to us all the time.'"

Appreciating Trees

The shapes of trees also tell their biographies: "Their entire life histories are recorded in their forms," says the Arnold Arboretum's Peter Del Tredici. It's especially interesting to look at trees in winter when the leaves have fallen and you can really see how the forces of nature have shaped the organism.

How it bends to the light, the shape and size of its branches, for example, are all part of the tree's shape or biological history which tell botanists something about the life processes the tree has undergone. "There's no magic to learning about trees," Del Tredici says, "just slow down and observe."


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