Documents: Special Interest: Horticultural Therapy:

The Trend Toward Smaller or Medium Sized Trees and Our Health
by Thomas Ogren
by Thomas Ogren


Thomas Leo Ogren is the author of Allergy-Free Gardening. His most recent book, Safe Sex in the Garden, was published March 2003, by Ten Speed Press. Two of his previous books, both novels, were published by New Readers Press, and are used nationwide in adult literacy programs.

Tom does consulting work for the USDA, the American Lung Association, for, and recently for county asthma coalitions. He is considered to be the leading authority on pollen-producing male cultivars, and on their opposites: pollen-free female plants. He is currently working on a book about lawns, for AOL Time Warner Books.

Tom and his wife, Yvonne, have four children. They live in San Luis Obispo, California.

July 8, 2001

The modern trend with street trees is to stop using so many trees that will grow really large. Emphasis for years now has been on planting smaller or medium-sized street trees. These trees don’t break up the sidewalks as much as the bigger trees did and besides, they’re easier to keep pruned.
Is there a downside to all these small or medium sized trees, replacing the big city trees of old? You bet there is! The leaves of trees do many things for us. They do a good job of catching and trapping a huge amount of airborne pollen and other small allergy-causing particulates. A smaller tree traps less, cleans less air.
A big tree’s leaves evaporate water into the air. A large tree has a very large, extensive root system. These roots hold the soil together, channel excess rain water into the soil instead of just letting it vanish as runoff. The roots of big trees also tap into deep sources of water and then release water vapor into the air through their leaves when the weather is hot. The hotter the day, the more water the big trees release into the air. This moisture serves to cool down the tree and to also cool down the surrounding area. Big trees are very effective air conditioners but small trees are just too little to do the work that the big trees do. 
There are other beneficial effects of these big city trees. Large trees cast a huge amount of shade in the summer. This shade keeps everything under them cooler. The automobiles are cooler, the people, the pets, the houses and other buildings are all cooler if in the shade of a large tree. The cooler the buildings are, the less air conditioning that is needed. Less air conditioning means less petrochemical fuel burned, which equals to saved money, cleaner air, cleaner water, good effects all around. 
Big trees provide much more habitat for wildlife than do small trees. Birds can feel safe making their nests high up in these big trees. Squirrels run through their branches and bees and butterflies rest high above among their leaves. Smaller trees simply provide less wildlife habitat. 
Big trees convert huge amounts of carbon dioxide into much needed oxygen. People, and all animals, breathe in oxygen and breathe out carbon dioxide. We can not live without enough oxygen and we need the big trees to produce it. The more oxygen in our air, the better. Smaller trees simply produce smaller amounts of oxygen. In most cases the amount is dramatic. One large tree may easily produce as much oxygen as a dozen or more smaller trees. 
Global warming. Think about it. Big trees cool down the air in our cities and help us fight against global warming. Forests everywhere have always provided us with needed buffer zones, areas that produced large amounts of oxygen and that keep the air cooler. But every year we cut down more and more of our forests, our jungles. We pave over more prime farmland every year and continue to clear-cut our forests of big trees. With all of this in mind, we simply MUST grow a huge amount of large trees in our cities. Our very future could well depend on it.
With all that said, let’s keep one other thing in mind: all trees are different and some species provide us with much greater benefits than do others. We need to use as many of the very best kinds of big trees in our cities-the ones that don’t cause allergies and don’t drown us in VOCs. We need to use tons of these big Super Trees. 

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