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Why trees are good- and bad

Trees and people belong together.
Trees and people belong together.
Kim Willis

When you walk into a forest you immediately notice the smell, a woodsy aroma that’s soothing and invigorating at the same time. That aroma comes from gases that trees give off and it’s most noticeable when trees are concentrated in groups, but all trees emit gases. The gases contain biogenic aerosols -- particulate matter that originates from plants.(Tree poop) When exposed to oxygen in the atmosphere these particulates cling to other particulates in the air, gradually growing larger, forming clouds that reflect the suns heat, cooling the earth and forming raindrops. This function of trees emitting gases is helpful, and works to offset global warming, because as the world warms, trees produce even more gases.

In forested areas of the world gas emission by trees can reduce the effect of global warming by about 30%. Rain falls more frequently and regularly where there are forests because of favorable cloud formation. And even in urban areas where they are not as concentrated, trees provide a cooling effect as well as making your property more valuable and desirable.

A tree’s bad breath

But there is a bad side to the gases that trees emit also. Trees emit isoprene, a chemical manufactured to protect leaves from oxygen damage and temperature fluctuations. Isoprene is abundant in the atmosphere but it’s not helpful to humans. A study recently completed by researchers at the University of North Carolina at Chapel Hill found that isoprene unites with air pollutants such as nitrogen oxides emitted by cars and coal burning plants to form harmful particulates at least partially responsible for lung cancer, asthma, and other lung disorders. These isoprene-nitrogen oxide combinations also produce smog and are damaging to the environment in other ways. It’s important to remember that without humans producing those nitrogen oxide emissions though, isoprene would be harmless.

And here’s some more news about the breath of trees. When trees are attacked by insects, particularly bark beetles, the trees produce chemicals to protect themselves and these chemicals are released into the atmosphere. The chemicals are predominately a monoterpene called ß-phellandrene. This chemical is also part of unhealthy smog and haze and harmful to those breathing it.

It gets worse. Researchers at Queensland University of Technology in Australia found that trees produce electrically charged ions in their vicinity. Charged ions are more likely to attract other molecules to stick to them and they are also more likely to “stick” to your lungs when breathed in. One of the molecules that charged ions like to collect is radon.

Radon is produced when rocks containing radioactive material gradually breakdown in the soil. It is water soluble and radon is often found in ground water. Radon emissions from the soil as a gas and from the ground water vary in concentration from one area to the next depending on the rocks underlying the area. Michigan has some high radon areas, where radon seeps into basements. Radon is known to cause cancer.

Here’s the connection between radon and trees. Trees with deep root systems act as pumps for radon, bringing the radon in ground water and from the soil to the surface, discharging it attached to charged ions in their gas admissions when they transpire (or breathe). Recent research suggests in areas with deep rooted trees, tree transpiration accounts for more than a third of the radon in the air.

So why plant a tree?

So far this article has been telling you about all the bad things trees do. In fact there is more reason than ever to plant trees. Trees help regulate the climate and they become even more important as our climate changes. The small health risks associated with trees breathing are generally caused by our own pollution of the planet. While trees may emit radon when they emit water vapor during transpiration they are also unlocking water stored in the soil and returning it to the atmosphere. Without rain we couldn’t grow crops very well and our world would be a dry and barren place.

A real estate agent will tell you that large, mature, well cared for trees add value to your property and make it easier to sell. And who can deny the beauty of trees as well as all the useful products they supply us with, including food? It is said that you plant trees for your grandchildren. All I know is that I continue to plant trees because I like them. One should always replace a tree that you have had to remove, maybe with a more suitable tree for the area, but always keeping the balance Nature is working to achieve.

Several studies of urban trees have found that more than 2/3 of the trees in cities were from natural regeneration, they grew from seeds floating in the air or carried in by animals and were not deliberately planted. However the remaining third of trees in a city, those planted by us, had a somewhat higher survival rate and were healthier overall. This is probably because they were more valuable specimens, and because if you plant something you tend to care for it.

Trees for small gardens

Love trees but have little room in the garden to plant them? There are many small trees that won’t hit the power lines and are easy to fit into small spaces. Trees add vertical lines to the garden and pleasing structure. Here are some to consider. Maples, some Japanese maple varieties and other maples ( Snake bark, (A. grosseri), Moose bark, A. griseum are good for small areas. Redbuds, ninebark, magnolia, strawberry tree, birches, dogwoods, hawthorns, sand cherries ( purple leaf cherries),weeping cherries, weeping peach, wisteria trained to tree form, crabapples( height varies, look for smaller types), Golden Chain Tree, cotoneaster, bristly locust, “Lace Lady” or “Twisty Baby” locust, purple smoke tree, witch hazel, Japanese stewartia.

There are also many dwarf or slow growing evergreens that can be added to the garden. Narrow, columnar evergreens are also useful in the landscape. Don’t forget that tub trees, (marginally hardy or tropical trees) can be added to the garden in the summer and stored in a suitable place over the winter. These would include figs, pomegranates, citrus trees, bay laurel and olive trees.

There is a tree for every garden and every garden should have trees. While trees affect the natural world in ways that may not always be good for us, we are only one species on the planet. And the value of trees far outweighs their bad breath.

Here are some additional articles you may want to read.

Herbal uses of the rose

How to grow Jacobs Ladder

How to help save the Monarch Butterfly

You can contact the author by emailing her at

You can see her garden blog at

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