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Sequestering carbon with trees

KENNEWICK -- The older I get, the more I realize that there are no easy answers.

The other day I came across an article from ScienceDaily that reported a research team from the College of Environmental Science and Forestry at the State University of New York, or SUNY, in Syracuse, N.Y., had determined the right mix of urban trees to help fight global warming in their region.

For years I've been an ardent tree enthusiast, urging tree planting for their many benefits.

One of those many benefits is the uptake of carbon dioxide (CO2) and sequestration or "tying-up"of its carbon.

Now, it's not so simple as just planting a tree. It has to be the right "mix" of trees. Apparently some trees do a better job of sequestering carbon than others.

Generally you want fast growing, long-lived (with a lifespan of 50 years or more) large (25 feet tall or more when mature) tree with dense wood. A tree that's not prone to attack by health compromising insects or disease is best, as well as a tree with broadleaves instead of needles.

Some of the trees recommended by the SUNY study for the central New York area included American basswood (linden), dogwood, Eastern white pine, Eastern red cedar, red maple and river birch.

Other trees deemed to be good at tying-up carbon include sycamore, sweet gum, and tulip tree. They recommended avoiding invasive trees like Tree-of-Heaven, fast growing trees with weak wood like silver maple, and trees with dense shade that prevents grass growth underneath like Norway maple.

Our tree management practices can negatively offset the sequestering benefits we attain from our planting the right trees. The SUNY report recommends avoiding trees that require high maintenance using fossil-fueled equipment, such as chain saws, leaf-blowers, and chippers.

Another consideration is what happens to the tree when it eventually dies. In a forest, it will decompose, releasing it's carbon back into the atmosphere. In urban situations, dead or declining trees are removed and then chipped or burned, again releasing the carbon. No net benefit there.

However, if the tree is kept as wood in the form of lumber or wood products, the carbon continues to be stored.

Last year, one of my colleagues had to cut down several walnuts on her property that had been attacked by the walnut twig borer. She was able to offset the cost of removal by selling the wood for its use as lumber.

I would be remiss if I failed to mention that trees provide additional benefits in reducing CO2 with their proper placement to provide shade that reduces energy needs for cooling in the summer.

Of course, that's assuming that the energy being used is generated with the help of fossil fuels.

The SUNY study recommended a specific mix of 31 trees for the city of Syracuse to help increase carbon sequestration by their urban forest. Their recommendations included a mix of different trees with no more that 10 per cent of one species, no more than 20 per cent of one genus, and no more than 30 per cent of one family.

The recommended mix eventually will increase carbon sequestration by 86 per cent, but it may take up to 40 years for the city to fully realize these benefits. These benefits will be even greater if more longer lived shade trees are planted and provided with proper management to keep them healthy and alive.

As I mentioned earlier, there are no easy answers, but planting the right trees, such as a red maple or even a sycamore, can help.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension.

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