Tree Study Criticized; Researcher Defends It
Stephanie Corchnoy, who conducted research reported in the story, responds:
Regarding Mary Welty’s letter: Hydrocarbons react with oxides of nitrogen to produce smog. Cars, power plants and all forms of vegetation emit hydrocarbons, so it is reasonable to compare the smog-producing potential of a tree to that of a car or power plant. The important aspect is that because cars emit both hydrocarbons and oxides of nitrogen, they can produce smog in an environment where no trees exist. In contrast, trees, which emit hydrocarbons only, cannot contribute to smog production in an environment where no cars exist.
The scope of our research project was to rank selected shade trees according to emission rate so that the lowest emitters could be recommended for planting. While we do discourage the planting of high-emitting trees, we have at no time suggested that existing trees be cut down. Also note that the South Coast Air Quality Management District is implementing tough restrictions on other sources of hydrocarbons such as fuels and lighter fluid.
William Corbin’s letter: Smog production is dependent on sunlight. Because of this, significantly decreased amounts of smog are produced during winter months regardless of how much hydrocarbons are being emitted. Emphasis is then shifted to summer months (the “smog season”) when smog production is at a maximum due to high intensity sunlight. The fact that the liquidambar is deciduous does not therefore improve its ranking because its emission rate during the smog season is extremely high.
All trees have their benefits, but none of the benefits can “neutralize” hydrocarbon emissions when Federal Ozone Standards need to be met for the health of life on this planet.
Saul Kay’s letter: Some hydrocarbons are only emitted in the presence of sunlight, so their emission rates were based on a 12-hour day. The emission rate for such hydrocarbons during the 12 hours of darkness is zero.
All trees can benefit the atmosphere by taking in carbon dioxide, but there is very little variance in rates. So for a similar carbon dioxide removal rate, you can choose between a tree that emits very little hydrocarbons or quite a bit of hydrocarbons.
As the article states, “a large Teflon bag . . . inflated with purified air” surrounds the tree. All measures are taken to stimulate a natural environment for the tree during sampling, and at no time is the tree “gasping for air.”
Thomas Courtright’s letter:
In urban areas trees that emit hydrocarbons are bad for the environment; hydrocarbons react to produce smog. Improving air quality means controlling emissions from all sources.
I agree that planting only a few species of trees is ecologically dangerous. More emissions inventories need to be conducted to expand the list of low-emitting trees. However, we cannot use this as an excuse to ignore the potential of high-emitting trees to adversely impact control strategies for controlling urban ozone. Smog is a serious threat today; planting low-emitting trees is a way to avoid worsening this problem for the future.
