Photo by Jaako
I was a little surprised to see this article in Wired Magazine:
Old-Growth Forests Can Actually Contribute to Global Warming
It reads like a press release for the logging industry.
The author, Matt Power, asserts that “over its lifetime, a tree shifts from being a vacuum cleaner for atmospheric carbon to an emitter.” Thus, he argues, mature forests contribute to global warming and we should log them, managing the trees like crops.
He refers to a study by the Canadian government that (according to Power) “found that during many years, Canadian forests actually give up more carbon from decomposing wood than they lock down in new growth.”
Not so fast.
I’ll quote directly from the article that Power links to as a source:
between 1990 and 2005 Canada’s managed forest was an overall sink except during five years when it was an overall source, due mainly to emissions from extensive natural forest fires. As well, since 1999, the mountain pine beetle (Dendroctonus ponderosae) has killed trees in about 10 million ha in central British Columbia, increasing emissions as the trees decay.
In other words, for 10 of the past 15 years, Canadian forests have absorbed more carbon than they emitted. The other five years, the forests emitted carbon because of forest fires and a disastrous pine beetle infestation. Nowhere in the article does it say that the forests emit carbon because they have matured.
Another paper used as a source in Power’s article also contradicts his main conclusion. This one says:
After logging or fires, the forest emits carbon due to the mineralisation of the organic debris left on the soil. This emission does not compensate for the C sequestration by the young trees, which gives a negative balance. The net C sequestration becomes positive after a variable lapse of time, from 2 to 10 years for fast-growing species like maritime pine and several dozen years for certain boreal forests. Then, the net sequestration decreases when the trees grow old.
It takes “several dozen years” for boreal forests to start absorbing carbon after logging. I don’t see how Power could have come to the conclusion that logging those forests would be a good way to fight atmospheric CO2.
Power even uses this same article to back up his claim that: “Left untouched, it [the tree] ultimately rots or burns and all that CO2 gets released.” I read and reread the abstract and didn’t see a single reference to trees releasing carbon because of age.
Of course, rotting trees do release carbon. But in a mature forest, the trees don’t die and decay all at once. A clearcut, however, releases about two thirds of its carbon immediately.
It turns out that forests hundreds of years old can continue to actively absorb carbon, holding great quantities in storage. Resprouting clear-cuts, on the other hand, often emit carbon for years, despite the rapid growth rate of young trees. This is because decomposer microbes in the forest soil, which release CO2 as they break down dead branches and roots, work more quickly after a stand is logged. On the dry eastern face of the Cascades, for example, where trees grow slowly, a replanted clear-cut gives off more CO2 than it absorbs for as much as 20 years.
Quoted from “The Giving Trees.”
Mature forests offer many other benefits. They are havens for biological diversity. Many species will only live in mature, contiguous forests. Healthy ecosystems will better withstand the stresses of global warming.
Forests regulate the water cycle, filtering it, slowing it, and preventing erosion and flooding. Trees serve aquatic life by shading streams and rivers.
And the effect of forests on weather and climate go beyond carbon sequestration. Trees serve as windbreaks and absorb sunlight (heat).
I wish Wired would fact check their writers before publishing such misleading and irresponsible opinions.