Explaining why carbon dioxide is 'sequestered'

Most people think of carbon dioxide as either the stuff they breathe out after taking in oxygen or the stuff that will eventually turn the planet into a furnace through global warming. But to ecological scientists like Princeton postdoctoral fellow John Caspersen, carbon is part of an age-old natural cycle and a telling indication of forest development.

In last week's issue of "Science" magazine, Caspersen published an article dealing with the carbon that is taken up by biomass — the collective term for the trees and other plants that absorb carbon dioxide.

"The gist of the article concerns why forests are sequestering carbon in the U.S.," Caspersen explained. Sequestering is the term used for the accumulation of carbon by forests. This sequestering is accomplished through photosynthesis — the process by which plants absorb carbon dioxide and use it to make sugars for energy.

According to Caspersen, only about half of the carbon that is released into the atmosphere will remain there. It can otherwise follow one of two alternatives. Some of the carbon will be absorbed into the ocean. The majority of it, however, is absorbed by biomass through photosynthesis.

The result is a considerably lower amount of carbon dioxide in the atmosphere than would be expected. Ecologists call this the "carbon sink."

This reduction of carbon dioxide in the atmosphere is viewed positively by scientists concerned with the effects that carbon can often have. It is a greenhouse gas that traps radiation inside the planet's atmosphere, resulting in global warming. It also mixes with water vapor in the upper atmosphere to produce acid rain.

Caspersen pointed out that there is much uncertainty regarding the cause of the carbon sink.

There are two hypotheses for the reasons behind the sequestering of carbon in U.S. forests.

The forest could be accumulating the carbon as a result of changing land use in the United States. This would mean that forests that had been damaged or destroyed through either natural or human-induced disasters are re-growing.

Thus the carbon sink would be attributed to the forests' gaining biomass, a phenomenon that would occur only temporarily until the forests had reached fairly adult size. After such time, the carbon sequestering would be greatly reduced and there would be much more carbon dioxide left in the air.

The other possibility is a process that Caspersen called "growth enhancement." Growth enhancement — the increase in tree growth in forest ecosystems — can result from any of several different factors, including the deposit of nitrogen and climate change.

The thought, though, is that this process is happening because carbon dioxide fertilizes the soil. In this scenario, the carbon sink would continue for an indeterminate time, possibly indefinitely.

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"The difficulty has been to separate those two things," Caspersen explained.

Determining the relative contributions of both these factors was the task that lay before Caspersen. His first inclination was to simply measure the current growth rate of the forests and compare it to growth rates of the past. Given the relative novelty of the studies, though, these necessary records simply did not exist.

Caspersen decided instead to find the average growth rate of the forests. The amount of biomass in the forest was divided by the forest's age, which can easily be determined by counting the rings of tree stumps.

Collecting data from five states across the country — Minnesota, Michigan, Virginia, North Carolina and Florida — with the help of the national forest service, Caspersen gathered a myriad of raw information. After a series of calculations, he finally produced a result that he could compare to the growth rate.

And what he found is that "the current growth rate is more or less consistent with the amount of biomass."

Since there was little discrepancy between average and current growth rates in these areas, Caspersen concluded that the carbon sink was the result of forest re-growth — not growth enhancement.

Caspersen's findings will have implications for ecologists and the way they plan for the future. The theory of the carbon sink has already begun to change the way experts deal with carbon dioxide in the atmosphere.

Knowing that sink will not last, ecologists are now trying to determine how to handle the excess carbon dioxide that will eventually be seen in the atmosphere.

Caspersen is a postgraduate working in the ecology and evolutionary biology department at the University. The article's coauthors were professor Stephen Pacala and postdoctorate Paul Moorcroft, both of the same department.

As an undergraduate at Oberlin College, Caspersen majored in ecological and evolutionary biology and later earned a Ph.D. in the same field at the University of Connecticut.

His interest has always been in forest ecology and ecosystems. Caspersen took an interest in the carbon cycle while examining global warming's effects.

It was not long before he combined these interests and skills to answer the questions that were on many ecologists' minds.