Forest carbon-sink potential explored
If forests worldwide were to grow 25 percent faster in 50 years than they do now they could serve as a "sink" for about half of the expected emissions of carbon dioxide
May 14, 1999
Web posted at: 4:10 PM EDT
Experimental forest plots pumped up with atmospheric carbon dioxide at levels expected by the year 2050 experienced a 25 percent growth increase, but researchers doubt the plots can sustain such growth.
The initial findings of the research suggest that if forests worldwide were to grow 25 percent faster in 50 years than they do now they could serve as a "sink" for about half of the expected emissions of carbon dioxide from the burning of fossil fuels.
However, the researchers, who reported their findings today in the journal Science, caution that such a high sustained uptake is unlikely. Open-air studies at an Italian hot springs that naturally emits increased carbon dioxide suggest that carbon dioxide-inspired growth spurts will decline within a few years.
"The crux of the matter is that vegetation can respond to higher CO2 and act as a carbon sink," said William Schlesinger, a Duke University botany professor and co-author of the paper. "The 25 percent growth increase is probably an upper limit for what the world's vegetation can do. Nevertheless, it's interestingly high."
The forest where the study is being conducted is dominated by 13-year-old loblolly pines -- among the fastest growing tree species -- at their peak growing age. Through the use of specially designed equipment, the trees are receiving atmospheric concentrations of carbon dioxide at 560 parts per million. Today's concentration is 360 parts per million.
The experiment is "fully replicated," meaning that it is being repeated at three locations. Three other identical tower-ringed forest plots that receive no extra CO2 are serving as controls so their responses can be compared to the three active sites.
In 1997, the first complete year of the replicated study, the overall growth rate of the dominant pine trees and underlying hardwoods, shrubs and vines increased 16 percent in the extra-CO2 plots when compared to the control plots, the authors reported in Science.
In 1998, that increase swelled to 25 percent, an addition that to some degree reflected the inclusion of fine root growth that was not measured in 1997.
Both Evan DeLucia, a University of Illinois plant biologist and the other co-author of the article, and Schlesinger noted that those results occurred during two growing seasons that were affected by droughts.
"The growth rates declined in the control plots as a result of the drought," while the high-CO2 plots "were able to make up for the drought," Schlesinger said.
It may be that plants growing at higher carbon dioxide levels can better conserve water, he said. Separate studies have shown that plant stomata -- leaf pores that regulate water release -- do not open as widely in an environment of elevated CO2.
"In the few studies that have looked at how trees, such as those growing next to natural springs of CO2, respond over long periods of time, growth response is strong at first, then decays," DeLucia said. "Other models also predict this response. The mechanisms for this slowing of the growth response are not entirely clear. One reason is that trees may acclimate by slowing their photosynthesis rates."
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