7 August 2007
Carbon Sink Sunk
by Kate Melville
Results from a decade-long experiment into how much carbon dioxide (CO2) pine trees can soak up show that the trees need plentiful water and large scale fertilization to act as efficient carbon-sinks. Scientists from Duke University involved in the Free Air Carbon Enrichment (FACE) experiment told attendees at the Ecological Society of America conference that proposals to "bank" extra CO2 in trees may depend on the vagaries of rainfall and large scale forest fertilization efforts.
"If water availability decreases to plants at the same time that carbon dioxide increases, then we might not have a net gain in carbon sequestration," said Ram Oren, a professor of ecology at Duke's Nicholas School of the Environment and Earth Sciences and FACE project director.
"In order to actually have an effect on the atmospheric concentration of CO2, the results suggest a future need to fertilize vast areas," he added. "And the impact on water quality of fertilizing large areas will be intolerable to society. Water is already a scarce resource."
The FACE experiment consisted of four forest plots receiving extra CO2 from computer-controlled valves mounted on rings of towers, and four other matched plots receiving no extra gas. Encouragingly, some trees that were treated produced about 20 percent more biomass. But since the amounts of available water and nitrogen nutrients varied substantially from plot to plot, using averages could be misleading. "In some areas, the growth is maybe 5 or 10 percent more, and in other areas it's 40 percent more," Oren explained. "So in sites that are poor in nutrients and water we see very little response. In sites that are rich in both we see a large response."
The researchers also found that extra carbon dioxide had no effect on "self thinning" - the tendency of less-successful trees to die off as the most-successful grow bigger. "We didn't find that elevated CO2 caused any deviation from this standard relationship," said co-researcher Heather McCarthy. Also unchanged by the CO2 enrichment were the proportions of carbon atoms that found their way to various components of plant systems - wood, leaves, roots and underlying soil. Only a few of those components will store carbon over time, noted Oren and McCarthy.
"Carbon that's in foliage is going to last a lot shorter time than carbon in the wood, because leaves quickly decay," McCarthy said. "So elevated CO2 could significantly increase the production of foliage but this would lead to only a very small increase in ecosystem carbon storage."
Source: Duke University