Although ozone slows plant growth, the beneficial effect of the carbon dioxide more than compensates for this effect, Leakey found. His unpublished results predict an increase in soy yields of 13% by 2050. US farmers currently plant about 150 million acres of soybean a year.
The following press release emphasies that the increased plant growth in the presence of higher CO2 is not enough to take all the CO2 out of the atmosphere. But the fact that the trees and plants grow more rapidly is economically valuable.
OAK RIDGE, Tenn., Feb. 16, 2004 -- Trees absorb more carbon dioxide when the amount in the atmosphere is higher, but the increase is unlikely to offset the higher levels of CO2, according to results from large-scale experiments conducted at Oak Ridge National Laboratory and elsewhere.
"Some people have used carbon dioxide fertilization to argue that this is a boon of the fossil fuel era and that it will lead to greater agricultural yields and carbon sinks," said Richard Norby of the Department of Energy's ORNL. "Some recent experiments, however, have suggested that there will be no lasting effect of carbon dioxide fertilization. As is often the case, the truth may lie in between."
Norby is among several scientists participating in a panel discussion titled "CO2 Fertilization: Boon or Bust?" Feb. 16 at the American Association for the Advancement of Science annual meeting in Seattle.
For the last six years, Norby and colleagues at ORNL have examined the responses to elevated carbon dioxide levels in a stand of sweetgum trees a few miles from ORNL. The experiment consisted of pumping tons of carbon dioxide into the plots, raising the concentration of carbon dioxide in the tree stand from the ambient level of about 370 parts per million to 550 ppm, and studying the effects.
In every year since the FACE project began, net primary productivity, which is the total amount of carbon dioxide fixed into organic matter such as leaves, stems and roots, has been higher in plots given extra carbon dioxide. The average increase has been 24 percent, and there is no indication that the increase will not continue. But, Norby notes, while his colleagues have observed a sustained increase in leaf photosynthesis, the response to carbon dioxide fertilization would not be apparent if only above-ground growth were measured. Wood production increased significantly during only the first year of treatment.
While Norby and colleagues have learned a great deal about above-ground allocation of carbon dioxide, in recent years they have focused their efforts on impacts on fine roots and soil sequestration of carbon dioxide. Fine root production has increased substantially in response to elevated carbon dioxide.
Fine roots are important for water and nutrient uptake, but they have a short life and their carbon returns to the soil within a year. Initial results suggest that the increase in carbon supply to fine roots has increased the carbon content of the soil. Norby cautions, however, that the positive effect of carbon dioxide fertilization is insufficient to halt the rising level of atmospheric carbon dioxide.
If some types of forest trees will grow more rapidly then higher atmospheric CO2 holds the prospect of lowering timber costs and hence of lowering housing and furniture costs.
SEATTLE -- A futuristic Duke University simulation of forest growth under the carbon dioxide-enriched atmosphere expected by 2050 does not reinforce the optimism of those who believe trees can absorb that extra CO2 by growing faster, said a spokesman for the experiment.During seven years of exposure to carbon dioxide concentrations 1½ times higher than today's, test plots of loblolly pines have indeed boosted their annual growth rates by between 10 and 25 percent, found the researchers. But "the highest responses have been in the driest years, and the effect of CO2 has been much less in normal and wet years," said William Schlesinger, a professor of biogeochemistry and dean of Duke's Nicholas School of the Environment and Earth Sciences.
These counterintuitive findings suggest that nitrogen deficiencies common to forest soils in the Southeastern United States may limit the abilities of loblolly pine forests to use the extra CO2 to produce more tissues as they take in more of the gas, he said.
"In a dry year trees naturally grow less so the amount of nitrogen doesn't make any difference," he said. "In a wet year, when there's plenty of water, the amount of nitrogen does make a difference." Tree growth depends on the availability of nitrogen, which foresters routinely add to Southeastern soils in the form of fertilizer when they plant trees, he added.
One advantage the plants may have in dry years is that with more CO2 in the atmosphere the leaves do not have to open their pores as much to let in the CO2. This reduces water loss from evaporation and allows plants to grow in dry environments. This explanation has been put forward to explain plant growth into the Negev desert in Israel.
The really bad news? More poison ivy:
Meanwhile, some other species in Duke's CO2-bathed forest plots have grown at faster rates than the loblolly pines, scientists report. Still-unpublished data shows 70 percent growth increases for poison ivy, according to Schlesinger.
It seems likely that the growth increase caused by higher CO2 will differ by tree species. Some will experience larger increases in growth rates and others will benefit from higher CO2 to a lesser extent. Also, since water is more of a rate-limiting factor in some areas and less in other areas the extent of the benefit of higher CO2 in terms of faster growth in lower water conditions will be greater in some geographic regions and less in other regions. Higher CO2 probably will increase total tree cover in drier areas and may even make it possible to grow trees into deserts as appears to be happening with the Negev.
Another factor to consider: It should be possible to select for or genetically engineer crop plants that will grow even faster in higher CO2 conditions. So the extent of the benefit of high CO2 seen with existing crop plants understates the size of the benefit likely to be achievable in the longer run.
Of course, higher atmospheric CO2 levels will cause many other effects. If higher CO2 raises global temperatures it could change precipitation patterns, total global precipitation, length of growing seasons (generally longer), wind patterns, and other many other factors. How will all this work out in terms of benefits and costs? It seems impossible at this point to hazard a guess that will have any degree of accuracy. But it seems clear that rising atmospheric CO2 will generate not just costs but benefits as well.
|Share |||Randall Parker, 2004 February 17 03:22 PM Climate Trends|