In most of these studies, the plants were exposed to roughly twice the amount of carbon dioxide currently in the atmosphere (the gas makes up about 0.036 percent of atmospheric gases today). This is the amount of the gas that many scientists predict will be present in the atmosphere in the year 2100, said Curtis.
"Unlike controversy surrounding global warming, the increase in atmospheric carbon dioxide itself is beyond argument," said Irakli Loladze, an ecologist at Princeton University in New Jersey.
The researchers found that elevated carbon dioxide levels had an overwhelmingly positive effect on the reproductive traits of crop plants. The analysis revealed a 25 percent increase in total seed weight, a 19 percent increase in the number of flowers, a 16 percent increase in number of seeds, and a 4 percent increase in individual seed weight.
The scientists detail their findings in the October issue of the botanical journal New Phytologist.
Paying the Price
The study confirms what many people expected about fruit and seed yields, said Curtis.
"The surprise is that nitrogen levels actually go down" with elevated carbon dioxide levels, reducing the nutritional value of these foods, he said. Nitrogen is most commonly present in the form of protein.
Nitrogen levels were 15 to 20 percent lower in the seeds of many crop species tested. There are several theories why the nitrogen content decreased. One is that the nitrogen is simply more diluted amongst the larger mass of fruits and seeds, said Curtis.
Alternatively, it could be that the plants become more efficient at photosynthesisconverting sunlight and carbon dioxide into glucose, providing the energy needed for growingunder high carbon dioxide levels, reducing their requirement for nitrogen.
Some crop species known as legumesincluding peas and soybeansdidn't follow this trend. These plants showed an increase in yield and no change in nutritional value.
The increase in productivity does not make up for the fall in nutritional value of the crops, said Loladze. "Plants provide 84 percent of worldwide calorie intake, but they also are the major source of essential nutrients," he said.
Loladze's own research has shown that levels of other micronutrients such as iron, zinc, and iodine may also fall in carbon dioxide enriched species. High carbon dioxide levels may result in less "essential [dietary] elements per calorie," he said.
While both crops and wild species showed an average of 30 percent total increase in growth, wild plants tested didn't allocate this growth in the same way as crop plants. They were more likely to use the additional resources for protection in the form of chemical deterrents or toughened leaves, than in increased reproductive output, the researchers say.
"Six thousand to 10,000 years of breeding have caused domestic plants to put all their energy into seeds and fruits; this leaves them very poorly equipped to deal with the vagaries of nature," said Curtis.
"But that's okay because we protect them from insects, browsing animals, and disease. Wild plants do not have this luxury," he said.
In addition, the composition of wild plant communities may be adversely affected by increasing carbon dioxide levels as some plants will be better able to take advantage of the resources than others, said the researchers.
"There will be winners and losers," said Curtis. "We may end up with a bunch of fast growing weedy species."
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