"Based on earlier single-treatment studies with elevated CO2, we initially hypothesized that, with the combination of all four treatments together, the response would be additional growth," said W. Rebecca Shaw, a researcher with the Nature Conservancy of California and lead author of the Science study.
But results from the third year of the experiment revealed a more complex scenario. While treatments involving increased temperature, nitrogen deposition or precipitation alone or in combination promoted plant growth, the addition of elevated CO2 consistently dampened those increases.
"The three-factor combination of increased temperature, precipitation and nitrogen deposition produced the largest stimulation [an 84 percent increase], but adding CO2 reduced this to 40 percent," Shaw and her colleagues wrote.
The mean net plant growth for all treatment combinations with elevated CO2 was about 4.9 tons per acre compared to roughly 5.5 tons per acre for all treatment combinations in which CO2 levels were kept normal. However, when higher amounts of CO2 gas were added to plots with normal temperature, moisture and nitrogen levels, aboveground plant growth increased by nearly a third.
Why would elevated CO2 in combination with other factors have a suppressive effect on plant growth? The researchers aren't sure, but one possibility is that excess carbon in the soil is allowing microbes to outcompete plants for one or more limiting nutrients.
"By applying all four treatments, we may be repositioning the ecosystem so that another environmental factor becomes limiting to growth," Field observed. "For example, by increasing plant growth as a result of adding water or nitrogen, the ecosystem may become more sensitive to limitation by another mineral nutrient such as phosphorous, potassium or something else we hadn't been measuring."
A new five-year experi
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Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
5-Dec-2002