Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/25/2007
Publication Date: 9/11/2007
Citation: Morgan, J.A., Milchunas, D.G., Lecain, D.R., West, M.S., Mosier, A. 2007. Carbon dioxide enrichment alters plant community structure and accelerates promotes shrub growth in the shortgrass steppe. Proceedings of the National Academy of Sciences 104(37):14724-14729. Interpretive Summary: Rising concentrations of greenhouse gases in Earth’s atmosphere are causing many meteorologists to project a warming of Earth’s climate, with some potentially catastrophic impacts for human society. While such changes in climate undoubtedly have important consequences for Earth’s ecosystems, those ecosystems may also respond directly to one of the more important greenhouse gases, carbon dioxide (CO2), since it is a substrate for photosynthesis, and also because increase in its concentration tend to increase plant water use efficiency. These responses can be perceived as being positive, resulting in a sort of CO2-induced greening of the Earth. However, we are finding that there are possible negative outcomes of this fertilization. In the present experiment, we used large open-top chambers to increase the ambient CO2 concentration over native Colorado shortgrass steppe vegetation for five years to levels we anticipate will be experienced later in this century. Because of different species’ sensitivities to CO2, we observed a shift in the plant species of the native shortgrass community, favoring a couple of species with low forage quality. One of the CO2-favored species, fringed sage, is a woody plant, and its expansion under artificially elevated CO2 confirms earlier predictions, but never substantiated in a field experiment, that rising CO2 concentrations in the past 200 years since the beginning of the industrial revolution have been partly responsible for the expansion of woody vegetation into many world grasslands. These findings of species shifts in response to CO2 suggest that the problem of woody plants taking over many world rangelands may be at least partly attributed to greenhouse gas emissions and their direct effects on plant physiology.
Technical Abstract: Doubling atmospheric carbon dioxide concentrations [CO2] over native Colorado shortgrass steppe progressively increased plant cover over a five year period in Artemisia frigida Willd, fringed sage, a common shrub of North American and Asian native grasslands. Plant community species composition in plots exposed to doubled [CO2] and present ambient [CO2] became increasing dissimilar over time, although commonly-used indices of species diversity were insensitive to CO2 treatment. The results suggest that rising atmospheric CO2 has the potential to cause substantive changes in world grasslands like the shortgrass steppe due to disproportional functional group and species sensitivities to CO2.