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Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe

Morgan, Jack A., Milchunas, Daniel G., LeCain, Daniel R., West, Mark, Mosier, Arvin R.
Proceedings of the National Academy of Sciences of the United States of America 2007 v.104 no.37 pp. 14724
elevated atmospheric gases, carbon dioxide, grasslands, steppes, plant ecology, community structure, shrubs, grasses, plant growth, environmental impact, community ecology, species diversity, vegetation cover, dry matter accumulation, Artemisia frigida, semiarid zones, botanical composition, Colorado
A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO₂], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO₂, and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO₂. However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO₂] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO₂]. Growth at the doubled [CO₂] resulted in an [almost equal to]40-fold increase in aboveground biomass and a 20-fold increase in plant cover of Artemisia frigida Willd, a common subshrub of some North American and Asian grasslands. This CO₂-induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO₂] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.