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item Polley, Herbert
item Johnson, Hyrum
item Mayeux Jr, Herman
item Tischler, Charles

Submitted to: Proceedings Symposium Shrubland Ecosystem Dynamics Changing Climate
Publication Type: Proceedings
Publication Acceptance Date: 8/1/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: The abundance of shrubs on many rangelands increases as rainfall increases. The relationship of woody abundance to rainfall is determined in part by the amount of growth or carbon uptake that plants realize per unit of water consumed (water use efficiency). Water use efficiency may increase as the concentration of carbon dioxide gas in the air rises as the result of human burning of fossil fuels, potentially increasing the abundance of woody plants on some rangelands. Carbon uptake per unit of water used by three shrubs that recently have increased in abundance on rangelands in the southwestern U.S. (mesquite, huisache, and threadleaf snakeweed) approximately doubled for a doubling in carbon dioxide concentration in air, both over low carbon dioxide concentrations representative of the past and over elevated levels expected in the future. For snakeweed, the increase in water use efficiency resulted mainly from slower rates of water use. For mesquite and huisache, the increase in water use efficiency at higher carbon dioxide levels resulted at least as much from faster carbon uptake as from slower water usage. By increasing carbon uptake and growth or reducing water usage, rising carbon dioxide concentrations may favor an increase in the abundance of woody plants on some rangelands that formerly were too dry to support shrubs.

Technical Abstract: Water availability controls plant productivity and the abundance of shrubs on many rangelands. That control is mediated by factors such as atmospheric CO2 concentration that influence plant water use efficiency. Increasing CO2 over past and projected levels elicited a similar (mesquite, Prosopis glandulosa; huisache, Acacia smallii) or possibly greater (threadleaf snakeweed, Gutierrezia microcephala) relative increase in intrinsic water use efficiency (photosynthesis/conductance) of shrubs that recently have proliferated on Southwestern rangelands. The contribution of higher photosynthesis to the increase differed among species, however. By increasing productivity and competition for light, higher water use efficiency may favor taller woody plants over grasses on some rangelands. Rising CO2 may most benefit shrubs that realize much of the increase in water use efficiency as higher photosynthesis.