Pine needles gathered by Great Basin packrats during the past 30,000 years show evidence of physiological response to increasing atmospheric carbon dioxide at the end of the last Ice Age. Measurements from nearly 1200 limber pine needles preserved in packrat middens from Idaho, Utah, Nevada and Arizona show that the number of stomata on leaf surfaces decreased as carbon dioxide levels increased during the last deglaciation. (Stomata are the leaf pores through which carbon dioxide is gained for photosynthesis and water is lost in evapotranspiration.) Results indicate that a 30% increase in atmospheric carbon dioxide between 15,000 and 12,000 years ago produced a 17% decrease in stomatal density and a 12% increase in the water use efficiency of limber pine. The new evidence about direct plant responses to changing carbon dioxide effects paleoclimatic estimations that are based on former plant distributions since traditional, paleobotanical evidence has been translated into temperature and precipitation estimates without regard to direct carbon dioxide effects. Thus, during the last Ice Age, limber pine, and not ponderosa pine, occurred across large expanses of the Great Basin lowlands and Colorado Plateau. Did this happen because ponderosa pine could not tolerate the dry summers, or because limber pine can grow at much lower water-use efficiencies (carbon dioxide levels) than ponderosa pine? To address this question, Betancourt and his colleagues are now examining other plant species found in packrat middens.

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For additional information and references, see the project description, Biotic Response to Climatic Variability and Human Impacts in Arid Lands , or contact:
Julio L. Betancourt
U.S. Geological Survey, Desert Laboratory, 1675 W. Anklam Road, Tucson, AZ 85745
Email: jlbetanc@usgs.gov
Phone: 602-670-6821