ARS | Publication request: THE ROLE OF WATER RELATIONS IN DRIVING GRASSLAND ECOSYSTEM RESPONSES TO RISING ATMOSPHERIC CO2

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 20, 2005
Publication Date: September 26, 2005
Citation: Morgan, J.A., Pendall, E., Mosier, A., Milchunas, D., King, J., Parton, B., Korner, C. 2005. The role of water relations in driving grassland ecosystem responses to rising atmospheric CO2. Seventh International CO2 Conference, Sept. 25-30, Boulder, CO.

Technical Abstract: While rising atmospheric CO2 is known to be an important contributor to radiative forcing of Earth’s climate, more direct effects of this gas on photosynthesis and plant water relations have been underway for more than a century, and likely have already contributed to important ecosystem changes. In semi-arid grasslands, CO2-induced changes in plant production and plant community species composition may be driven more by water relations than to the widely-accepted photosynthetic response. Stomata of most herbaceous plant species close in response to rising CO2 concentration, a reaction that results in increased leaf and plant water use efficiency, higher plant and soil water status, and altered seasonal evapotranspirational dynamics. Field CO2 enrichment experiments conducted in grasslands over the past 15 years confirm the importance of water relations in driving ecosystem plant production and species responses to CO2, and suggest that semi-arid regions like the Colorado shortgrass steppe may be among the world’s more responsive to rising atmospheric CO2. Further, differential species sensitivities either to elevated CO2 or to the resultant improvement in water relations can cause significant shifts in plant community species composition with important ecological and management consequences. CO2-enhanced plant productivity is generally accompanied by lower plant N concentration, due in large part to the inability of soil N release to keep pace with increased productivity. A widening of the ecosystem C/N ratio may eventually result from this dilution of plant N, and may feed-back and limit plant responses to CO2. A major challenge in CO2 enrichment research is in determining how to interpret short-term experiments which are conducted as small elevated CO2 islands in otherwise present-day environments. Such conditions do not incorporate land-atmosphere feed-backs to CO2 and future climate change, nor do they reflect the long-term soil nutrient dynamics which ultimately will determine ecosystem responses to global change. These problems are especially relevant to semi-arid ecosystems where CO2 responses are driven primarily by water, and will need to be addressed by combined modeling/experimental approaches to more realistically scale our small plot CO2 enrichment experimental results to a future CO2 enriched world.