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Water Use Efficiency of Temperate Grassland Species Soars in Elevated CO2 Environment
Engloner, A.I., Kovacs, D., Balogh, J. and Tuba, Z.  2003.  Anatomical and eco-physiological changes in leaves of couch-grass (Elymus repens L.), a temperate loess grassland species, after 7 years growth under elevated CO2 concentration.  Photosynthetica 41: 185-189.

What was done
Grassland monoliths with their original soils to a depth of 40 cm were removed from a xeric temperate loess grassland and relocated to open-top chambers at the Global Climate Change and Plants Experimental Ecological Research Station outside Budapest, Hungary, where they were exposed for seven years to either ambient air or air enriched to a CO2 concentration of 700 ppm.  Measurements of leaf photosynthesis and transpiration rates of a dominant member of this temperate loess grassland (steppe) ecosystem - couch-grass (Elymus repens (L.) Gould.) - were conducted throughout the experiment according to protocols described by Tuba et al. (1994, 1996), while measurements of starch and soluble sugars were measured as described by Tuba et al. (1994).

What was learned
Rates of net photosynthesis increased by an average of 194% in response to the ~90% increase in atmospheric CO2 concentration, leading to starch and soluble sugar increases of approximately 50 and 72%, respectively.  Simultaneously, leaf transpiration rates declined by about 18%, leading to a whopping 345% increase in water use efficiency.

What it means
For a plant described as a xeric grassland species, a three-and-a-half-fold increase in the efficiency with which it uses water to produce organic matter has got to be like manna from heaven, providing it with an incredible amount of "staying power" in the face of the woody-plant invasions of grasslands that are currently occurring throughout the world in response to the historical and still-ongoing rise in the air's CO2 content [see Trees (Range Expansions) in our Subject Index for more on this topic].  This phenomenon should also allow couch-grass to expand its own range into much drier areas than it has ever before been able to colonize.  It will be interesting to see how it and earth's many other plants, essentially all of which are benefited by higher atmospheric CO2 concentrations, spread abroad and interact with each other in the creation of new and more complex ecosystems in the years and decades ahead.

Tuba, Z., Szente, K. and Koch, J.  1994.  Response of photosynthesis, stomatal conductance, water use efficiency and production to long-term elevated CO2 in winter wheat.  Journal of Plant Physiology 144: 551-668.

Tuba, Z., Szente, K., Nagy, Z., Csintalan, Z. and Koch, J.  1996.  Responses of CO2 assimilation, transpiration and water use efficiency to long-term elevated CO2 in perennial C3 xeric loess steppe species.  Journal of Plant Physiology 148: 356-361.

Reviewed 28 January 2004