By Robert C. Balling Jr., Ph.D.
Arizona State University

Drought continues to be a hot topic these days—and more than a few greenhouse apocalysts say its effects are worse when atmospheric carbon dioxide increases. But the biological literature is alive with articles showing that, throughout the world, elevated CO₂ will help plants throughout the world cope with dry conditions.

In one study, Norway spruce and beech saplings grew for four years at natural CO₂ levels and natural plus 200 parts per million (ppm); soil nitrogen was varied, along with underlying soil type. Higher CO₂ substantially increased the water-use efficiency from 14 percent to 25 percent depending on soil type and nitrogen fertilization.

French scientist Alain Cousson grew a wildflower at natural (350 ppm) and doubled (700 ppm) CO₂ levels, discovering via plant dissection that the latter reduced the plants pores by more than half, which would undoubtedly reduce water loss and plant stress from potentially harmful gaseous molecules (e.g., ozone) and small aerosols.

British scientists grew potatoes at CO₂ levels of 370 ppm, 550 ppm, and 680 ppm, with varying ozone (O₃) concentrations, discovering that the elevated CO₂ reduced seasonal mean leaf conductance by 41 percent to 55 percent. Water-use efficiency increased progressively with elevated CO₂, whatever the concentration of O₃. Elevated CO₂ also caused a 12 percent increase in total leaf thickness. They write: "Such effects may have important beneficial implications for crop production under future climatic conditions, which are expected to be warmer and drier and involve [increased CO₂]."

Wullschleger and colleagues elevated the atmospheric CO₂ concentration in a 12-year-old stand of sweetgum trees from 390 ppm to 540 ppm. The team reported that the elevated CO₂ resulted in a 7 percent reduction in evapotranspiration—yet further evidence that elevated CO₂ substantially increases the water-use efficiency, not only saving water but also protecting against drought-related stress.

These four recent studies, when added to the thousands of similar reports, show us that elevated CO₂ decreases stomatal conductance and increases photosynthesis and biomass, combining to give plants a substantial increase in water-use efficiency. That allows plants to use less water, cope better with drought, and survive in areas currently too dry to support their growth.

References:

Bucher-Wallin, I.K., et al., 2000. Effects of elevated CO₂, increased nitrogen deposition and soil on evapotranspiration and water use efficiency of spruce-beech model ecosystems. Phyton, 40, 49–60.

Cousson, A. 2002. Carbon dioxide and ferricyanide parallel each other to inhibit Commelina stomatal opening in a putative Ca²⁺-independent fashion. Journal of Plant Physiology, on-line early release.

Lawson, T., J. et al., 2002. Impact of elevated CO₂ and O₃ on gas exchange parameters and epidermal characteristics in potato (Solanum tuberosum L.). Journal of Experimental Botany, 53, 737–746.

Wullschleger, S.D., et al., 2002. Sensitivity of stomatal and canopy conductance to elevated CO₂ concentration—interacting variables and perspectives of scale. New Phytologist, 153, 485–496.