More CO2 Means Less Water Stress for Plants
Another major benefit of atmospheric CO2 enrichment is that plants exposed to elevated levels of atmospheric CO2 generally do not open their leaf stomatal pores - through which they take in carbon dioxide and give off water vapor - as wide as they do at lower CO2 concentrations. In addition, they tend to produce less of these pores per unit area of leaf surface at higher levels of atmospheric CO2. Both of these changes tend to reduce most plants' rates of water loss by transpiration; and the amount of carbon they gain per unit of water lost - or water-use efficiency - therefore typically rises, greatly increasing their ability to withstand drought.
The reduction in leaf evaporation rate produced by this phenomenon can be as much as a third for a doubling of the air's CO2 content; and combining this effect with the simultaneous 30 to 50% CO2-induced increase in plant productivity, which is described in More CO2 Means More Plant Growth, dramatically increases the efficiency with which individual leaves utilize water to produce organic matter. In many cases, in fact, a doubling of the atmospheric CO2 concentration actually doubles leaf water use efficiency.
A second way by which atmospheric CO2 enrichment improves plant water relations is by increasing plant turgor, which is essential for proper vegetative growth and development. Very briefly, this phenomenon begins with a CO2-induced increase in leaf carbohydrate concentration, which then enhances leaf osmotic potential and ultimately helps to maintain adequate leaf water contents for continued photosynthesis and growth in the face of declining soil moisture availability.
The many ramifications of these several atmospheric CO2 enrichment effects on plant water relations are truly impressive. As the atmosphere's carbon dioxide concentration rises ever higher in the years ahead, nearly all plants should be able to grow where it is presently too dry for them, enabling the most drought-resistant species to reclaim great tracts of land previously lost to desertification. Greater vegetative cover should also reduce the adverse effects of soil erosion caused by the ravages of wind and rain.