As the CO2 content of air rises, many plants exhibit increased rates of net photosynthesis and biomass production. Plant water use efficiency is defined as the amount of carbon gained per unit water lost per unit leaf area. Plant water use efficiency among many field crops and grasses is directly related with atmospheric CO2 enrichment. Thus, from agriculture's perspective, as CO2 content of air continues to rise, plant life in general should exhibit increases in water use efficiency and biomass production. In Canada, Rosenzweig et al. (1993) analyzed model scenarios and found that a 2 °C increase in temperature with no precipitation change resulted in wheat yield increases with the direct effect of CO2 taken into account.
The productive capacity of a plant is the net resultant of two processes, pho-tosynthetic fixation of carbon dioxide and its release by respiration (Rosenberg, 1974). Crop species vary in their response to CO2.Wheat, rice, soybeans, cotton, oats, barley, and alfalfa belong to a physiological class called C3 plants that respond readily to increased CO2 levels. Corn, sorghum, sugarcane, millet, and other tropical grasses are C4 plants, though more efficient photosynthetically than C3 plants at present levels of CO2, tend to be less responsive to enriched concentrations. C3 crops show an average increase in et primary production of approximately 33% for a doubling of atmospheric CO2 (Koch and Mooney, 1996). Some field studies show C4 plants responded to elevated CO2 due to increased water use efficiency (Owensby et al., 1993).
Elevated atmospheric CO2 concentrations tend to reduce the size of open stom-atal pore space on leaf surfaces. Thus, plants tend to display lower stomatal conductances which effectively reduces the amount of water lost to the atmosphere via transpiration. As a result, the soil moisture content would likely rise with increased atmospheric CO2 content, given no change in precipitation trend.
However, the reduced transpiration may be offset by higher evaporation at higher temperatures. While water use efficiency may increase under this scenario, the effects of higher temperatures may negate any beneficial effects. For example, increased temperatures may accelerate the rate at which plants release CO2 through respiration, resulting in less tan optimal conditions for net growth (Rosenzweig and Hillel, 1995).
Highly productive forest ecosystems have the greatest potential for absolute increases in productivity due to CO2 effects. Studies have shown a stimulation of photosynthesis of about 60% for a doubling of CO2 (Saxe et al., 1998; Norby et al., 1999). A fast growing young pine forest showed an increase in 25% in net primary production for an increase in atmospheric CO2 to 560 ppm (DeLucia et al., 1999). Slowing deforestation and promoting natural forest regeneration and afforestation could increase CO2 storage.
The direct biological effects of atmospheric CO2 enrichment tend to mimic a warming and moistening of the environment. This is expected because plant optimum temperatures and water use efficiencies both rise. Studies have shown that woody species, such as oak trees, will gradually expand onto arid and semiarid grasslands due to increased precipitation patterns, especially during the summer growing season. This is a likely scenario in the southwestern United States where climate models generally predict a tendency toward increasing precipitation.
The rise in atmospheric CO2 should also have a significant positive effect upon pasture and rangeland productivity, based on research studies. CO2 enrichments appear to slightly augment the legume content of grasslands, providing more nitrogen to the ecosystem which promotes the nutritive quality of the forage. Increases in the air's CO2 content should enable pasture and rangeland plants to better cope with water deficits.
Experiments have also shown that elevated CO2 consistently enhanced rates of net photosynthesis in upland cotton (Ready et al., 1999). These studies indicated that although elevated CO2 did not significantly impact boll size or maturation, it did increase boll numbers by about 40% regardless of temperatures, without changing fiber properties. The results infer that if air temperatures in cotton-growing areas of the United States increase in future years, the predicted rise in the air's CO2 content will enhance cotton photosynthesis rates, boll production, and fiber yields without altering fiber quality. It remains unclear, however, if the enhanced photosynthesis due to the direct effect of higher CO2 will be offset at least partially by projected higher moisture stress in some areas.
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