The increasing atmospheric CO2 concentration probably will have significant direct effects on vegetation whether predicted changes in climate occur or not. Averaging over many prior greenhouse and growth chamber studies, plant growth and yield have typically increased more than 30%, with a doubling of CO2 concentration. Such a doubling also causes stomatal conductance to decrease about 37 which typically increases leaf temperatures more than 1-degrees-C, and which may decrease evapotranspiration, although increases in leaf area counteract the latter effect. Interactions between CO2 and climate variables also appear important. In one study the growth increase from near-doubled CO2 ranged from minus 60% at 12-degrees-C to 0% at 19-degrees-C to plus 130% at 34-degrees-C, suggesting that if the climate warms, the average growth response to doubled CO2 could be consistently higher than the 30% mentioned above. Even when growing in nutrient-poor soil, the growth response to elevated CO2 has been large, in contrast to nutrient solution studies which showed little response. Several studies have suggested that under water-stress, the CO2 growth stimulation is as large or larger than under wellwatered conditions. Therefore, the direct CO2 effect will compensate somewhat, if not completely, for a hotter drier climate. And if any climate change is small, then plant growth and crop yields will probably be significantly higher in the future high-CO2 world.
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