Figure 24: Calculated growth rate enhancement of wheat, young orange and very young pine trees already taking place as a result of atmospheric enrichment by CO2 during the past two centuries (a) and expected to take place as a result of atmospheric enrichment by CO2 to a level of 600 ppm (b).
In this case, these values apply to pine trees during their first two years of growth and orange trees during their 4th through 10th years of growth. As is shown in figure 20, the effect of increased CO2 gradually diminishes with tree age, so these values should not be interpreted as applicable over the entire tree lifespans. There are no longer-running controlled CO2 tree experiments. Yet, even 2,000 year old trees still respond significantly as is shown in figure 17.