What was done
The percentage growth enhancement resulting from a 300 ppm increase in the air's CO2 content was analyzed for three tree species rooted in the ground and exposed to elevated CO2 for a number of years to determine the long-term response of woody perennials to atmospheric CO2 enrichment.  In addition, numerous CO2 enrichment studies employing woody species grown in pots or other containers were analyzed for comparison.

What was learned
In analyzing 176 CO2 enrichment experiments, wherein woody species were grown in containers with finite rooting volumes, it was discovered that almost any conceivable growth response could be obtained within a few short years.  However, such responses declined dramatically with time, suggesting that any CO2-induced growth enhancements observed in root-restricted plants were unlikely to persist beyond five years.

In contrast, in analyzing the results of four CO2 enrichment studies, wherein trees were rooted in the ground and not restricted by rooting volumes, it was discovered that their CO2-induced growth enhancements were approximately 90% after five years of treatment.  In addition, the data suggested that after 30 years of exposure to atmospheric CO2 enrichment, their growth responses would still be as high as 25%.

What it means
As the CO2 content of the air continues to increase, woody species growing naturally in the ground will likely exhibit increases in productivity that will continue to persist for decades.  Although large initial stimulations may decrease over time, it is likely that elevated CO2 will still exert a positive influence on tree growth even after a century.  Thus, earth's forests and woody perennials have an extended opportunity to remove large quantities of carbon from the air and sequester it away in their tissues for long periods of time.