What was done
Seedlings of eight woody species, varying widely in life-form and habit (four evergreen and four deciduous), were grown for approximately two months in experimental chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm to determine if CO2-induced growth responses could be correlated with, and thus predicted by, an easily measurable foliar characteristic.

What was learned
The relative growth rates of all eight woody species increased significantly after two months of atmospheric CO2 enrichment, with the greatest proportional increases occurring for species exhibiting the lowest absolute dry weights.  As anticipated, the four deciduous species used in this study exhibited greater growth responses to elevated CO2 than did the four evergreen species.

After measuring various leaf characteristics and correlating them with observed CO2-induced growth responses, it was determined that the specific leaf area (SLA- ratio of leaf area to leaf weight) of plants growing at ambient CO2 concentration positively correlated with CO2-induced growth responses, accounting for 88% of the observed variation in relative growth rates measured among species exposed to elevated CO2.  Thus, it was not surprising to discover that the deciduous species had higher SLAs than the evergreen species, which commonly exhibit lower SLAs due to greater investments in structural carbohydrates and other leaf components that protect and lengthen the lifespans of individual leaves.

What it means
As the CO2 concentration of the air continues to increase, it is likely that woody plants will exhibit substantial increases in growth that vary with life form and habit.  Due to the strong positive correlation that exists between SLA and CO2-induced relative growth rates for woody plants, it may be possible to predict their growth responses to future increases in the air's CO2 content.  However, caution must be used when making such predictions, as this relationship was derived from plants grown individually in isolation and not in competition with neighboring plants in mixed assemblages.