What was done
Over a period of four years and within 16 open-top chambers (half maintained at the mean ambient atmospheric CO2 concentration of 379 ppm and half at a mean elevated CO2 concentration of 704 ppm), the authors studied the effects of elevated CO2 on the aboveground biomass of three species of oak -- Quercus myrtifolia, Q. geminata, and Q. chapmanni -- in a fire-regenerated scrub-oak ecosystem on a barrier island off the coast of Florida, USA, by means of allometric relationships between stem diameter and aboveground biomass, which they derived from destructive measurements made on trees growing on an adjacent site.

What was learned
In the words of the authors, "increased aboveground biomass in elevated CO2 was apparent after eight months (44%), and the relative stimulation increased over time, from 55% at the end of 1997, 66% at the end of 1998, to 75% at the end of 1999."  At the time of last measurement, the aboveground biomass of the dominant Q. myrtifolia had increased by 73%, while that of the subdominant Q. geminata had increased by only 23%, but that of the subdominant Q. chapmanni had risen in excess of 150%.  With respect to individual years, they also note that in spite of the fact the mean increase in total aboveground oak biomass during the drought year of 1998 was 51% lower than it was during 1997 and 54% lower than it was in 1999, "elevated CO2 significantly increased annual increment in aboveground biomass by 122% during the drought year 1998, compared to a 65% increase in 1997 and a 116% increase in 1999."

What it means
This long-term study of oak trees growing in a natural setting provides strong parallel evidence for the reality of the large CO2-induced growth stimulation that has long been evident in the sour orange tree study of Idso and Kimball (2001).  It also provides strong evidence for the observation of Curtis et al. (2003) that the meta-analysis of Jablonski et al. (2002) suggests that drought does not diminish the positive effects of atmospheric CO2 enrichment on plant growth and development.  Indeed, it indicates that it actually tends to enhance this stimulatory phenomenon, as was earlier demonstrated by the meta-analysis of Idso and Idso (1994).

References
Curtis, P.S., Jablonski, L.M. and Wang, X.  2003.  Assessing elevated CO2 responses using meta-analysis.  New Phytologist 160:6.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

Jablonski, L.M., Wang, X. and Curtis, P.S.  2002.  Plant reproduction under elevated CO2 conditions: a meta-analysis of reports on 79 crop and wild species.  New Phytologist 156: 9-26.