What was done
Three leguminous tree seedlings native to the Indo-Malayan region of Southeast Asia - Intsia palembanica Miq., Sindora echinocalyx (Bentham) and Adenanthera pavonina L. - were grown in 12-L pots in large growth containers in a CO2-controlled glasshouse at ambient (350 ppm) and elevated (700 ppm) atmospheric CO2 concentrations for 150 days, after which their spectral reflectances in the visible (400-700 nm) waveband were determined.

What was learned
Quoting the authors, "each of the study species exhibited pronounced increases in leaf spectral reflectance under elevated CO2 (P = 0.004), with relative increases summed across 400-700 nm ranging from 9% to 23%," which findings, in their words, "are also consistent with the limited empirical data available from prior studies (Pinter et al., 1992; Carter et al., 2000)."

What it means
Thomas calculates that "the estimated decrease in net radiative forcing resulting from increased leaf reflectance at 700 ppm CO2 ranges from 0.1 to 0.4 W m^-2," and that "if one extrapolates such a change to terrestrial vegetation generally, this would represent 2-8% of the total projected increase in radiative forcing between 1990 and 2100," which he notes "is comparable with the increase in radiative forcing over this period expected from one of the 'major' greenhouse gases (N2O)," as per Hansen and Sato (2001).  This phenomenon is one of the many negative biospheric feedbacks that tend to counter the impetus for warming provided by anthropogenic CO2 emissions (see Feedback Factors (Biophysical) in our Subject Index).

References
Carter, G.A., Bahadur, R. and Norby, R.J.  2000.  Effects of elevated atmospheric CO2 and temperature on leaf optical properties in Acer saccharum.  Environmental and Experimental Botany 43: 267-273.

Hansen, J.E. and Sato, M.  2001.  Trends of measured climate forcing agents.  Proceedings of the National Academy of Sciences, USA 98: 14,778-14,783.

Pinter, P.J., Anderson, R.J., Kimball, B.A. et al.  1992.  Evaluating cotton response to free-air carbon-dioxide enrichment with canopy reflectance observations.  Critical Reviews in Plant Sciences 33: 241-249.