What was done
Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed in branch chambers for two years (1993-1994) to ambient air and air enriched to 175 and 350 ppm CO2 above ambient.  In a second experiment that assessed many of the same carbon based secondary compounds (CBSCs), needles were exposed in both branch and open-top chambers to ambient air and air enriched to 200 ppm CO2 above ambient throughout the 1998 growing season.

What was learned
In the initial branch chamber study, in the words of the authors, "concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated CO2 treatments," although "there were no significant treatment effects on foliar lignin concentrations" and "nitrogen concentrations were about 10% lower in needles from the elevated CO2 treatments."  In the second study, the authors report that "treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this ? study N concentrations were not significantly affected by the CO2 treatments in either the branch or open-top chambers."

The authors note that the total soluble phenolic and PA concentrations they observed appear to be related to "the balance between carbohydrate sources and sinks: the greater the source:sink ratio, the greater the concentration of phenolic compounds (Herms and Mattson, 1992; Peņuelas and Estiarte, 1998)."  With respect to the catechin concentration increases they documented, they note that similar results have been observed "in Scots pine (P. sylvestris L.) needles (Ivonis, 1990) and in leaves of willow (Salix myrinifolia Salisb.) and in European white birch (Betula pendula Roth) seedlings ... (Julkunen-Titto and Tahvanainen, 1993; Lavola and Julkunen-Titto, 1994)."  They also note that "Zuker (1983) has argued that proanthocyanidins are primarily defensive against microbes and can impede decomposition of plant litter," which is believed to be brought about "by inactivating enzymes and precipitating proteins (Tiarks et al., 1989)."

What it means
The results of this study indicate, in the words of the authors, that "soluble phenolic, catechin and PA concentrations were generally higher in needle extracts from elevated CO2 treatments than from ambient CO2 treatments," noting that these responses "could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling."

References
Herms, D.A. and Mattson, W.J.  1992.  The dilemma of plants: to grow or defend.  Quarterly Review of Biology 67: 283-335.

Ivonis, I.Y.  1990.  Effect of a polyethylene cover and supplementary CO2 feeding on the content of growth regulators and (+)-catechin in Scotch pine needles.  Soviet Forest Science 4: 82-86.

Julkunen-Titto, R. and Tahvanainen, J.  1993.  Increased CO2 and nutrient status changes affect phytomass and the production of plant defensive secondary chemicals in Salix myrsinifolia (Salisb.).  Oecologia 95: 495-498.

Lavola, A. and Julkunen-Titto.  1994.  The effect of elevated carbon dioxide and fertilization on primary secondary metabolites in birch, Betula pendula (Roth).  Oecologia 99: 315-321.

Peņuelas, J. and Estiarte, M.  1998.  Can elevated CO2 affect secondary metabolism and ecosystem function?  Trees 13: 20-24.

Tiarks, A.E., Bridges, J.R., Hemingway, R.W. and Shoulders, E.  1989.  Condensed tannins in southern pines and their interactions with the ecosystem.  In: Chemistry and Significance of Condensed Tannins.  Hemingway, R.W. and Karchesy, J.J. (Eds.). Plenum Press, New York, pp. 369-390.

Zucker, W.V.  1983.  Tannins: Does structure determine function? An ecological perspective.  American Naturalist 121:335-365.