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The Significance of CO2-Induced Increases in Leaf Mass per Unit Area
Ishizaki, S., Hikosaka, K. and Hirose, T.  2003.  Increase in leaf mass per area benefits plant growth at elevated CO2 concentration.  Annals of Botany 91: 905-914.

What was done
The authors grew specimens of the perennial herb Polygonum cuspidatum Sieb. et Zucc. (Japanese knotweed; syn. Reynoutria japonica Houttuyn) four to a 4-liter pot from seeds collected from a single clone growing on Mt. Fuji, Japan, in open-top chambers at three soil (washed river sand) nitrogen (N) concentrations in ambient (370 ppm CO2) and elevated (700 ppm CO2) air, measuring a number of plant parameters at various times throughout the growing season.

What was learned
Elevated CO2 significantly increased plant dry mass: by 24.4% at low N, by 45.5% at medium N, and by 91.5% at high N.  It also significantly increased leaf mass per unit leaf area (MLA) at all N levels and ages by an average of 29% and leaf net assimilation rate by an average of 27%.  Based on a sensitivity analysis of an equation they developed for plant relative growth rate (R), the authors additionally determined that the CO2-induced increase in MLA "led, on average, to a 13.0% increase in R at elevated CO2 as compared with the lower MLA observed at ambient CO2."

What it means
The authors note that "many studies have shown that MLA consistently increases under elevated CO2 (Poorter et al., 1996; Yin, 2002)" and that "this increase has been primarily explained by accumulation of non-structural carbohydrates (Poorter et al., 1997)."  They now also have demonstrated that this response "contributes to growth enhancement under elevated CO2," possibly because "it would contribute to increasing leaf N content on an area basis."

Poorter, H., Roumet, C. and Campbell, B.D.  1996.  Interspecific variation in the growth response of plants to elevated CO2: a search for functional types.  In: Korner, C. and Bazzaz, F.A., Eds. Carbon Dioxide, Populations, and Communities.  Academic Press, San Diego, CA, pp. 375-412.

Poorter, H., Van Berkel, Y., Baxter, B., Den Hertog, J., Dijkstra, P., Gifford, R.M., Griffin, K.L., Roumet, C., Roy, J. and Wong, S.C.  1997.  The effect of elevated CO2 on the chemical composition and construction costs of leaves of 27 C3 species.  Plant, Cell and Environment 20: 474-482.

Yin, X.  2002.  Responses of leaf nitrogen concentration and specific leaf area to atmospheric CO2 enrichment: a retrospective synthesis across 62 species.  Global Change Biology 8: 631-642.

Reviewed 16 July 2003