As the atmospheric concentration of CO2 rises, plants commonly exhibit reduced foliar concentrations of the nitrogen-rich photosynthetic enzyme rubisco, which is typically present in excess amounts at ambient CO2 concentrations (see Acclimation in our Subject Index).  Consequently, exposure to elevated CO2 frequently reduces foliar nitrogen concentrations as well, which allows excess nitrogen to be mobilized away from the photosynthetic process and into processes more limiting to growth.

In the study of Norby et al. (2000), a 300-ppm increase in the air's CO2 content reduced the nitrogen content in leaves of red and sugar maple by 19 and 25%, respectively.  Similarly, Rey and Jarvis (1998) reported that young silver birch trees exposed to twice-ambient levels of atmospheric CO2 displayed leaf nitrogen contents that were 13% less than those observed in leaves of ambiently-growing trees.  This phenomenon has also been noticed in the needles of coniferous trees.  Gielen et al. (2000), for example, documented a 33% reduction in the needle nitrogen content of young Scots pines growing at 750 ppm CO2 compared to needle nitrogen contents of control trees growing at 350 ppm CO2.

Elevated CO2 can also mobilize other limiting resources away from the photosynthetic process and into other processes important to plant growth and development.  In the study of Ormrod et al. (1999), for example, a mere 180-ppm increase in the air's CO2 content caused 19 and 25% reductions in the chlorophyll a and b concentrations of Douglas-fir needles, respectively.  Similar results were reported in young Scots pines, which exhibited a 26% reduction in total needle chlorophyll concentration in response to a 350-ppm increase in the CO2 content of the air (Gielen et al., 2000).  However, Carter et al. (2000) noted that elevated CO2 (+300 ppm) had no effect on leaf chlorophyll concentrations in sugar maple.

Excess carbohydrates resulting from CO2-enhanced photosynthesis are often used close to their site of production to increase leaf growth.  In reviewing the results of several peer-reviewed papers related to this topic, Taylor et al. (2001) concluded that elevated CO2 consistently enhanced leaf extension rates in poplar species, which phenomenon positively correlates with increased wood production.  Similarly, Ferris et al. (2001) reported that a 200-ppm increase in the air's CO2 concentration boosted leaf area in three poplar species by approximately 40%.  In addition, a doubling of the atmospheric CO2 concentration enhanced leaf size in a native British tree species (Alnus glutinosa) by 17% (Poole et al., 2000).  In all of these cases, it is likely that CO2-induced increases in leaf turgor pressure (Tognetti et al., 2000) contributed to the CO2-induced enhancements in leaf growth, as mediated through enhanced cell division and elongation.

Sometimes, excess carbohydrates are used to enhance the biosynthesis of secondary carbon compounds within leaves, which often results in greater specific leaf areas (Cornelissen et al., 1999) and defensive resistance to pathogens and herbivores.  In the study of Hattenschwiler et al. (1999), for example, a 280-ppm increase in the air's CO2 concentration significantly increased needle concentrations of tannins and phenolics in spruce trees.  Similarly, a 350-ppm increase in the CO2 content of the air enhanced total phenolics in needles of loblolly pine by 21% (Gebauer et al., 1998).  Likewise, leaves of various Mediterranean forest species grown at 710 ppm CO2 displayed 18% greater lignin concentrations than leaves from species grown at 350 ppm CO2 (De Angelis et al., 2000).  However, Heyworth et al. (1998) did not observe any significant effects of elevated CO2 on tannin concentrations in needles of Scots pine exposed to 700 ppm CO2, nor did King et al. (2001) for aspen leaves exposed to the same atmospheric CO2 concentration.  And in the study of Schaffer et al. (1997), twice-ambient levels of atmospheric CO2 stimulated the production of foliar carbon compounds in mango trees so dramatically that the concentrations of several leaf minerals were actually decreased due to this "dilution effect."

Under certain conditions, excess carbohydrates can be used to enhance various leaf anatomical features.  Paoletti et al. (1998), for example, reported that white oak trees fumigated with air containing 750 ppm CO2 displayed leaf cuticles that were three times thicker than those of leaves on trees grown in air containing 350 ppm CO2.  In another study, Lin et al. (2001) reported that this same elevated CO2 concentration enhanced needle thickness in young Scots pines by increasing the area occupied by photosynthetic mesophyll tissue.  Such increases in leaf and cuticle thickness can increase resistance to herbivory and pathogenic attack, much as increased concentrations of secondary carbon compounds do.

In summation, it would appear that the current upward trend in atmospheric CO2 concentration will affect leaf characteristics of woody plants.  Fortunately, the available data suggest that the resulting changes will likely lead to greater and more efficient photosynthesis and growth rates, while increasing leaf resistance to herbivory and pathogenic attack.  Thus, tree productivity will likely increase in the future due to changes in foliar properties driven by the on-going rise in the air's CO2 content.

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.

Cornelissen, J.H.C., Carnelli, A.L. and Callaghan, T.V.  1999.  Generalities in the growth, allocation and leaf quality responses to elevated CO2 in eight woody species.  1999.  New Phytologist 141: 401-409.

De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S.  2000.  Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem.  Plant and Soil 224: 31-41.

Ferris, R., Sabatti, M., Miglietta, F., Mills, R.F. and Taylor, G.  2001.  Leaf area is stimulated in Populus by free air CO2 enrichment (POPFACE), through increased cell expansion and production.  Plant, Cell and Environment 24: 305-315.

Gebauer, R.L.E., Strain, B.R. and Reynolds, J.F.  1998.  The effect of elevated CO2 and N availability on tissue concentrations and whole plant pools of carbon-based secondary compounds in loblolly pine (Pinus taeda).  Oecologia 113: 29-36.

Gielen, B., Jach, M.E. and Ceulemans, R.  2000.  Effects of season, needle age and elevated atmospheric CO2 on chlorophyll fluorescence parameters and needle nitrogen concentration in (Pinus sylvestris L.).  Photosynthetica 38: 13-21.

Hattenschwiler, S. and Schafellner, C.  1999.  Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees.  Oecologia 118: 210-217.

Heyworth, C.J., Iason, G.R., Temperton, V., Jarvis, P.G. and Duncan, A.J.  1998.  The effect of elevated CO2 concentration and nutrient supply on carbon-based plant secondary metabolites in Pinus sylvestris L. Oecologia 115: 344-350.

King, J.S., Pregitzer, K.S., Zak, D.R., Kubiske, M.E., Ashby, J.A. and Holmes, W.E.  2001.  Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability.  Global Change Biology 7: 65-74.

Lin, J., Jach, M.E. and Ceulemans, R.  2001.  Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2.  New Phytologist 150: 665-674.

Norby, R.J., Long, T.M., Hartz-Rubin, J.S. and O'Neill, E.G.  2000.  Nitrogen resorption in senescing tree leaves in a warmer, CO2-enriched atmosphere.  Plant and Soil 224: 15-29.

Ormrod, D.P., Lesser, V.M., Olszyk, D.M. and Tingey, D.T.  1999.  Elevated temperature and carbon dioxide affect chlorophylls and carotenoids in Douglas-fir seedlings.  International Journal of Plant Science 160: 529-534.

Paoletti, E., Nourrisson, G., Garrec, J.P. and Raschi, A.  1998.  Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments.  Plant, Cell and Environment 21: 1071-1075.

Poole, I., Lawson, T., Weyers, J.D.B. and Raven, J.A.  2000.  Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa.  New Phytologist 145: 511-521.

Rey, A. and Jarvis, P.G.  1998.  Long-Term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees.  Tree Physiology 18: 441-450.

Schaffer, B., Whiley, A.W., Searle, C. and Nissen, R.J.  1997.  Leaf gas exchange, dry matter partitioning, and mineral element concentrations in mango as influenced by elevated atmospheric carbon dioxide and root restriction.  Journal of the American Society of Horticultural Science 122: 849-855.

Taylor, G., Ceulemans, R., Ferris, R., Gardner, S.D.L. and Shao, B.Y.  2001.  Increased leaf area expansion of hybrid poplar in elevated CO2.  From controlled environments to open-top chambers and to FACE.  Environmental Pollution 115: 463-472.

Tognetti, R., Rashi, A. and Jones, M.B.  2000.  Seasonal patterns of tissue water relations in three Mediterranean shrubs co-occurring at a natural CO2 spring.  Plant, Cell and Environment 23: 1341-1351.