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Elevated CO2 Reduces Fluctuating Asymmetry of Oak Leaves
Volume 7, Number 20: 19 May 2004

Fluctuating asymmetry is the terminology used to describe small variations from perfect symmetry in otherwise bilaterally symmetrical characters in an organism (Moller and Swaddle, 1997).  It is believed to arise in consequence of developmental instability experienced during ontogeny that is caused by various stresses, including both genetic and environmental factors, and has been studied extensively in animals but less so in plants (Moller and Shykoff, 1999).  Now, in the first study to address the effects of atmospheric CO2 enrichment on leaf asymmetry plus how herbivores respond to these effects, Cornelissen et al. (2004) open up a whole new window through which to view the world of the future in terms of the potential effects of the ongoing rise in the air's CO2 content on the plant and animal components of the biosphere.

The Cornelissen et al. study was conducted on a two-hectare section of a native scrub-oak community at the Kennedy Space Center, Titusville, Florida, USA.  This site has served as the home base for a number of important studies we have reviewed previously (Day et al., 1996; Stiling et al., 1999; Li et al., 2000; Buckley, 2001; Dilustro et al., 2001; Lodge et al., 2001; Ainsworth et al., 2002; Dijkstra et al., 2002; Hungate et al., 2002; Stiling et al., 2002), and, hence, we will not describe it in any detail here, other than to say that the site employs open-top chambers maintained at atmospheric CO2 concentrations of approximately 370 and 700 ppm, and that the study of Cornelissen et al. concentrated on the two schlerophyllous oaks that dominate the site's vegetation: myrtle oak (Quercus myrtifolia) and sand live oak (Quercus geminata).

Based on measurements of (1) distances from the leaf midrib to the left and right edges of the leaf at its widest point and (2) leaf areas on the left and right sides of the leaf midrib, Cornelissen et al. determined that "asymmetric leaves were less frequent in elevated CO2, and, when encountered, they were less asymmetric than leaves growing under ambient CO2."  In addition, they found that "Q. myrtifolia leaves under elevated CO2 were 15.0% larger than in ambient CO2 and Q. geminata leaves were 38.0% larger in elevated CO2 conditions."  As a bonus, they also determined that "elevated CO2 significantly increased tannin concentration for both Q. myrtifolia and Q. geminata leaves" and that "asymmetric leaves contained significantly lower concentrations of tannins than symmetric leaves for both Q. geminata and Q. myrtifolia."

In commenting on their primary findings of reduced percentages of leaves experiencing asymmetry in the presence of elevated levels of atmospheric CO2 and the lesser degree of asymmetry exhibited by affected leaves in the elevated CO2 treatment, Cornelissen et al. say that "a possible explanation for this pattern is the fact that, in contrast to other environmental stresses, which can cause negative effects on plant growth, the predominant effect of elevated CO2 on plants is to promote growth with consequent reallocation of resources (Docherty et al., 1996)."  Another possibility they discuss "is the fact that CO2 acts as a plant fertilizer," and, as a result, that "elevated CO2 ameliorates plant stress compared with ambient levels of CO2," which is one of the well-documented biological benefits of atmospheric CO2 enrichment (Idso and Idso, 1994).

With respect to the ancillary finding of CO2-induced increases in tannin concentrations in the leaves of both oaks (a mean increase of approximately 35% for Q. myrtifolia and 43% for Q. geminata), we note that this phenomenon may provide both species with greater protection against herbivores, and that part of that protection may be associated with the observed CO2-induced reductions in the amount and degree of asymmetry in the leaves of the CO2-enriched trees.  Consistent with this hypothesis, for example, Stiling et al. (1999, 2002) found higher abundances of leaf miners in the leaves of the trees in the ambient CO2 chambers, where asymmetric leaves were more abundant, while in the current study it was determined that leaf miners attacked asymmetric leaves more frequently than would be expected by chance alone in both CO2 treatments.

Another benefit associated with the extra tannins produced in the leaves of CO2-enriched trees occurs when the leaves are eaten by ruminants.  As described in our Editorial of 7 Aug 2002, when eating foliage containing higher concentrations of condensed tannins, "belching ruminants" typically experience reduced methane emissions; and since methane is a far more powerful greenhouse gas than is CO2 on a molecule-to-molecule basis, this phenomenon leads to a significant reduction in atmospheric greenhouse-gas forcing of climate, which results in the application of a significant biological brake (or negative feedback) on the rate of CO2-induced global warming.

More studies of the many ramifications of reduced asymmetry in the leaves of CO2-enriched plants are clearly warranted, both to provide a better idea of what the high-CO2 world of the future holds for the biosphere and to probe deeper into the nature of the CO2 dependency of the several processes by which the portended changes are likely to be produced.  Of particular interest to us in this regard are the relationships that may exist among various environmental stresses, fluctuating asymmetry, and plant antioxidant levels and their many and diverse relationships to human health [see our second Major Report Enhanced or Impaired? Human Health in a CO2-Enriched Warmer World], as well as whatever relationship might exist between fluctuating asymmetry and the CO2-induced ultra-enhancement of early spring branch growth that we have observed in sour orange trees (Idso et al., 2000, 2001).  Other scientists will undoubtedly have their own pet projects come to mind.  In all instances, however, fluctuating asymmetry may prove a stimulus for new ways of looking at old observations, all the way down to the genetic level, which could prove fruitful.

Sherwood, Keith and Craig Idso

References
Ainsworth, E.A., Davey, P.A., Hymus, G.J., Drake, B.G. and Long, S.P.  2002.  Long-term response of photosynthesis to elevated carbon dioxide in a Florida scrub-oak ecosystem.  Ecological Applications 12: 1267-1275.

Buckley, P.T.  2001.  Isoprene emissions from a Florida scrub oak species grown in ambient and elevated carbon dioxide.  Atmospheric Environment 35: 631-634.

Cornelissen, T., Stiling, P. and Drake, B.  2004.  Elevated CO2 decreases leaf fluctuating asymmetry and herbivory by leaf miners on two oak species.  Global Change Biology 10: 27-36.

Day, F.P., Weber, E.P., Hinkle, C.R. and Drake, B.G.  1996.  Effects of elevated atmospheric CO2 on fine root length and distribution in an oak-palmetto scrub ecosystem in central Florida.  Global Change Biology 2: 143-148.

Dijkstra, P., Hymus, G., Colavito, D., Vieglais, D.A., Cundari, C.M., Johnson, D.P., Hungate, B.A., Hinkle, C.R. and Drake, B.G.  2002.  Elevated atmospheric CO2 stimulates aboveground biomass in a fire-regenerated scrub-oak ecosystem.  Global Change Biology 8: 90-103.

Dilustro, J.J., Day, F.P. and Drake, B.G.  2001.  Effects of elevated atmospheric CO2 on root decomposition in a scrub oak ecosystem.  Global Change Biology 7: 581-589.

Docherty, M., Hurst, D.K., Holopainem, J.K. et al.  1996.  Carbon dioxide-induced changes in beech foliage cause female beech weevil larvae to feed in a compensatory manner.  Global Change Biology 2: 335-341.

Hungate, B.A., Reichstein, M., Dijkstra, P., Johnson, D., Hymus, G., Tenhunen, J.D., Hinkle, C.R. and Drake, B.G.  2002.  Evapotranspiration and soil water content in a scrub-oak woodland under carbon dioxide enrichment.  Global Change Biology 8: 289-298.

Idso, C.D., Idso, S.B., Kimball, B.A., Park, H.-S., Hoober, J.K. and Balling Jr., R.C.  2000.  Ultra-enhanced spring branch growth in CO2-enriched trees: Can it alter the phase of the atmosphere's seasonal CO2 cycle?  Environmental and Experimental Botany 43: 91-100.

Idso, K.E., Hoober, J.K., Idso, S.B., Wall, G.W. and Kimball, B.A.  2001.  Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves.  Environmental and Experimental Botany 48: 199-211.

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.

Li, J.-H., Dijkstra, P., Hymus, G.J., Wheeler, R.M., Piastuchi, W.C., Hinkle, C.R. and Drake, B.G.  2000.  Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment.  Global Change Biology 6: 727-733.

Lodge, R.J., Dijkstra, P., Drake, B.G. and Morison, J.I.L.  2001.  Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd.  Plant, Cell and Environment 24: 77-88.

Moller, A.P. and Shykoff, P.  1999.  Morphological developmental stability in plants: patterns and causes.  International Journal of Plant Sciences 160: S135-S146.

Moller, A.P. and Swaddle, J.P.  1997.  Asymmetry, Developmental Stability and Evolution.  Oxford University Press, Oxford, UK.

Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G.  2002.  Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest.  Oecologia DOI 10.1007/s00442-002-1075-5.

Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B.  1999.  Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack.  Ecological Applications 9: 240-244.