Background
Naturally-occurring CO2 springs in Tuscany, Italy, provide a unique opportunity to study the effects of long-term atmospheric CO2 enrichment on plant growth and development.  However, they also emit higher-than-normal concentrations of the major phytotoxic air pollutants H2S and SO2 (Schulte et al., 1999).  Consequently, the springs provide a perfect setting in which to study the relative strengths of two competing phenomena, i.e., the growth-promoting effects of elevated CO2 and the growth-retarding effects of elevated H2S and SO2.

What was done
Capitalizing upon this situation, Grill et al. analyzed various properties of leaves and acorns produced on two species of oak trees (Quercus ilex L. and Quercus pubescens L.) growing at double-to-triple normal atmospheric CO2 concentrations near the CO2 springs, as well as the same characteristics of leaves and acorns growing on similar trees located some distance away in ambient-CO2 air.  In addition, they analyzed several characteristics of seedlings they sprouted from acorns produced by the CO2-enriched and ambient-treatment trees; and they used chromosome stress tests "to investigate whether alterations in sulphur-regime have negative consequences for seedlings."

What was learned
In the words of the authors, "acorns from CO2 springs contained significantly higher sulphur concentrations than controls (0.67 vs. 0.47 mg g^-1 dry weight in Q. ilex cotyledons and 1.10 vs. 0.80 in Q. pubescens)," indicative of the fact that the trees were indeed significantly affected by the H2S- and SO2-enriched air in the vicinity of the CO2-emitting springs.  They also report that Q. ilix seedlings grown from CO2 spring acorns showed elevated rates of chromosomal aberrations in root tips, suggestive of the presence of a permanent stress.  Nevertheless, as demonstrated by the results of several studies conducted on mature trees from these sites, the CO2-enriched air - even in the presence of significantly elevated concentrations of phytotoxic H2S and SO2 - tremendously enhanced the trees' photosynthetic prowess: by 26-69% (Blaschke et al., 2001), 36-77% (Stylinski et al., 2000), and a whopping 175-510% (Tognetti et al., 1998).

What it means
The growth-promoting effects of a doubling-to-tripling of the air's CO2 concentration can more than compensate for the deleterious consequences of similarly large increases in the concentrations of the planet's major sulphur-based air pollutants in two species of oak trees, and probably in most other plants as well, although more research will clearly be required to demonstrate the veracity of this latter assumption.

References
Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A.  2001.  Photosynthesis, soluble and structural carbon compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations.  Plant Biology 3: 288-297.

Schulte, M., Raiesi, F.G., Papke, H., Butterbach-Bahl, K., van Breemen, N. and Rennenberg, H.  1999.  CO2 concentration and atmospheric trace gas mixing ratio around natural CO2 vents in different Mediterranean forests in central Italy.  In: Raschi, A., Vaccori, F.P. and Miglietta, F. (Eds.).  Ecosystem Response to CO2: The Maple Project Results.  European Communities, Brussels, Belgium, pp. 168-188.

Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A.  2000.  Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques.  Plant, Cell and Environment 23: 1353-1362.

Tognetti,R., Johnson, J.D., Michelozzi, M. and Raschi, A.  1998.  Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2.  Environmental and Experimental Botany 39: 233-245.