Taking advantage of one of these natural situations (CO2-emitting springs near Pisa, Italy), Tognetti et al. (2000a) studied the water relations of three woody shrubs (Erica arborea, Myrtus communis and Juniperus communis) growing at small distances from the springs, where atmospheric CO2 concentrations of approximately 700 ppm prevailed, as well as at further distances from the springs where normal concentrations of 360 ppm prevailed at the time of their study.  Two common responses were evident in all of the shrubs: the CO2-enriched air reduced leaf stomatal conductances and increased leaf water potentials, i.e., made them less negative and, therefore, less stressful.  Consequently, the group of five scientists concluded that the CO2-induced adjustments in the shrubs' internal water relations would likely allow them "to endure severe periodic drought."

In a contemporary report on other aspects of the same study, Tognetti et al. (2000b) reported that the plants growing in the CO2-enriched air closer to the springs experienced increased leaf turgor pressure, particularly during the warmer summer months, which is also indicative of better plant water relations.  And in yet another study of the same shrubs at the same location, Tognetti et al. (2002) found that elevated CO2 altered the elastic cell-wall properties of all three shrubs in such a way as to endow the shrubs with greater capacities for water uptake from the soil than are currently possessed by control plants growing in ambient air.  In addition, the CO2-enriched shrubs displayed greater relative water contents than did ambiently-grown plants as leaf water potentials declined with available soil moisture.

Moving from shrubs to trees, Bartak et al. (1999) studied various physiological processes of mature Arbutus unedo trees growing in the general vicinity of CO2-emitting vents located in central Italy.  At different distances from the vents, physiological measurements were made on trees that had been exposed to average atmospheric CO2 concentrations of approximately 355 ppm (ambient) and 465 ppm (CO2-enriched) throughout the entire 30 years of their existence.  This modest 30% increase in atmospheric CO2 concentration was determined by Bartak et al. to have boosted net photosynthetic rates in the perennial evergreen species by 110 to 140%, depending upon light intensity.  In addition, their work revealed that the CO2-enriched trees had experienced no photosynthetic acclimation to the extra vent-derived CO2 to which they had been continuously exposed.

At the high end of the CO2 concentration spectrum, Fernandez et al. (1998) studied a number of the effects of very high CO2 levels produced by natural CO2 springs on an indigenous tree during the rainy and dry seasons in Venezuela.  They found, first of all, that the ultra-high CO2 concentrations - some as much as 100 times greater than the current global mean - were in no way detrimental to the trees, as some might have thought they would be.  Instead, photosynthesis was stimulated by the high CO2 in all seasons and in spite of the likely presence of toxic hydrocarbons and sulfur gasses that are typically released to the air along with CO2 in such situations.  During the dry season, in fact, trees growing away from the springs at ambient CO2 levels displayed net losses of carbon from their leaves, while trees growing near the springs at elevated CO2 concentrations exhibited net carbon gains.  In addition, the high CO2 concentrations reduced leaf stomatal densities by about 70%, causing the water-use efficiency of the trees to rise 2-fold and 19-fold, respectively, during the rainy and dry seasons, when measured at a CO2 concentration of 1,000 ppm compared to an ambient concentration of 350 ppm, which represents less than a tripling of the air's CO2 content.  Consequently, and because of the trees' long-term exposure to these high CO2 concentrations under totally natural conditions, Fernandez et al. conclude that their work provides "a positive answer to the question of whether increases in carbon assimilation will be sustained throughout the growing season and over multiple seasons" in a high-CO2 world of the future.

In light of these several observations, it is our opinion that a doubling, tripling or even greater enhancement of the atmosphere's CO2 concentration will only further improve the productivities and water use efficiencies of earth's woody plants.  Consequently, what many IPCC-inspired climate alarmists and a growing group of similarly-motivated bio-alarmists view with horror (erroneously, of course), we welcome with open arms: more fossil fuel-derived CO2 for the atmosphere of our undernourished biosphere.

References
Bartak, M., Raschi, A. and Tognetti, R.  1999.  Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2.  Photosynthetica 37: 1-16.

Fernandez, M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Rengifo, E. and Herrera, A.  1998.  Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia.  New Phytologist 138: 689-697.

Tognetti, R., Minnocci, A., Penuelas, J., Rashi, A. and Jones, M.B.  2000a.  Comparative field water relations of three Mediterranean shrub species co-occurring at a natural CO2 vent.  Journal of Experimental Botany 51: 1135-1146.

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

Tognetti, R., Raschi, A. and Jones M.B.  2002.  Seasonal changes in tissue elasticity and water transport efficiency in three co-occurring Mediterranean shrubs under natural long-term CO2 enrichment.  Functional Plant Biology 29: 1097-1106.