Reference
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.
What was done
The authors studied a number of effects of very high CO2 levels (some as great as 35,000 ppm, produced from natural CO2 springs) on an herb and a tree during the rainy and dry seasons in Venezuela, comparing them with control plants that were located a mere 24 m from the plants growing adjacent to the high CO2 sources. Because natural sites, such as this one, contain vegetation that may have persisted for generations at elevated CO2 concentrations, they provide data that is highly indicative of future plant responses to the rising CO2 content of earth's atmosphere.
What was learned
High CO2 concentrations --some as much as 100 times greater than the current global mean-- did not inhibit photosynthesis in these species. Instead, photosynthesis was stimulated by elevated 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 natural situations such as this one. During the dry season, in fact, plants growing away from the springs at ambient CO2 levels displayed net losses of carbon from their leaves, while plants growing near the springs at elevated CO2 concentrations exhibited net carbon gains. In addition, the high CO2 concentrations reduced stomatal densities by about 70% in both species, causing plant water-use efficiency in the herb to rise nearly 3-fold and 4-fold, respectively, during the rainy and dry seasons, while the water-use efficiency of the tree rose 2-fold and 19-fold during the same periods, when measured at a CO2 concentration of 1,000 ppm as opposed to an ambient concentration of 350 ppm.
What it means
These Venezuelan herbs and trees should grow very well indeed as the CO2 content of the air continues to rise. And because of their long-term exposures to high CO2 under totally natural conditions, they provide, in the words of the authors, "a positive answer to the question of whether increases in carbon assimilation will be sustained throughout the growing season and over multiple seasons."