The first chapter of the book introduces this idea and briefly describes some study sites on three different continents where naturally-emitted CO2 is used to investigate plant responses to long-term atmospheric CO2 enrichment.  The next three chapters describe the formation, movement and accumulation of CO2 and other volatile gasses within the ground and their efflux to the atmosphere, including that facilitated by volcanic discharges and "limnic eruptions" from CO2-charged lakes.

The remaining chapters primarily report various types of plant responses to atmospheric CO2 enrichment that have been observed at several of the study sites.  Ehleringer et al., for example, describe C3 vegetation growing along a CO2 concentration gradient near burning coal seams in southern Utah, USA, where carbon isotope data suggest that a mere 70 ppm increase in the air's CO2 content is likely doubling the vegetation's water-use efficiency relative to that of control plants 10 km distant from the high CO2 source.

Most of the rest of the plant responses to long-term atmospheric CO2 enrichment reported in this book come from experiments conducted near natural CO2 springs in Italy.  At the Bossoleto site in central Italy, decreases in stomatal density were reported in a grass (van Gardingen et al.) and oak species (Paoletti et al.) growing at high CO2 concentrations relative to nearby control plants.  Complementing their work, Woodward and Beerling showed a close similarity between stomatal density changes in various historic plant samples and certain species growing near high CO2 springs in Florida, USA.  Taken together, these findings indicate that stomatal density has dropped as the CO2 content of the air has risen over the past few centuries; and this phenomenon should have resulted in a contemporaneous increase in plant water-use efficiency, which is something that Johnson et al. report for another oak species growing in high CO2 air at the Bossoleto site.

Photosynthetic down regulation, which is typically observed in plants growing in artificially-elevated CO2 experiments of short duration, was observed in one grass species (van Gardingen et al.) but not in oak trees (van Gardingen et al.), sedges (Bettarini et al.), or another grass and an herb (Fordham et al.) growing in, or obtained from, naturally high CO2 environments.  Also, Selvi observed monospecific stands of a certain grass that grew with "outstanding vegetative vigor" near natural CO2 vents in central-western Italy on peat-like soils that had a soluble aluminum content "well above the toxicity thresholds" known for this plant.

A particularly interesting study was conducted by Fordham et al., who collected seeds of a grass and an herb growing at ambient and naturally high CO2 concentrations in central Italy, germinated them and grew the resulting plants in controlled atmospheric CO2 concentrations of 350 and 700 ppm for approximately three months.  Their experiment demonstrated that plants grown from seeds collected from high CO2 environments responded better to artificial atmospheric CO2 enrichment than did plants grown from seeds collected at ambient CO2.  This observation suggests that plants long adapted to elevated CO2 concentrations have an intrinsically greater growth potential than those that have never been exposed to higher-than-normal CO2 levels.  Some of this benefit may be derived from the slightly larger seed size that was observed in the plants that grew naturally in the higher CO2 environments around the CO2 vents; while some of it may be explained by the results of Badiani et al., which suggest that plants growing near high CO2 springs experience less oxidative stress than those grown at ambient CO2 concentrations.

In summary, the editors of this book have compiled a nice collection of papers that demonstrate the feasibility of using naturally-emitting CO2 sources to explore the effects of elevated CO2 on plant growth and physiology.  Many of the individual studies also report results of original research that is valuable in its own right, independent of the method of atmospheric CO2 enrichment employed.