What was done
The authors exposed sections of a southern California chaparral ecosystem - dominated by the resprouting shrub Adenostoma fasciculatum and the obligate seeder Ceanothus greggii, a nitrogen fixer - to atmospheric CO2 concentrations ranging from 250 to 750 ppm in 100-ppm increments for a period of four years, using 12 naturally-lit 2x2x2-m null-balance glass chambers, within which they measured net ecosystem CO2 exchange (NEE) and emission rates of biogenic volatile organic compounds (BVOCs).

What was learned
NEE exhibited a marked linear increase in response to increasing atmospheric CO2 concentration, more than tripling its rate in going from 400 ppm to 700 ppm at 0400, 1200 and 1600 hours in June, and rising from moderate negative to weak positive values in December.

The main BVOCs were monoterpenes, with only traces of isoprene.  In winter, the authors report that "total trace gas emissions expressed on a ground area basis were low and did not respond to increasing CO2 concentrations," while in summer they found that the "BVOC emissions were of an order of magnitude greater than during winter, but still the different levels of CO2 did not affect the emission rates."

What it means
The authors conclude that "BVOC emission can remain nearly constant as rising CO2 reduces emission per unit leaf area while stimulating biomass growth and leaf area per unit ground area."  This finding is of great significance, because, as the authors note, "the production of these trace gases by the vegetation is of importance for local and regional tropospheric ozone pollution," being of such a nature that more BVOC emissions often lead to the production of more tropospheric ozone, which phenomenon has a number of deleterious consequences.  For more on this topic - and an even more impressive finding with respect to CO2 effects on isoprene emissions - see our Editorial of 15 Jan 2003.