What was done
In May 1987 a long-term in situ elevated CO2 experiment began in a North Atlantic wetland located at the Smithsonian Environmental Research Center on the Chesapeake Bay, USA.  After seventeen years, the authors evaluated the long-term effects of atmospheric CO2 enrichment on the net CO2 exchange, shoot density and shoot biomass of the wetland sedge, Scirpus olneyi, as well as how these effects have been influenced by salinity, which is one of the main environmental stressors of the wetlands.

What was learned
In every year of the past 17 years, the net CO2 exchange rate and shoot biomass and density of the plants growing in the CO2-enriched (ambient +340 ppm) air were all greater than they were among the plants growing in ambient air.  In the case of the net CO2 exchange rate, the extra CO2 boosted it by 80% in the first year of the study, but the enhancement declined to about 35% by the end of the third year and remained relatively constant at that value over the following 15 years.  Shoot biomass and density also increased, but whereas the CO2-induced stimulation of the net CO2 exchange rate remained essentially constant over the past 15 years, the CO2-induced stimulations of shoot biomass and density increased over time.  After 5 years of a nearly constant stimulation of 16%, for example, shoot density increased in near linear fashion to a value 128% above the ambient-air value at the end of year 17.  The response of shoot biomass to CO2 enrichment was also nearly linear, reaching a value approximately 70% above ambient at year 17.  What is more, the trends in shoot density and biomass do not appear to be leveling off, leading one to wonder just how high the CO2-induced stimulations will ultimately rise.

Salinity was closely correlated with net CO2 exchange, shoot density and shoot biomass, such that the higher the salinity, the more detrimental were its effects on these variables.  Nevertheless, even at the highest levels of salinity reported, atmospheric CO2 enrichment was able to produce a positive, albeit reduced, stimulatory effect on net CO2 exchange.  For shoot biomass and density, the responses were better still.  Not only did atmospheric CO2 enrichment essentially eradicate the detrimental effects of salinity, there was, in the words of Rasse et al., "circumstantial evidence suggesting that salinity stress increased the stimulation of shoot density by elevated atmospheric CO2 concentration."

What it means
Several important things are demonstrated by this experiment.  First, as the researchers state, their results "leave no doubt as to the sustained response of the salt march sedge to elevated atmospheric CO2 concentration."  Second, given the fact that the initial responses of the three growth variables declined or remained low during the first few years of the study, but leveled out or increased thereafter, it is clear that much more long-term research needs to be carried out if we are to ascertain the full and correct impacts of atmospheric CO2 enrichment on plants (see also, in this regard, our Editorial of 5 Mar 2003).  In the case of the wetland sedge of this study, for example, it took about ten growing seasons before an increasing trend in the shoot density could clearly be recognized.  Last of all, there is the authors "most important finding," i.e., "that a species response to elevated atmospheric CO2 concentration can continually increase when [it] is under stress and declining in its natural environment."  This result is highly significant and once again bears witness to the fact that earth's rising atmospheric CO2 concentration is not a catastrophic disaster, as climate alarmists would have one believe, but actually a boon to the biosphere for which we will all someday be extremely grateful.