A number of CO2 enrichment studies conducted in laboratories and greenhouses have suggested that the botanical benefits derived from elevated concentrations of atmospheric CO2 gradually decline over time. In field studies, however, where the experimental plants are rooted in the ground and free to explore the natural soil environment, this phenomenon is less commonly noticed. In many instances, in fact, just the opposite occurs: plants respond even better to elevated CO2 as time passes. Consequently, it is the opinion of most scientists that the down regulation of photosynthesis and growth that has been observed in certain circumstances will be neither prevalent nor complete in real-world situations.
In one of the longest studies of its kind ever to be conducted - and where down-regulation would be expected to appear if it were a truly ubiquitous consequence of long-term exposure to elevated CO2 - eight sour orange tree seedlings were planted directly into the ground at Phoenix, Arizona, in July of 1987 and enclosed in pairs within four open-top chambers made of clear polyethylene film. Then, in November of that year, air of 700 ppm CO2 began to be continuously pumped into two of the enclosures through perforated plastic tubes that lay upon the ground, while ambient air of 400 ppm CO2 was similarly pumped into the other two enclosures. This protocol was faithfully maintained for seventeen years; and the scientists in charge of the study documented the course of atmospheric CO2 enrichment effects in the trees as they progressed from tiny seedlings through the juvenile stage of development and into full maturity.
The results of this 17-year experiment were truly phenomenal. In commenting on their findings, Kimball et al. (2007) stated that "rather than a continual acclimation" - i.e., rather than a gradual long-term decline in the aerial fertilization effect of the extra 300 ppm of CO2 supplied to the CO2-enriched trees - "there was a sustained enhancement of about 70% in annual fruit and incremental wood production over the last several years of the experiment." This observation thus led them to conclude that "the effects of elevated CO2 on trees can be large and sustained for many years," as they indeed demonstrated to be the case with sour orange trees, there having been a 70% sustained increase in biomass production over the entire last decade of the study in response to the 75% increase in the air's CO2 content employed throughout the experiment.
Although not to be ruled out in all instances, down regulation of photosynthesis and growth does not appear to be a major impediment to the long-term effectiveness of the aerial fertilization effect of atmospheric CO2 enrichment in stimulating the productivity of Earth's vegetation. As Woodrow (1994) concluded in an earlier study after reviewing the subject in depth, "C3 plants probably possess the genetic feedback mechanisms required to efficiently 'smooth out' any imbalance within the photosynthetic system caused by a rise in atmospheric CO2," so that the beneficial effects of atmospheric CO2 enrichment are unlikely to be totally erased over time. In fact, it has been demonstrated in several species that down-regulation of photosynthesis is sometimes a direct consequence of reduced foliar nitrogen concentrations; and several other studies have shown that the foliar nitrogen contents of such CO2-enriched plants oftentimes overcome the initial depression in this parameter to ultimately return to levels characteristic of control plants growing in ambient air.
Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.
Woodrow, I.E. 1994. Optimal acclimation of the C3 photosynthetic system under enhanced CO2. Photosyn Res 39: 401-412.