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Plant Responses to Free-Air CO2 Enrichment
Volume 8, Number 19: 11 May 2005

Several techniques have been used to create elevated concentrations of atmospheric CO2 to study the impacts of CO2-enriched air on plant growth and development.  The most realistic of these techniques - in that it is thought to least perturb the natural environment - is generally acknowledged to be Free-Air CO2 Enrichment or FACE.  This outdoor technique utilizes computer-controlled vertical vent pipes arranged in circular arrays that are programmed to respond to changes in wind speed and direction so as to continuously release just the right amount of CO2 from the upwind pipes to keep the plants within the central portions of the circular arrays continuously supplied with air that is close to the desired CO2 concentration.

What have we learned from this technique?  In a review of 124 primary research papers describing results obtained from large-scale FACE experiments that have been conducted over the past fifteen years, Ainsworth and Long (2005) summarize a number of findings that they compare with findings obtained from other techniques.  In most of the FACE experiments, however, the degree of atmospheric CO2 enrichment above the background level was approximately 200 ppm, whereas in the great bulk of all other CO2 enrichment studies a 300-ppm enrichment of the air's CO2 content was typically employed.  Hence, to express the FACE results summarized by Ainsworth and Long on a comparable basis, we have multiplied them by the factor 300/200 = 1.5, since the review of Idso and Idso (1994) indicates that between these two enrichment levels, plant physiological responses to atmospheric CO2 enrichment are very close to linear.

So what did Ainsworth and Long learn from their comprehensive review of the FACE literature?  To what degree were various plant physiological processes and properties altered by a 300 ppm increase in the air's CO2 concentration?

Averaged across all FACE experiments and species, a 300-ppm increase in the air's CO2 concentration increased the light-saturated rate of leaf net photosynthesis by 47%, while it boosted diurnal carbon assimilation by 43%.  Leaf stomatal conductance was also altered in a way that had positive implications, decreasing by 30%, which decline helped to increase instantaneous transpiration efficiency (a leaf-level measure of water-use efficiency) by 81%.  And at the very end of the line, dry matter production and final yield (in the case of agricultural crops) rose by 26%.

The greatest CO2-induced benefits were accrued by trees, which experienced increases of 111% in leaf-level water-use efficiency and 42% in dry matter production in response to a 300-ppm increase in atmospheric CO2 concentration.  Among crops, for comparison, C4 sorghum posted a yield increase of only 7%, while the C3 crops rice and wheat exhibited yield increases of 16% and 22%, although cotton (which is actually a woody perennial and, therefore, more like a tree than a rice or wheat plant) exhibited a yield increase of 63%.  Also, whereas averaged across all plants studied there was a 7% decrease in nitrogen content per unit leaf area, trees exhibited a 3% increase in this parameter.

Another interesting aspect of the meta-analysis of Ainsworth and Long was its revelation that the mean value of the CO2-induced increase in the light-saturated rate of CO2 uptake for all studies conducted at temperatures less than 25C was 28%, while it was 45% for all studies conducted at temperatures above 25C.  This finding vividly illustrates the significant positive synergism that exists between concomitant increases in atmospheric CO2 concentration and temperature, which was demonstrated in the earlier review of non-FACE studies conducted by Idso and Idso (1994).  Also of great interest was the finding that the presence of ozone, a noxious air pollutant, actually enhanced the CO2-induced increase in light-saturated CO2 uptake, boosting it from 55% to 89%.

These findings bode well for the future of earth's biosphere, as they indicate we can expect significant increases in plant growth as the air's CO2 content continues to rise, even in the presence of significant ozone pollution and perhaps actually aided by rising air temperatures.

Sherwood, Keith and Craig Idso

References
Ainsworth, E.A. and Long, S.P.  2005.  What have we learned from 15 years of free-air CO2 enrichment (FACE)?  A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2New Phytologist 165: 351-372.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.