Carbon Dioxide Concentration of the Atmosphere Above Illinois Forest and Grassland

The statements. are commonly made in biological text books and lectures tnat the carbon dioxide content of the air approximates .03 to .04% and that carbon dioxide is the limiting factor in photosynthesis. These generalizations are basically sound and are scientifically justifiable, but their unqualified acceptance may lead to a dogmatic assurance that they are always true. Actually, the carbon dioxide content of the air varies at different levels and the traditional value of .03-.04%o for carbon dioxide concentration is an average; the carbon dioxide content of the air infrequently falls below .03%, but often exceeds .04%. Under conditions of low light intensity and a relatively high carbon dioxide concentration of the air, light intensity, rather than carbon dioxide concentration, becomes the limiting factor in photosynthesis.