During photosynthesis, plants discriminate against the heavier isotopes of carbon, taking up proportionally less C-13 and C-14 than is available in their carbon reservoir. The result is isotopic fractionation, and it is passed along to the consumers of the plants (the herbivores) and to their consumers (the carnivores). In fact, additional fractionation occurs when herbivores eat the plants and when carnivores eat the herbivores. It is believed that all organisms discriminate against C-14 about twice as much as against C-13, and the ratio between the stable C-12 and C-13 atoms can be used to correct for the initial depletion of C-14. Radiocarbon dates can be corrected for isotopic fractionation, a correction called normalization. The amount of isotopic fractionation depends on the photosynthetic pathway used by the plant. Most flowering plants, trees, shrubs and temperate zone grasses are known as C3 plants, because they create a molecule with three carbon atoms using the Calvin-Benson photosynthetic cycle. Grasses that are adapted to arid regions, such as buffalo grass (Bouteloua) and maize (Zea), are known as C4 plants, because they create a molecule with four carbon atoms using the Hatch-Slack cycle. C3 plants discriminate against the heavier carbon isotopes more strongly than do C4 plants.

Further reading:

Chisholm, B.S.
1989 Variation in diet reconstructions based on stable carbon isotope evidence. In The Chemistry of Prehistoric Human Bone, edited by T.D. Price. Cambridge: Cambridge University Press, pp. 10-37.

Vermaas, W.
1998 An introduction to photosynthesis and its applications. http://photoscience.la.asu.edu/photosyn/education/photointro.html