C₄ photosynthesis is favored over C₃ photosynthesis under conditions of high temperature and/or low atmospheric CO₂. Below we provide a 3-D graphic of how the ratio of photorespiration-to-photosynthesis increases as temperature increases and/or CO₂ decreases.

Web Figure 9.8.A   Changes in the ratio of photorespiration-to-photosynthesis as a function of both CO₂ concentration and temperature for a plant with C₃ photosynthesis. (Courtesy of Ehleringer and Cerling 2001.)

As described in Chapter 9 of the textbook, the rate of photorespiration in a leaf increases because RuBP carboxylase is more likely to react with O₂ instead of CO₂ as temperature increases and/or CO₂ decreases. The result of an increase in photorespiration is a decrease in photosynthetic quantum efficiency. This leads to environmental conditions where C₄ plants are favored over C₃ and vice versa.

Web Figure 9.8.B   Predicted realms of atmospheric CO₂ and growing season temperature that favor plants with C₃ or C₄ photosynthesis. (Courtesy of Ehleringer et al. 1997.)

This model leads to predictions that the abundances of C₃ and C₄ plants should vary spatially (as a function of latitude as discussed in Web Topic 9.3) and as a function of atmospheric CO₂ history (as discussed in Web Topic 9.5). The secret to evaluating this model is to go back and look at plant fossils over time and to relate photosynthesis pathway with prehistoric atmospheric CO₂. This is a difficult task.

Reconstruction of the expansion of C₄ plants over the past 10–15 million years can be a challenge, because individual plants are not well recorded in the fossil record in many locations. Most plant fossils are thought to have formed as a result of submersion into an anaerobic, aquatic environment. The vast majority of plants do not become fossils under these conditions, but instead are decomposed by microbial activities leading to diminished numbers of fossils that we can use to reconstruct the expansion of C₄ plants over time. However, there are good proxies.

Carbon isotope ratios (δ¹³C) of animal tissues reflect the foods that they ate. For modern animals, it is possible to choose from among, hair, muscle, bone collagen, and tooth enamel for isotope analyses in order to determine the proportions of C₃ and C₄ food sources in their diets. Hair provides a sequential record of the animal’s diet during the time of hair production. Muscle and bone collagen analyses provide an integrated measure of the animal’s current diet as shown in Web Figure 9.8.C below.

Web Figure 9.8.C   Carbon isotope ratios of common herbivores in African savannas. (Courtesy of Ehleringer and Cerling 2001.)

The δ¹³C of animal teeth faithfully record the carbon isotope ratios of food sources during the period of tooth development. The carbon in tooth enamel is actually carbonate that has condensed from CO₂ that was produced as a byproduct of metabolizing food. After an animal has died and decomposition has occurred, about the only remaining tissue that remains and preserves the C₃/C₄ dietary signals is the enamel in the animal’s teeth.

For many herbivores, such as cows and horses, tooth growth is continuous and provides a longer-term dietary record than for animals with a fixed tooth growth period (such as humans). As teeth are preserved for millions of years, δ¹³ of tooth enamel can be used to reconstruct the abundances of C₃ and C₄ plants eaten by mammalian grazers over extended time periods. In the graphic below, we plot the frequency distributions of modern animal teeth with that observed for C₃ and C₄ plants:

Web Figure 9.8.D   Frequency histograms of leaves with C₃ and C₄ photosynthesis and of the teeth of herbivores feeding off those plants. (Courtesy of Ehleringer and Cerling 2001.)

Note that there is an “ε” offset of 14.1‰. This is the isotope enrichment associated with CO₂ condensing to form carbonate. Now we have a tool—the carbon isotope ratio of animal tooth enamel will record the abundance of C₃ versus C₄ food sources.

Thure Cerling and his colleagues at the University of Utah have applied the “tooth enamel” tool to reconstruct the historical abundances of C₃/C₄ plants. Their results are very interesting, as shown in Web Figure 9.8.E below.

Web Figure 9.8.E   Frequency histograms of the carbon isotope ratios of tooth enamel of herbivores in several different regions of the globe. The isotope ratio data are separate into two time periods to illustrate the emergence of C₄-dominated ecosystems in the warmer regions about 6–8 million years ago. (Courtesy of Ehleringer and Cerling 2001.)

Until 8 million years ago, C₄ plants were not an important part of ecosystems. Then, between 6 and 9 million years ago, C₄ plants became important parts of many ecosystems, particularly at tropical and semi-tropical latitudes. These results are consistent with model predictions above. C₄ photosynthesis is predicted not to occur on Earth until the atmospheric CO₂ level falls below a critical threshold value. Even then it should appear first in those locations with the warmest temperatures during the growing season.

There is also extensive evidence presently to show that fluctuations in the atmospheric CO₂ concentration values between glacial (180 ppm) and interglacial (280 ppm) periods were large to influence the local abundances of C₃/C₄ plants. Shown below (Web Figure 9.8.F) are the time series analyses of carbon in bog in tropical Africa. At about 10,0000 to 12,000 years ago, the vegetation surrounding these bogs switched from being dominated by C₄ plants to being dominated by C₃ plants. This shift is evident by the dramatic change is carbon isotope ratios of materials feeding into the bog, especially at the time that the glacial period ended and we entered our current inter-glacial period.

Web Figure 9.8.F   Carbon isotope ratios of bog materials deposited by the surrounding vegetation as a function of time in three different lakes systems in tropical Africa. Note that about 10,000–12,000 years ago, there was a shift from C₄-dominated to C₃ dominated ecosystems. Shown also is a reconstruction of the Byrd ice core data showing that the increase in atmospheric CO₂ coincides with the changes from C₄-to C₃-dominated ecosystems. (Courtesy of Ehleringer and Cerling 2001.)

Together these historical pieces of information paint an interesting history. The C₃ photosynthetic pathway dominated for much of Earth’s history. And it is only relatively recently in Earth’s (6–8 million years ago) history that we have seen an expansion of C₄-dominated ecosystems. What the future holds is unclear, since anthropogenic burning of fossil fuels is rapidly increasing in atmospheric CO₂ to levels far exceeding those observed on Earth over the past 1 million years.