The objectives of this synthesis are (1) to review the factors that influence the ecological, geographical, and palaeoecological distributions of plants possessing C-4 photosynthesis and (2) to propose a hypothesis/model to explain both the distribution of C-4 plants with respect to temperature and CO2 and why C-4 photosynthesis is relatively uncommon in dicotyledonous plants (hereafter dicots), especially in comparison with its widespread distribution in monocotyledonous species (hereafter monocots). Our goal is to stimulate discussion of the factors controlling distributions of C-4 plants today, historically, and under future elevated CO2 environments. Understanding the distributions of C-3/C-4 plants impacts not only primary productivity, but also the distribution, evolution, and migration of both invertebrates and vertebrates that graze on these plants. Sixteen separate studies all indicate that the current distributions of C-4 monocots are tightly correlated with temperature: elevated temperatures during the growing season favor C-4 monocots. In contrast, the seven studies on C-4 dicot distributions suggest that a different environmental parameter, such as aridity (combination of temperature and evaporative potential), more closely describes their distributions. Differences in the temperature dependence of the quantum yield for CO2 uptake (light-use efficiency) of C-3 and C-4 species relate well to observed plant distributions and light-use efficiency is the only mechanism that has been proposed to explain distributional differences in C-3/C-4 monocots. Modeling of C-3 and C-4 light-use efficiencies under different combinations of atmospheric CO2 and temperature predicts that C-4-dominated ecosystems should not have expanded until atmospheric CO2 concentrations reached the lower levels that are thought to have existed beginning near the end of the Miocene. At that time, palaeocarbonate and fossil data indicate a simultaneous, global expansion of C-4-dominated grasslands. The C-4 monocots generally have a higher quantum yield than C-4 dicots and it is proposed that leaf venation patterns play a role in increasing the light-use efficiency of most C-4 monocots. The reduced quantum yield of most C-4 dicots is consistent with their rarity, and it is suggested that C-4 dicots may not have been selected until CO2 concentrations reached their lowest levels during glacial maxima in the Quaternary. Given the intrinsic light-use efficiency advantage of C-4 monocots, C-4 dicots may have been limited in their distributions to the warmest ecosystems, saline ecosystems, and/or to highly disturbed ecosystems. All C-4 plants have a significant advantage over C-3 plants under low atmospheric CO2 conditions and are predicted to have expanded significantly on a global scale during full-glacial periods, especially in tropical regions. Bog and lake sediment cores as well as pedogenic carbonates support the hypothesis that C-4 ecosystems were more extensive during the last glacial maximum and then decreased in abundance following deglaciation as atmospheric CO2 levels increased.
