title: C4photosynthesis, atmospheric CO2, and climate.
abstract: The objectives of this synthesis are (1) to review the factors that influence the ecological, geographical, and palaeoecological distributions of plants possessing C4photosynthesis and (2) to propose a hypothesis/model to explain both the distribution of C4plants with respect to temperature and CO2and why C4photosynthesis 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 C4plants today, historically, and under future elevated CO2environments. Understanding the distributions of C3/C4plants 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 C4monocots are tightly correlated with temperature: elevated temperatures during the growing season favor C4monocots. In contrast, the seven studies on C4dicot 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 CO2uptake (light-use efficiency) of C3and C4species relate well to observed plant distributions and light-use efficiency is the only mechanism that has been proposed to explain distributional differences in C3/C4monocots. Modeling of C3and C4light-use efficiencies under different combinations of atmospheric CO2and temperature predicts that C4-dominated ecosystems should not have expanded until atmospheric CO2concentrations 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 C4-dominated grasslands. The C4monocots generally have a higher quantum yield than C4dicots and it is proposed that leaf venation patterns play a role in increasing the light-use efficiency of most C4monocots. The reduced quantum yield of most C4dicots is consistent with their rarity, and it is suggested that C4dicots may not have been selected until CO2concentrations reached their lowest levels during glacial maxima in the Quaternary. Given the intrinsic light-use efficiency advantage of C4monocots, C4dicots may have been limited in their distributions to the warmest ecosystems, saline ecosystems, and/or to highly disturbed ecosystems. All C4plants have a significant advantage over C3plants under low atmospheric CO2conditions 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 C4ecosystems were more extensive during the last glacial maximum and then decreased in abundance following deglaciation as atmospheric CO2levels increased.
authors.affiliation: Stable Isotope Ratio Facility for Environmental Research, Department of Biology, University of Utah, Salt Lake City, UT 84112, USA Fax: 801-581-4665; e-mail: ehleringer@bioscience.utah.edu, , , , , , US.
authors.forename: James R
authors.initials: JR
authors.lastname: Ehleringer
authors.affiliation: Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA, , , , , , US.
authors.forename: Thure E
authors.initials: TE
authors.lastname: Cerling
authors.affiliation: Stable Isotope Ratio Facility for Environmental Research, Department of Biology, University of Utah, Salt Lake City, UT 84112, USA Fax: 801-581-4665; e-mail: ehleringer@bioscience.utah.edu, , , , , , US.
authors.forename: Brent R
authors.initials: BR
authors.lastname: Helliker
keywords: Carbon isotope ratio; Global change; Key words Photosynthesis; Light-use efficiency; Quantum yield