Climate and atmosphere interact to drive vegetation change

Abstract

About seven Mya, the carbon isotope composition of fossil tooth enamel and paleosol carbonates from Pakistan, North America, Africa and South America shifted to higher values. The shift recorded a transition from ecosystems dominated by plants using the C3 photosynthetic pathway (most plants, including all woody species and many grasses) to those using C4 (including some grasses). C3 plants have lower carbon isotope values than do C4 plants, and this difference is reflected in the soil in which they grow and the herbivores that eat them. Because C4 plants are competitively favored under warm, dry conditions, the ecological shift was initially attributed to climate change. However, the global near-synchronism of the isotopic shift led to the hypothesis that the C3–C4 transition was driven by falling atmospheric CO2 concentrations. New carbon isotope records from Africa and Asia suggest that patterns of regional vegetation change might be controlled by a combination of factors.