THE LION OR THE UNICORN? DO WE NEED TO INVOKE VARIATIONS IN ATMOSPHERIC CO2 IN ORDER TO EXPLAIN LATE QUATERNARY CHANGES IN THE TROPICAL LAND BIOSPHERE?

During the Last Glacial Maximum (LGM), the atmospheric partial pressure of CO2 decreased to a physiologically-limiting level of 190 - 200 microatm, rising to its preindustrial level of 270 – 280 microatm by the beginning of the Holocene. There is no modern analogue, even at high altitudes, for the reduced CO2/O2 ratio that prevailed at the LGM. Experiments in growth chambers suggest that it would have increased photorespiration and decreased water-use efficiency in C3 trees, shrubs and herbs; thus providing a competitive advantage to C4 plants, notably graminoids, and CAM cacti and succulents, which are drought-adapted and possess carbon-concentrating mechanisms that inhibit photorespiration. Although the Earth’s orbital configuration was similar to today, other ice-age boundary conditions such as lower sea-surface temperatures were associated with widespread cooling and regional precipitation changes over tropical land areas. In contrast, during the early to mid-Holocene, increased (decreased) climatic seasonality in the Northern (Southern) Hemisphere should have resulted in an enhanced inter-hemispheric gradient in summer-monsoon precipitation, at a time when air temperatures and CO2 were close to their preindustrial levels. This paper will review the case for significant changes in the terrestrial carbon cycle in Africa and the tropical Americas during the Late Quaternary, focussing particularly on evidence derived from stable carbon isotopes and fossil grass cuticles. Significant increases in the proportion of C4 biomass at the LGM were restricted to sites at which the effects of lower atmospheric CO2 were enhanced by decreased precipitation. Furthermore, the C4/C3 ratio increased during the early to mid-Holocene in areas subject to drought or to an enhanced proportion of summer rainfall. These findings suggest that regional climate has been more important as a control on the past distribution of C3 and C4 plants than atmospheric CO2, although the latter may have been responsible for some of the puzzling features of the palaeoecological record, such as the occurrence of no-analogue plant assemblages and altered vegetation-canopy structure.

Huang, Y, Street-Perrott, F.A. et al. 2001. Climate change as the dominant control on glacial-interglacial variations in C3 and C4 plant abundance. Science 293, 1647-1651.