Simulated effects of low atmospheric CO2 on structure and composition of North American vegetation at the Last Glacial Maximum

1. Physiological experiments have indicated that the lower CO2 levels of th e last glaciation (200 mu mol mol(-1)) probably reduced plant water-use eff iciency (WUE) and that they combined with increased aridity and colder temp eratures to alter vegetation structure and composition at the Last Glacial Maximum (LGM). 2. The effects of low CO2 on vegetation structure were investigated using B IOME3 simulations of leaf area index (LAI), and a two-by-two factorial expe rimental design (modern/LGM CO2, modern/ LGM climate). 3. Using BIOME3, and a combination bf lowered CO2 and simulated LGM climate (from the NCAR-CCM1 model), results in the introduction of additional xeri c vegetation types between open woodland and closed-canopy forest along a l atitudinal gradient in eastern North America. 4. The simulated LAI of LGM vegetation was 25-60% lower in many regions of central and eastern United States relative to modern climate, indicating th at glacial vegetation was much more open than today. 5. Comparison of factorial simulations show that low atmospheric CO2 has th e potential to alter vegetation structure (LAI) to a greater extent than LG M climate. 6. If the magnitude of LAI reductions simulated for glacial North America w ere global, then low atmospheric CO2 may have promoted atmospheric warming and increased aridity, through alteration of rates of water and heat exchan ge with the atmosphere.