The effects of late Quaternary climate and pCO2 change on C4 plant abundance in the south-central United States

Abstract

The late Quaternary was a time of substantial environmental change, with the past 70,000 years exhibiting global changes in climate, atmospheric composition, and terrestrial floral and faunal assemblages. We use isotopic data and couple climate and vegetation models to assess the balance between C 3 and C 4 vegetation in Texas during this period. The carbon isotope composition of fossil bison, mammoth, and horse tooth enamel is used as a proxy for C 3 versus C 4 plant consumption, and indicates that C 4 plant biomass remained above 55% through most of Texas from prior to the Last Glacial Maximum (LGM) into the Holocene. These data also reveal that horses did not feed exclusively on herbaceous plants, consequently isotopic data from horses are not reliable indicators of the C 3 –C 4 balance in grassland biomes. Estimates of C 4 percentages from coupled climate–vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide (CO 2 ) concentrations in shaping the regional C 4 signal. C 4 percentages estimated using observed modern climate–vegetation relationships and late Quaternary climate variables (simulated by a global climate model) are much lower than those indicated by carbon isotope values from fossils. When the effect of atmospheric CO 2 concentration on the competitive balance between C 3 and C 4 plants is included in the numerical experiment, however, estimated C 4 percentages show better agreement with isotopic estimates from late Quaternary mammals and soils. This result suggests that low atmospheric CO 2 levels played a role in the observed persistence of C 4 plants throughout the late Quaternary.