C3C4 Dynamics during Glacial Interglacial Periods

The quantum-yield model predicts that important changes in the global proportions of C4 biomass occurred during the Pleistocene glacial-interglacial transitions. Figure 1 shows that at very low atmospheric C02 levels, C4 plants can be favored even at moderately low temperatures. The oscillation between glacial and interglacial conditions reflected an oscillation between about 180 and 280 ppmV (Fig. 2, middle), respectively, based on the CO, concentrations in the Antarctic ice cores (Petit et al, 1999). The temperature change between the glacial and interglacial intervals varied globally, with estimated changes in temperature from about 5°C in the tropics (Stute et al, 1995) to >15°C in the polar regions (Cuffey et al, 1995). Therefore, the dCO,/dT gradient in the tropics was about 20 ppm/°C, compared to about 7 ppm/°C at high latitudes. Based on the slope of the C, /C4 crossover at low atmospheric C02 levels (Fig. 1), it is possible that in some regions greater C4 abundance would be expected in glacial conditions relative to interglacial conditions, because the "CO, starvation" effect would be more decisive than the "temperature" effect.

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Lake Bosumtwi, Ghana I Talbot and Johannessen (1992)

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Lake Barombl Mbo,

West Cameroon Giresse etal. (1994)

CO2 record I- ," ice core Antarctica Neftel etal (1988)

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Lake Bosumtwi, Ghana I Talbot and Johannessen (1992)

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Lake Barombl Mbo,

West Cameroon Giresse etal. (1994)

CO2 record I- ," ice core Antarctica Neftel etal (1988)

FIGURE 6 Chronological profiles of the carbon isotope ratio values of organic matter from lake sediments and bogs in central Africa. The data indicate that these areas all had more extensive C4 biomass during the last glacial maximum (30-20 ka B.P. than during the Flolocene (10 ka B.P. to present). Data are from Talbot and Johannessen (1992), Giresse et al. (1994) Aueour et al. (1994), and Neftel et al. (1988). Adopted from Cerling et al. (1998a).

FIGURE 6 Chronological profiles of the carbon isotope ratio values of organic matter from lake sediments and bogs in central Africa. The data indicate that these areas all had more extensive C4 biomass during the last glacial maximum (30-20 ka B.P. than during the Flolocene (10 ka B.P. to present). Data are from Talbot and Johannessen (1992), Giresse et al. (1994) Aueour et al. (1994), and Neftel et al. (1988). Adopted from Cerling et al. (1998a).

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FIGURE 7 Predicted relative carbon gain by the quantum-yield model and therefore predicted competitive success by C,- and C4-grass canopies across the Great Plains of North America under today's atmospheric carbon dioxide levels. Noted are the predicted cross-over points from Cr to C4-dominance based on the quantum-yield model and the observations for soil organic matter (Tieszen etal., 1997) and for aboveground harvests (Epstein etal, 1997). Adopted from Ehleringer (1978).

Ehleringer et al (1997) examined published reports of 8L,C in peat bogs and lakes from Central Africa in regions that are currently dominated by rain-forest ecosystems. The available data strongly support the hypothesis of extensive C4 expansion during the last full glacial (Aocour et al, 1993; Hillaire etal, 1989 )(Fig. 6). This implies extensive retreat of the African rain-forest ecosystems and has important implications for réfugia during the Pleistocene which are discussed below. Farther east in Africa, sedimentary data from Sacred Lake in Kenya also show that C4 grasses were much more common during the glacial period when C3 vegetation would have been "C02 starved" (Street-Perrott etal, 1995; 1997; Huang et al, 1995; 1999). Following déglaciation, the C4 abundances in the Sacred Lake region exhibited a dramatic decline correlated with the increases in atmospheric C02 levels.

Within North American ecosystems, there is also evidence that C4 ecosystems were more extensive during the last glacial period than they are today. Soil carbonates from the southwestern portions of North America show that C4 plants dominated the landscape during glacial periods, but are less abundant in these aridland ecosystems today (Cole and Monger, 1994; Liu et al, 1996; Monger et al, 1998). Dietary analyses of fossil herbivores from western North America also provide convincing evidence of widespread C4 abundance in regions that have a near absence of C4 grasses today (Con-nin et al, 1998). While the mechanisms for the observed decline in C4 abundance in North America require further study, the dramatic decrease in C4 plants is correlated with the transition out of the glacial and the abrupt increases in atmospheric C02 levels.

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FIGURE 7 Predicted relative carbon gain by the quantum-yield model and therefore predicted competitive success by C,- and C4-grass canopies across the Great Plains of North America under today's atmospheric carbon dioxide levels. Noted are the predicted cross-over points from Cr to C4-dominance based on the quantum-yield model and the observations for soil organic matter (Tieszen etal., 1997) and for aboveground harvests (Epstein etal, 1997). Adopted from Ehleringer (1978).

FIGURE 8 Predicted distributions of C, and C4 grasses in steppe and savanna ecosystems of the world. These are the only two ecological regions where grasses are a significant fraction of the vegetation. Distribution of ecological regions is based on Bailey (1998) and the partitioning of photosynthetic pathways is based on the synthesis in Sage and Monson (1999).

FIGURE 8 Predicted distributions of C, and C4 grasses in steppe and savanna ecosystems of the world. These are the only two ecological regions where grasses are a significant fraction of the vegetation. Distribution of ecological regions is based on Bailey (1998) and the partitioning of photosynthetic pathways is based on the synthesis in Sage and Monson (1999).

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