The Effects of Late Quaternary Climate and Pco2 Change on C4 Plant Abundance in the South-Central United States Paul L

Palaeogeography, Palaeoclimatology, Palaeoecology 207 (2004) 331–357 www.elsevier.com/locate/palaeo The effects of late Quaternary climate and pCO2 change on C4 plant abundance in the south-central United States Paul L. Kocha,*, Noah S. Diffenbaugha,1, Kathryn A. Hoppeb,2 a Department of Earth Sciences, University of California, Santa Cruz, CA 95064, USA b Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA Received 28 July 2003; accepted 25 September 2003 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 C3 and C4 vegetation in Texas during this period. The carbon isotope composition of fossil bison, mammoth, and horse tooth enamel is used as a proxy for C3 versus C4 plant consumption, and indicates that C4 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 C3 –C4 balance in grassland biomes. Estimates of C4 percentages from coupled climate–vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide (CO2) concentrations in shaping the regional C4 signal. C4 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 CO2 concentration on the competitive balance between C3 and C4 plants is included in the numerical experiment, however, estimated C4 percentages show better agreement with isotopic estimates from late Quaternary mammals and soils. This result suggests that low atmospheric CO2 levels played a role in the observed persistence of C4 plants throughout the late Quaternary. D 2004 Elsevier B.V. All rights reserved. Keywords: C3;C4; Pleistocene; Holocene; Mammal; Soil; Paleosol; Carbon isotope; Oxygen isotope; Vegetation; GCM; Texas 1. Introduction east, arid subtropical in the west, and temperate/ continental in the north. Temperature varies strongly Today, Texas exhibits strong gradients in climate from north to south, whereas rainfall changes from and vegetation. Climate is humid subtropical in the east to west. Intersecting climatic gradients couple with geology and topography to create vegetation zones (Fig. 1, Appendix A). Moving west across * Corresponding author. Tel.: +1-831-459-5861. northern and central Texas, the pine and hardwood E-mail addresses: [email protected] (P.L. Koch), forests of the east give way to oak woodlands mixed [email protected] (N.S. Diffenbaugh), [email protected] (K.A. Hoppe). with tallgrass prairie, and then to mixedgrass and 1 Tel.: +1-831-459-3504. shortgrass prairie intermingled with shrublands on 2 Tel.: +1-650-723-9191. the Texas Panhandle (Diamond et al., 1987). The 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2003.09.034 332 P.L. Koch et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 207 (2004) 331–357 Fig. 1. Map showing localities and modern Texas vegetation zones. Vegetation zones are described in Appendix A. The area marked with gray shading on the Texas–New Mexico border is the region from which soil samples from Holliday (2000) were collected (Locality 21). BP— Blackland Prairie; OWP—Oak Wood and Prairie; CSP—Coast Sand Plains. coast has scattered forests, prairie, and wetlands. Like many parts of the globe, the south-central US Shrublands occur inland of the coast in southern was subject to large environmental fluctuations in the Texas, and woodlands and shrublands occur on the Quaternary. Noble gas analyses suggest that the mean plateaus of central Texas. In mountainous western annual temperature in the south-central US was f 5 Texas, which is within the Chihuahuan desert, basins jC lower at the Last Glacial Maximum (LGM) (Stute have lowland desert grass- and shrublands and higher et al., 1995). Quantitative estimates of past precipita- altitudes have forests. tion are unavailable, but lake levels, fossil assemb- P.L. Koch et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 207 (2004) 331–357 333 lages, and speleothem growth rates offer estimates periods with lower amounts of moisture, and C3 plants that are qualitative and variable (Mock and Bartlein, should dominate wetter areas or periods (Polley et al., 1995; Wilkins and Currey, 1997; Musgrove et al., 1993; Huang et al., 2001). Differences in WUE no 2001). From the last interglacial to the Holocene, the doubt contribute to dominance by C3 trees in areas concentration of atmospheric CO2 was lower than pre- with substantial rainfall, like eastern Texas. Yet within Industrial values ( f 180 vs. 280 ppmV), though grasslands, which are at least seasonally dry, recent values have risen sharply to >360 ppmV in the last work has shown that C4 grass production is positively two centuries due to human activities (Leuenberger et correlated with mean annual and growing season al., 1992; Petit et al., 1999; Monnin et al., 2001). precipitation (Paruelo and Lauenroth, 1996; Epstein Given the strong climatic gradients that in part shape et al., 1997; Yang et al., 1998).C4 grasses are a vegetation zones in the region today, we might expect greater fraction of biomass in grasslands that are that regional biomes would be sensitive to the large wetter, not drier. C4 dicots are more abundant in dry climatic and atmospheric shifts of the Quaternary. areas, but they comprise a small fraction of biomass Pollen and plant macrofossils provide the most direct (2% to 5%) (Ehleringer et al., 1997). Thus, the measure of how vegetation responded to Pleistocene distribution of C3 and C4 plants on grasslands is climate and atmospheric changes. Unfortunately, pol- affected by moisture, but not as expected from simple len and plant macrofossil sites are uncommon in Texas, ideas about differences in WUE. and the state often ‘falls between the cracks’ in synoptic Carbon-concentrating ability also makes C4 plants studies of past climate and vegetation (e.g., Thompson less prone to photorespiration, a process in which and Anderson, 2000; Williams et al., 2000). Prior work fixed carbon is oxidized without an energy yield for does reveal two points of interest, however. First, the plant. Photorespiration rates in C3 plants rise with pollen data have led to conflicting views of the LGM temperature, but are low and invariant in C4 plants. As vegetation of northern and central Texas as either a a result, the quantum yield (i.e., carbon gain per pine-spruce woodland or a grassland (Bryant and photon absorbed) for C3 plants drops as temperature Holloway, 1985; Hall and Valastro, 1995). Second, rises, but remains constant for C4 plants (Ehleringer et the type of grasses comprising Pleistocene biomes is al., 1997). This temperature sensitivity in yield likely unclear. Plants can use the C3,C4, or Crassulacean acid explains why C4 grasses dominate grasslands with a metabolism (CAM) photosynthetic pathways. These warm growing season (>22 jC), whereas C3 grasses plants differ in many key attributes that affect, among dominate where the growing season is cool (Ehler- other things, biogeography, competitive abilities, rates inger, 1978; Paruelo and Lauenroth, 1996; Tieszen et of carbon fixation, and susceptibility to predation al., 1997). By similar logic, we might expect that C3 (Ehleringer et al., 1997).C4 photosynthesis is common grasses would dominate under cool Pleistocene cli- in grasses, but also occurs in sedges and weedy herbs, mates. The situation is complicated, however, because and rarely in woody dicots. Most trees, shrubs, and experiments have shown that quantum yield is affect- herbs, and many grasses are C3 plants. CAM occurs ed by atmospheric pCO2 as well as temperature. chiefly in succulent plants. Because of differences in Quantum yield drops with decreasing pCO2 in C3 their sensitivities to environmental factors, plants using plants, but is insensitive to pCO2 changes in C4 plants C3 versus C4 photosynthetic pathways may have had (Ehleringer et al., 1997). Thus, lower pCO2 in the different geographic ranges in the Quaternary (Ehler- Pleistocene would have favored C4 plants, whereas inger et al., 1997). lower temperatures would have favored C3 plants. C4 plants have structural and enzymatic adapta- Given these complex interactions, predicting the tions that allow them to concentrate CO2 at the site of proportions of C3 and C4 plants will require quantita- carbon fixation. As a consequence, C4 plants have tive modeling. Collatz et al. (1998) conducted a global greater water use efficiency (WUE) than C3 plants. climate–vegetation-modeling study of C4 plant distri- That is, photosynthetic carbon gain relative to tran- bution under lower pCO2 with a LGM climate simu- spirational water loss is higher in C4 than in C3 plants. lated using a general circulation model. The south- If this greater efficiency translates to a competitive central US was the only area in North America where advantage, C4 plants should dominate areas or time they simulated a change in %C4 biomass between the 334 P.L. Koch et al. / Palaeogeography, Palaeoclimatology,

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