title: A late-Quaternary perspective, on atmospheric pCO2, climate, and fire as drivers of C4-grass abundance.
abstract: Various environmental factors, including atmospheric CO2 (pCO2), regional climate, and fire, have been invoked as primary drivers of long-term variation in C4 grass abundance. Evaluating these hypotheses has been difficult because available paleorecords often lack information on past C4 grass abundance or potential environmental drivers. We analyzed carbon isotope ratios (delta13C) of individual grains of grass pollen in the sediments of two East African lakes to infer changes in the relative abundance of C3 vs. C4 grasses during the past 25 000 years. Results were compared with concurrent changes in pCO2, temperature, moisture balance, and fire activity. Our grass-pollen delta13C analysis reveals a dynamic history of grass-dominated vegetation in equatorial East Africa: C4 grasses have not consistently dominated lowland areas, and high-elevation grasses have not always been predominantly C3. On millennial timescales, C4 grass abundance does not correlate with charcoal influx at either site, suggesting that fire was not a major proximate control of the competitive balance between C3 and C4 grasses. Above the present-day treeline on Mt. Kenya, C4 grass abundance declined from an average of approximately 90% during the glacial period to less than approximately 60% throughout the Holocene, coincident with increases in pCO2 and temperature, and shifts in moisture balance. In the lowland savanna southeast of Mt. Kilimanjaro, C4 grass abundance showed no such directional trend, but fluctuated markedly in association with variation in rainfall amount and seasonal-drought severity. These results underscore spatiotemporal variability in the relative influence of pCO2 and climate on the interplay of C3 and C4 grasses and shed light on an emerging conceptual model regarding the expansion of C4-dominated grasslands in Earth's history. They also suggest that future changes in the C3/C4 composition of grass-dominated ecosystems will likely exhibit striking spatiotemporal variability as a result of varying combinations of environmental controls.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Urban
authors.forename: David M
authors.initials: DM
authors.lastname: Nelson
authors.forename: F Alayne
authors.initials: FA
authors.lastname: Street-Perrott
authors.forename: Dirk
authors.initials: D
authors.lastname: Verschuren
authors.forename: Feng Sheng
authors.initials: FS
authors.lastname: Hul
mesh_terms: D001272:Atmosphere; D002244:Carbon / Q000737:chemistry; D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000032:analysis*; D002247:Carbon Isotopes / Q000378:metabolism; D002980:Climate; D005390:Fires; D065948:Grassland; D007630:Kenya; D010163:Paleontology; D006109:Poaceae / Q000737:chemistry / Q000502:physiology*; D011156:Population Density
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide; D002244:Carbon