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A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance

Michael A Urban, David M Nelson, F Alayne Street-Perrott, Dirk Verschuren (UGent) and Feng Sheng Hu
(2015) ECOLOGY. 96(3). p.642-653
Author
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Abstract
Various environmental factors, including atmospheric CO2 (pCO(2)), regional climate, and fire, have been invoked as primary drivers of long-term variation in C-4 grass abundance. Evaluating these hypotheses has been difficult because available paleorecords often lack information on past C-4 grass abundance or potential environmental drivers. We analyzed carbon isotope ratios (C-13) of individual grains of grass pollen in the sediments of two East African lakes to infer changes in the relative abundance of C-3 vs. C-4 grasses during the past 25000 years. Results were compared with concurrent changes in pCO(2), temperature, moisture balance, and fire activity. Our grass-pollen C-13 analysis reveals a dynamic history of grass-dominated vegetation in equatorial East Africa: C-4 grasses have not consistently dominated lowland areas, and high-elevation grasses have not always been predominantly C-3. On millennial timescales, C-4 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 C-3 and C-4 grasses. Above the present-day treeline on Mt. Kenya, C-4 grass abundance declined from an average of 90% during the glacial period to less than 60% throughout the Holocene, coincident with increases in pCO(2) and temperature, and shifts in moisture balance. In the lowland savanna southeast of Mt. Kilimanjaro, C-4 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 pCO(2) and climate on the interplay of C-3 and C-4 grasses and shed light on an emerging conceptual model regarding the expansion of C-4-dominated grasslands in Earth's history. They also suggest that future changes in the C-3/C-4 composition of grass-dominated ecosystems will likely exhibit striking spatiotemporal variability as a result of varying combinations of environmental controls.
Keywords
ISOTOPE, VEGETATION, POLLEN, MT-KENYA, ICE CORES, MOUNT-KENYA, LAKE RUTUNDU, CARBON-DIOXIDE, C-4 GRASSLANDS, EAST-AFRICA, grasslands, fire, East Africa, climate change, carbon isotopes, C-3 grasses, C-4 grasses, atmospheric pCO(2)

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Citation

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MLA
Urban, Michael A., et al. “A Late-Quaternary Perspective on Atmospheric PCO(2), Climate, and Fire as Drivers of C-4-Grass Abundance.” ECOLOGY, vol. 96, no. 3, 2015, pp. 642–53, doi:10.1890/14-0209.1.
APA
Urban, M. A., Nelson, D. M., Street-Perrott, F. A., Verschuren, D., & Hu, F. S. (2015). A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance. ECOLOGY, 96(3), 642–653. https://doi.org/10.1890/14-0209.1
Chicago author-date
Urban, Michael A, David M Nelson, F Alayne Street-Perrott, Dirk Verschuren, and Feng Sheng Hu. 2015. “A Late-Quaternary Perspective on Atmospheric PCO(2), Climate, and Fire as Drivers of C-4-Grass Abundance.” ECOLOGY 96 (3): 642–53. https://doi.org/10.1890/14-0209.1.
Chicago author-date (all authors)
Urban, Michael A, David M Nelson, F Alayne Street-Perrott, Dirk Verschuren, and Feng Sheng Hu. 2015. “A Late-Quaternary Perspective on Atmospheric PCO(2), Climate, and Fire as Drivers of C-4-Grass Abundance.” ECOLOGY 96 (3): 642–653. doi:10.1890/14-0209.1.
Vancouver
1.
Urban MA, Nelson DM, Street-Perrott FA, Verschuren D, Hu FS. A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance. ECOLOGY. 2015;96(3):642–53.
IEEE
[1]
M. A. Urban, D. M. Nelson, F. A. Street-Perrott, D. Verschuren, and F. S. Hu, “A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance,” ECOLOGY, vol. 96, no. 3, pp. 642–653, 2015.
@article{7238092,
  abstract     = {{Various environmental factors, including atmospheric CO2 (pCO(2)), regional climate, and fire, have been invoked as primary drivers of long-term variation in C-4 grass abundance. Evaluating these hypotheses has been difficult because available paleorecords often lack information on past C-4 grass abundance or potential environmental drivers. We analyzed carbon isotope ratios (C-13) of individual grains of grass pollen in the sediments of two East African lakes to infer changes in the relative abundance of C-3 vs. C-4 grasses during the past 25000 years. Results were compared with concurrent changes in pCO(2), temperature, moisture balance, and fire activity. Our grass-pollen C-13 analysis reveals a dynamic history of grass-dominated vegetation in equatorial East Africa: C-4 grasses have not consistently dominated lowland areas, and high-elevation grasses have not always been predominantly C-3. On millennial timescales, C-4 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 C-3 and C-4 grasses. Above the present-day treeline on Mt. Kenya, C-4 grass abundance declined from an average of 90% during the glacial period to less than 60% throughout the Holocene, coincident with increases in pCO(2) and temperature, and shifts in moisture balance. In the lowland savanna southeast of Mt. Kilimanjaro, C-4 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 pCO(2) and climate on the interplay of C-3 and C-4 grasses and shed light on an emerging conceptual model regarding the expansion of C-4-dominated grasslands in Earth's history. They also suggest that future changes in the C-3/C-4 composition of grass-dominated ecosystems will likely exhibit striking spatiotemporal variability as a result of varying combinations of environmental controls.}},
  author       = {{Urban, Michael A and Nelson, David M and Street-Perrott, F Alayne and Verschuren, Dirk and Hu, Feng Sheng}},
  issn         = {{0012-9658}},
  journal      = {{ECOLOGY}},
  keywords     = {{ISOTOPE,VEGETATION,POLLEN,MT-KENYA,ICE CORES,MOUNT-KENYA,LAKE RUTUNDU,CARBON-DIOXIDE,C-4 GRASSLANDS,EAST-AFRICA,grasslands,fire,East Africa,climate change,carbon isotopes,C-3 grasses,C-4 grasses,atmospheric pCO(2)}},
  language     = {{eng}},
  number       = {{3}},
  pages        = {{642--653}},
  title        = {{A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance}},
  url          = {{http://doi.org/10.1890/14-0209.1}},
  volume       = {{96}},
  year         = {{2015}},
}
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