, Volume 52, Issue 3, pp 358–370 | Cite as

Effect of the replacement of a native savanna by an African Brachiaria decumbens pasture on the CO2 exchange in the Orinoco lowlands, Venezuela

  • J. San José
  • R. Montes
  • N. Nikonova
  • J. Grace
  • C. Buendía
Original Papers


In the Orinoco lowlands, savannas have been often replaced by pastures composed of the C4 grass, Brachiaria decumbens Stapf. We addressed following questions: (1) How does the replacement of the native vegetation affect CO2 exchange on seasonal and annual scales? (2) How do biophysical constraints change when the landscape is transformed? To assess how these changes affect carbon exchange, we determined simultaneously the CO2 fluxes by eddy covariance, and the soil CO2 efflux by a chamber-based system in B. decumbens and herbaceous savanna stands. Measurements covered a one-year period from the beginning of the dry season (November 2008) to the end of the wet season (November 2009). During the wet season, the net ecosystem CO2 exchange reached maximum values of 23 and 10 μmol(CO2) m−2 s−1 in the B. decumbens field and in the herbaceous savanna stand, respectively. The soil CO2 efflux for both stands followed a temperature variation during the dry and wet seasons, when the soil water content (SWC) increased above 0.087 m3 m−3 in the latter case. Bursts of CO2 emissions were evident when the dry soil experienced rehydration. The carbon source/sink dynamics over the two canopies differed markedly. Annual measurements of the net ecosystem production indicated that the B. decumbens field constituted a strong carbon sink of 216 g(C) m−2 y−1. By contrast, the herbaceous savanna stand was found to be only a weak sink [36 g(C) m−2 y−1]. About 53% of the gross primary production was lost as the ecosystem respiration. Carbon uptake was limited by SWC in the herbaceous savanna stand as evident from the pattern of water-use efficiency (WUE). At the B. decumbens stand, WUE was relatively insensitive to SWC. Although these results were specific to the studied site, the effect of land use changes and the physiological response of the studied stands might be applicable to other savannas.

Additional key words

Eddy covariance soil CO2 eflux water-use efficiency 



apparent photosynthetic flux


humidity mole fraction deficit


soil CO2 efflux at 10°C


soil CO2 efflux


gross primary production


leaf area index


net ecosystem CO2 exchange


net ecosystem flux


net ecosystem production


photosynthetic photon flux


photosynthetic photon flux density


ecosystem respiration


mean friction velocity


water-use efficiency


apparent quantum yield (i.e., initial slope)


evapotranspiration flux


CO2 storage


soil water content


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Copyright information

© The Institute of Experimental Botany 2014

Authors and Affiliations

  • J. San José
    • 1
  • R. Montes
    • 2
  • N. Nikonova
    • 1
  • J. Grace
    • 3
  • C. Buendía
    • 1
  1. 1.Ecology CenterVenezuelan Institute for Scientific ResearchCaracasVenezuela
  2. 2.Environment Studies DepartamentSimón Bolívar UniversityCaracasVenezuela
  3. 3.School of GeoSciencesThe University of EdinburghEdinburghUK

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