Contributions of woody and herbaceous vegetation to tropical savanna ecosystem productivity: a quasi-global estimate
Jon Lloyd (1, 2), Michael I. Bird (3, 4), Lins Vellen (3, 5), Antonio Carlos Miranda (6), Elmar M. Veenendaal (7), Gloria Djagbletey (8), Heloisa S. Miranda (6), Garry Cook (9) and Graham D. Farquhar (5)
1. Earth and Biosphere Institute, School of Geography, University of Leeds, Leeds, LS2 9JT, U.K. / 2. Corresponding author () / 3. Research School of Earth Sciences, Institute of Advanced Studies, Australian National University, Canberra 0200, Australia / 4. Present address: School of Geography and Geosciences, Irvine Building, University of St. Andrews, St. Andrews, Fife, KY16
9AL, U.K. / 5. Environmental Biology Group, Research School of Biological Sciences, Institute of Advanced Studies, Australian National University,
Canberra 0200, Australia / 6. Departmento de Ecologia, Universidade de Brasília, Brasília DF 70910, Brazil / 7. Nature Conservation and Plant Ecology Group Wageningen University, Bornse Steeg 69, 6708PD Wageningen, The Netherlands / 8. Forestry Research Institute of Ghana, P.O. Box 63, Kumasi, Ghana / 9. CSIRO, Sustainable Ecosystems, PMB 44, Winnellie, Northern Territory 0822, Australia / Received July 29, 2006; accepted November 29, 2006; published online January 2, 2008
Summary
To estimate the relative contributions of woody and herbaceous vegetation to savanna productivity, we measured the 13C/12C isotopic ratios of leaves from trees, shrubs, grasses and the surface soil carbon pool for 22 savannas in Australia, Brazil
and Ghana covering the full savanna spectrum ranging from almost pure grassland to closed woodlands on all three continents.
All trees and shrubs sampled were of the C3 pathway and all grasses of the C4 pathway with the exception of Echinolaena inflexa (Poir.) Chase, a common C3 grass of the Brazilian cerrado. By comparing the carbon isotopic compositions of the plant and carbon pools, a simple model
relating soil δ13C to the relative abundances of trees + shrubs (woody plants) and grasses was developed. The model suggests that the relative
proportions of a savanna ecosystem’s total foliar projected cover attributable to grasses versus woody plants is a simple
and reliable index of the relative contributions of grasses and woody plants to savanna net productivity. Model calibrations
against woody tree canopy cover made it possible to estimate the proportion of savanna productivity in the major regions of
the world attributable to trees + shrubs and grasses from ground-based observational maps of savanna woodiness. Overall, it
was estimated that 59% of the net primary productivity (Np) of tropical savannas is attributable to C4 grasses, but that this proportion varies significantly within and between regions. The C4 grasses make their greatest relative contribution to savanna Np in the Neotropics, whereas in African regions, a greater proportion of savanna Np is attributable to woody plants. The relative contribution of C4 grasses in Australian savannas is intermediate between those in the Neotropics and Africa. These differences can be broadly
ascribed to large scale differences in soil fertility and rainfall.
Keywords:
carbon isotopes, cerrado, grasses, photosynthetic pathway, soil carbon pool.
