Articles | Volume 12, issue 10
Biogeosciences, 12, 2927–2951, 2015
Biogeosciences, 12, 2927–2951, 2015

Research article 21 May 2015

Research article | 21 May 2015

Structural, physiognomic and above-ground biomass variation in savanna–forest transition zones on three continents – how different are co-occurring savanna and forest formations?

E. M. Veenendaal1, M. Torello-Raventos2, T. R. Feldpausch3, T. F. Domingues4, F. Gerard5, F. Schrodt3, G. Saiz2,25, C. A. Quesada6,3, G. Djagbletey7, A. Ford8, J. Kemp9, B. S. Marimon10, B. H. Marimon-Junior10, E. Lenza10, J. A. Ratter11, L. Maracahipes10, D. Sasaki12, B. Sonké13, L. Zapfack13, D. Villarroel14, M. Schwarz15, F. Yoko Ishida16,6, M. Gilpin3, G. B. Nardoto17, K. Affum-Baffoe18, L. Arroyo14, K. Bloomfield3, G. Ceca1, H. Compaore19, K. Davies2, A. Diallo20, N. M. Fyllas3, J. Gignoux21, F. Hien20, M. Johnson3, E. Mougin22, P. Hiernaux22, T. Killeen14,23, D. Metcalfe8, H. S. Miranda17, M. Steininger24, K. Sykora1, M. I. Bird2, J. Grace4, S. Lewis3,26, O. L. Phillips3, and J. Lloyd16,27 E. M. Veenendaal et al.
  • 1Centre for Ecosystem Studies, University of Wageningen, 6700 AA Wageningen, the Netherlands
  • 2School of Earth and Environmental Science, James Cook University, Cairns, Qld, Australia
  • 3Earth and Biosphere Institute, School of Geography, University of Leeds, England, UK
  • 4School of Geosciences, University of Edinburgh, Scotland, UK
  • 5Centre for Ecology & Hydrology, Wallingford, England, UK
  • 6Instituto Nacional de Pesquisas da Amazonia, Manaus, Brazil
  • 7Forest Research Institute of Ghana, Kumasi, Ghana
  • 8CSIRO Ecosystem Sciences Tropical Forest Research Centre, Atherton, Qld, Australia
  • 9Australian Tropical Forest Research Institute, Cairns, Australia
  • 10Universidade do Estado de Mato Grosso, Nova Xavantina, MT, Brazil
  • 11Royal Botanic Garden, Edinburgh, Scotland, UK
  • 12Fundação Ecológica Cristalino, Alta Floresta, Brazil
  • 13Department of Biology, University of Yaoundé, Yaoundé, Cameroon
  • 14Museo Noel Kempff Mercado, Santa Cruz, Bolivia
  • 15Fieldwork Assistance, Jena, Germany
  • 16School of Marine and Tropical Biology, James Cook University, Cairns, Qld, Australia
  • 17Universidade de Brasilia, DF, Brazil
  • 18Resource Management Support Centre, Forestry Commission of Ghana, Kumasi, Ghana
  • 19Institut de l'Environnement et de Recherches Agricoles, Ouagadougou, Burkina Faso
  • 20Centre National des Semences Forestières, Ouagadougou, Burkina Faso
  • 21Ecole Normale Supérieure, Paris CEDEX 05, France
  • 22Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, Toulouse, France
  • 23World Wildlife Fund, Washington DC, USA
  • 24Conservation International, Washington DC, USA
  • 25Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Garmisch-Partenkirchen, Germany
  • 26Department of Geography, University College London, England, UK
  • 27Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY, England, UK

Abstract. Through interpretations of remote-sensing data and/or theoretical propositions, the idea that forest and savanna represent "alternative stable states" is gaining increasing acceptance. Filling an observational gap, we present detailed stratified floristic and structural analyses for forest and savanna stands located mostly within zones of transition (where both vegetation types occur in close proximity) in Africa, South America and Australia. Woody plant leaf area index variation was related to tree canopy cover in a similar way for both savanna and forest with substantial overlap between the two vegetation types. As total woody plant canopy cover increased, so did the relative contribution of middle and lower strata of woody vegetation. Herbaceous layer cover declined as woody cover increased. This pattern of understorey grasses and herbs progressively replaced by shrubs as the canopy closes over was found for both savanna and forests and on all continents. Thus, once subordinate woody canopy layers are taken into account, a less marked transition in woody plant cover across the savanna–forest-species discontinuum is observed compared to that inferred when trees of a basal diameter > 0.1 m are considered in isolation. This is especially the case for shrub-dominated savannas and in taller savannas approaching canopy closure. An increased contribution of forest species to the total subordinate cover is also observed as savanna stand canopy closure occurs. Despite similarities in canopy-cover characteristics, woody vegetation in Africa and Australia attained greater heights and stored a greater amount of above-ground biomass than in South America. Up to three times as much above-ground biomass is stored in forests compared to savannas under equivalent climatic conditions. Savanna–forest transition zones were also found to typically occur at higher precipitation regimes for South America than for Africa. Nevertheless, consistent across all three continents coexistence was found to be confined to a well-defined edaphic–climate envelope with soil and climate the key determinants of the relative location of forest and savanna stands. Moreover, when considered in conjunction with the appropriate water availability metrics, it emerges that soil exchangeable cations exert considerable control on woody canopy-cover extent as measured in our pan-continental (forest + savanna) data set. Taken together these observations do not lend support to the notion of alternate stable states mediated through fire feedbacks as the prime force shaping the distribution of the two dominant vegetation types of the tropical lands.

Short summary
When nearby forest and savanna stands are compared, they are not as structurally different as first seems. Moreover, savanna-forest transition zones typically occur at higher rainfall for South America than for Africa but with coexistence confined to a well-defined edaphic-climate envelope. With interacting soil cation-soil water storage–precipitations effects on canopy cover also observed we argue that both soils and climate influence the location of the two major tropical vegetation types.
Final-revised paper