Biogeosciences, 12, 2927-2951, 2015
https://doi.org/10.5194/bg-12-2927-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
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 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.

Citation: Veenendaal, E. M., Torello-Raventos, M., Feldpausch, T. R., Domingues, T. F., Gerard, F., Schrodt, F., Saiz, G., Quesada, C. A., Djagbletey, G., Ford, A., Kemp, J., Marimon, B. S., Marimon-Junior, B. H., Lenza, E., Ratter, J. A., Maracahipes, L., Sasaki, D., Sonké, B., Zapfack, L., Villarroel, D., Schwarz, M., Yoko Ishida, F., Gilpin, M., Nardoto, G. B., Affum-Baffoe, K., Arroyo, L., Bloomfield, K., Ceca, G., Compaore, H., Davies, K., Diallo, A., Fyllas, N. M., Gignoux, J., Hien, F., Johnson, M., Mougin, E., Hiernaux, P., Killeen, T., Metcalfe, D., Miranda, H. S., Steininger, M., Sykora, K., Bird, M. I., Grace, J., Lewis, S., Phillips, O. L., and Lloyd, J.: Structural, physiognomic and above-ground biomass variation in savanna–forest transition zones on three continents – how different are co-occurring savanna and forest formations?, Biogeosciences, 12, 2927-2951, https://doi.org/10.5194/bg-12-2927-2015, 2015.
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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.
When nearby forest and savanna stands are compared, they are not as structurally different as...
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