Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 11, 4591–4636, 2014 Open Access www.biogeosciences-discuss.net/11/4591/2014/ Biogeosciences BGD doi:10.5194/bgd-11-4591-2014 Discussions © Author(s) 2014. CC Attribution 3.0 License. 11, 4591–4636, 2014 This discussion paper is/has been under review for the journal Biogeosciences (BG). Forest–savanna Please refer to the corresponding final paper in BG if available. transition zones Structural, physiognomic and E. M. Veenendaal et al. aboveground biomass variation in Title Page savanna-forest transition zones on three Abstract Introduction continents. How different are Conclusions References co-occurring savanna and forest Tables Figures formations? J I E. M. Veenendaal1, M. Torello-Raventos2, T. R. Feldpausch3, T. F. Domingues4, J I 5 3 2,25 3,6 7 8 F. Gerard , F. Schrodt , G. Saiz , C. A. Quesada , G. Djagbletey , A. Ford , Back Close J. Kemp9, B. S. Marimon10, B. H. Marimon-Junior10, E. Lenza10, J. A. Ratter11, L. Maracahipes10, D. Sasaki12, B. Sonké13, L. Zapfack13, D. Villarroel14, Full Screen / Esc M. Schwarz15, F. Yoko Ishida6,16, M. Gilpin3, G. B. Nardoto17, K. Affum-Baffoe18, L. Arroyo14, K. Bloomfield3, G. Ceca1, H. Compaore19, K. Davies2, A. Diallo20, Printer-friendly Version N. M. Fyllas3, J. Gignoux21, F. Hien20, M. Johnson3, E. Mougin22, P. Hiernaux22, Interactive Discussion 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 4591 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1Centre for Ecosystem Studies, University of Wageningen, 6700 AA, the Netherlands 2School of Earth and Environmental Science, James Cook University, Cairns, Qld, Australia BGD 3Earth and Biosphere Institute, School of Geography, University of Leeds, UK 11, 4591–4636, 2014 4School of Geosciences, University of Edinburgh, Scotland, UK 5Centre for Ecology & Hydrology, Wallingford, UK 6Instituto Nacional de Pesquisas da Amazonia, Manaus, Brazil Forest–savanna 7 Forest Research Institute of Ghana, Kumasi, Ghana transition zones 8CSIRO Ecosystem Sciences Tropical Forest Research Centre, Atherton Qld, Australia 9Australian Tropical Forest Research Institute, Cairns, Australia E. M. Veenendaal et al. 10Universidade do Estado de Mato Grosso, Nova Xavantina, MT, Brazil 11Royal Botanic Garden, Edinburgh, Scotland, UK 12 Fundação Ecológica Cristalino, Alta Floresta, Brazil Title Page 13Department of Biology, University of Yaoundé Cameroon 14Museo Noel Kempff Mercado, Santa Cruz, Bolivia Abstract Introduction 15 Fieldwork Assistance, Jena, Germany Conclusions References 16School of Marine and Tropical Biology, James Cook University, Cairns, Qld, Australia 17Universidade de Brasilia, DF, Brazil Tables Figures 18Resource Management Support Centre, Forestry Commission of Ghana, Kumasi, Ghana 19Institut de l’Environnement et de Recherches Agricoles, Ouagadougou, Burkina Faso J I 20Centre National des Semences Forestières, Ouagadougou, Burkina Faso 21 Ecole Normale Supérieure, Paris Cedex 05, France J I 22Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, Toulouse, France Back Close 23World Wildlife Fund, Washington DC, USA 24 Conservation International, Washington DC, USA Full Screen / Esc 25Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Garmisch-Partenkirchen, Germany Printer-friendly Version 26Department of Geography, University College London, UK 27 Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Interactive Discussion Road, Ascot, Berkshire SL5 7PY, UK 4592 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Received: 12 December 2013 – Accepted: 10 February 2014 – Published: 24 March 2014 Correspondence to: J. Lloyd ([email protected]) BGD Published by Copernicus Publications on behalf of the European Geosciences Union. 11, 4591–4636, 2014 Forest–savanna transition zones E. M. Veenendaal et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 4593 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD Through interpretations of remote sensing data and/or theoretical propositions, the idea that forest and savanna represent “alternative stable states” is gaining increas- 11, 4591–4636, 2014 ing acceptance. Filling an observational gap, we present detailed stratified floristic and 5 structural analyses for forest and savanna stands mostly located within zones of tran- Forest–savanna sition (where both vegetation types occur in close proximity) in Africa, South America transition zones and Australia. Woody plant leaf area index variation was related in a similar way to tree canopy cover for both savanna and forest with substantial overlap between the E. M. Veenendaal et al. two vegetation types. As total woody plant canopy cover increased, so did the contri- 10 bution of middle and lower strata of woody vegetation to this total. Herbaceous layer cover also declined as woody cover increased. This pattern of understorey grasses and Title Page herbs being progressively replaced by shrubs as canopy closure occurs was found for Abstract Introduction 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 Conclusions References 15 the savanna-forest species discontinuum is observed compared to that implied when Tables Figures 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 clo- sure. An increased contribution of forest species to the total subordinate cover is also J I observed as savanna stand canopy closure occurs. Despite similarities in canopy cover J I 20 characteristics, woody vegetation in Africa and Australia attained greater heights and stored a greater concentration of above ground biomass than in South America. Up to Back Close three times as much aboveground biomass is stored in forests compared to savannas Full Screen / Esc under equivalent climatic conditions. Savanna/forest transition zones were also found to typically occur at higher precipitation regimes for South America than for Africa. Printer-friendly Version 25 Nevertheless, coexistence was found to be confined to a well-defined edaphic/climate envelope consistent across all three continents with both soil and climate playing a role Interactive Discussion as the key determinants of the relative location of forest and savanna. Taken together these observations do not lend support the notion of alternate stable states mediated 4594 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | through fire-feedbacks as the prime force shaping the distribution of the two dominant vegetation types of the tropical lands. BGD 11, 4591–4636, 2014 1 Introduction Forest–savanna In general terms, “savannas” may be defined as woody vegetation formations types transition zones 5 having a fractional axylale (herbaceous) ground cover of at least 0.1; but also with a woody species composition quite distinct from “forests”; the latter also typically E. M. Veenendaal et al. (though not always) with axylales absent (Torello-Raventos et al., 2013). Together for- est and savanna dominate the tropical vegetated regions covering 0.15 to 0.2 of the earth’s surface (Walter and Mueller-Dombois, 1971). Title Page 10 At a broad scale it has long been recognized that the distributions of these two biomes are principally governed by precipitation and its seasonality (Schimper, 1903). Abstract Introduction Nevertheless, it is sometimes possible to find these different vegetation formation types Conclusions References in climatic zones where they are not usually occurring. For example, stands dominated by species usually associated with forest vegetation formation types (“forest outliers”) Tables Figures −1 15 may be found in Australia at mean annual precipitations (PA) of < 1.0 ma in both Aus- tralia (Fensham, 1995) and South America (Killeen et al., 2006). Conversely, savannas J I are often seen on sandy soils under precipitation regimes usually associated with forest −1 J I (PA > 2.0 ma ) with such “savanna inliers” having been reported for South America, Australia and Africa (Hopkins, 1992; Lloyd et al., 2008; Torello-Raventos et al., 2013). Back Close 20 There are also discrete regions where the two biomes intercept – often referred to as “ecotones” or “Zones of (Ecological) Tension” (ZOT) – where both forest and savanna Full Screen / Esc exist as discrete “patches” under similar climatic conditions. Although the influence of soil structure in shaping vegetation distributions within such ZOT has long been recog- Printer-friendly Version nised (Cochrane, 1989; Ratter, 1992; Thompson et al., 1992; Hoffmann et al., 2009; Interactive Discussion 25 Lehmann et al., 2011; Saiz et al., 2012), the observation that the artificial exclusion of fire from savanna areas within ZOT is followed by invasion of forest species has led to the idea that forest and savanna stands may represent alternate stable states modu- 4595 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | lated by fire-mediated feedbacks (Warman and Moles, 2009; Hoffmann et al., 2012a; Murphy and Bowman, 2012). Here the regular recurrence