Sharp ecotones spark sharp ideas: comment on "Structural, physiognomic and above-ground biomass variation in savanna–forest transition zones on three continents – how different are co-occurring savanna and forest formations?" by Veenendaal et al. (2015)

Abstract

Scientific progress occurs as ideas are developed, challenged and debated. Such debates between different schools of thought are plentiful in the history of ecology (Real and Brown, 1991). One emerging ecological paradigm is that tropical forest and savanna can be alternative stable states under the same environmental conditions. There is increasing consensus that savannas can be self-stabilizing through a positive feedback mechanism between fire and low tree cover. Also, the closed canopies of forests can prevent fire to occur by outshading flammable herbaceous vegetation and creating a humid microclimate (Hoffmann et al., 2012). Thus, under given climatic conditions, both forest and savanna can be present. Evidence for this forest–savanna bistability is derived from fire-exclusion experiments (Moreira, 2000; Higgins et al., 2007), vegetation mosaics observed in the field (Warman and Moles, 2009; Favier et al., 2012; Hoffmann et al., 2012; Dantas et al., 2013; Gray and Bond, 2015), vegetation shifts in the paleo-ecological record (Fletcher et al., 2014), mathematical models (Staver and Levin, 2012; Van Nes et al., 2014; Baudena et al., 2015; Staal et al., 2015) and analyses of remotely sensed estimates of tree cover (Hirota et al., 2011; Staver et al., 2011a, b). The latter studies have fueled this debate by showing that tree-cover frequency distributions across the global tropics are bimodal within a range of climatic conditions (with peaks around 20 and > 80 % cover and intermediate cover being rare). In a recent publication in this journal, Veenendaal et al. (2015) presented a global field study of tropical forest– savanna ecotones (or “zones of transition”), arguing that their data are inconsistent with the hypothesis that tropical forest and savanna can be alternative stable states through a feedback between fire and low tree cover. Here we assert that the results presented do not refute but rather support the emerging view of alternative stable states in the tropics and the role of fire therein. Nevertheless, we acknowledge that the picture is far from complete and believe that insights presented by the authors can contribute to a coherent understanding of forest–savanna dynamics. Veenendaal et al. (2015) investigated the effect of climate and soil conditions on vegetation structure of 61 one-hectare plots near forest–savanna ecotones in South America, Africa and Australia. Based on their extensive data collection, they provide two main arguments supporting inconsistency with the alternative stable states hypothesis. Firstly, in contrast to what is expected from discontinuities in the remote sensing data (Hirota et al., 2011; Staver et al., 2011b), they argue that woody plant cover in their field plots shows no signs of discontinuity. Secondly, they consider a soil–climatic envelope to be sufficient to explain the forest–savanna transition