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

Abstract

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.

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)

“…Nevertheless, here we show that the distribution of canopy cover from 41 field plots (Fig. 4 in Veenendaal et al, 2015), even including all canopy strata, is multimodal (Fig. 1).…”

“…In a recent publication in this journal, Veenendaal et al (2015) presented a global field study of tropical forestsavanna 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.…”

“…To support their first point of canopy-cover continuity, Veenendaal et al (2015) presented observations of the cover of different canopy layers (the upper, middle and lower strata), as opposed to the commonly used remote sensing product (MODIS VCF;DiMiceli et al, 2011), which can only detect coverage at heights above 5 m. The inclusion of all strata in canopy measurements is an advance to previous work. However, it is unfortunate that their plot locations were not randomly selected, which limits their capacity to correctly test continuity in canopy cover.…”

Soil or fire: what causes treeless sedgelands in Tasmanian wet forests?

“…A number of biogeographic and ecological theories have been proposed to explain forest -savanna mosaics, broadly championing the primacy of fire regimes (Bowman 2000), soil physical (Beard 1953) and/or chemical (Lloyd et al 2008) conditions, including some combinations of these edaphic factors (Veenendaal et al 2015;Langan et al 2017).These theories are fiercely contested. If forest -savanna boundaries are controlled by topographic and edaphic factors (such as soil drainage, texture, nutrient supply, water holding capacity, rooting depth), then fire disturbance is a secondary effect that acts to sharpen them and reinforce stable mosaics (Kellman 1984;Wilson and Agnew 1992;Wood and Bowman 2012).…”