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

[摘要] Through interpretations of remote-sensing data and/or theoretical propositions, the idea thatforest and savanna represent "alternative stable states" is gaining increasingacceptance. Filling an observational gap, we present detailed stratified floristic and structuralanalyses for forest and savanna standslocated mostly within zones of transition (where bothvegetation types occur in close proximity) in Africa, South America and Australia. Woody plantleaf area index variation was related to tree canopy cover in a similar way for both savanna andforest with substantial overlap between the two vegetation types. As total woody plant canopycover 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 understoreygrasses and herbs progressively replaced by shrubs as the canopy closes over was found forboth savanna and forests and on all continents. Thus, once subordinate woody canopy layers aretaken into account, a less marked transition in woody plant cover across the savanna–forest-speciesdiscontinuum is observed compared to that inferred when trees ofa basal diameter > 0.1 m are considered in isolation. This is especially thecase for shrub-dominated savannas and in taller savannas approaching canopy closure. Anincreased contribution of forest species to the totalsubordinate cover is also observed as savanna stand canopy closureoccurs. Despite similarities in canopy-cover characteristics, woody vegetation in Africa andAustralia attained greater heights and stored a greater amount of above-ground biomass thanin South America. Up to three times as much above-ground biomass is stored in forests compared tosavannas under equivalent climatic conditions. Savanna–forest transition zones were also found totypically occur at higher precipitation regimes for South America than for Africa. Nevertheless, consistent across all three continentscoexistence was found to be confined to a well-defined edaphic–climate envelopewith soil and climate the key determinants of therelative location of forest and savanna stands. Moreover, when considered in conjunction with the appropriatewater availability metrics, it emerges that soil exchangeable cations exert considerable controlon woody canopy-cover extent as measured in our pan-continental (forest + savanna) data set.Taken together these observations do not lend support to thenotion of alternate stable states mediatedthrough fire feedbacks as the prime force shaping thedistribution of the two dominant vegetation types of the tropicallands.

[学科分类] 地球化学与岩石