Clarifying the confusion: old-growth savannahs and tropical ecosystem degradation

Abstract

Ancient tropical grassy biomes are often misrecognized as severely degraded forests. I trace this confusion to several factors, with roots in the nineteenth century, including misinterpretations of the nature of fire in savannahs, attempts to reconcile savannah ecology with Clementsian succession, use of physiognomic (structural) definitions of savannah and development of tropical degradation frameworks focused solely on forests. Towards clarity, I present two models that conceptualize the drivers of ecosystem degradation as operating in both savannahs and forests. These models highlight how human-induced environmental changes create ecosystems with superficially similar physiognomies but radically different conservation values. Given the limitation of physiognomy to differentiate savannahs from severely degraded forests, I present an alternative approach based on floristic composition. Data from eastern lowland Bolivia show that old-growth savannahs can be reliably distinguished by eight grass species and that species identity influences ecosystem flammability. I recommend that scientists incorporate savannahs in tropical degradation frameworks alongside forests, and that savannah be qualified as old-growth savannah in reference to ancient grassy biomes or derived savannah in reference to deforestation. These conceptual advances will require attention not only to tree cover, but also to savannah herbaceous plant species and their ecologies.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

Abandoned pastures cannot spontaneously recover the attributes of old‐growth savannas

“…The time since livestock removal from the pastures varied between 3 and 25 years, according to the landowners or people responsible for the study areas. Despite the importance of fire for the maintenance of secondary savannas (Veldman, ), these pastures had been protected from fire since their abandonment because burning is illegal in Brazil (Durigan & Ratter, ). All areas presented a flat relief and relatively homogeneous soils (M. G. B. Cava, unpublished data) that are acidic (pH ranging from 3.7 to 4.8), sandy (total sand content ranging from 765 to 934 g/kg), with low fertility (degree of base saturation [V%] ranging from 8 to 47).…”

Understanding patterns of land-cover change in the Brazilian Cerrado from 2000 to 2015

“…However, frequently, secondary savannah does not fully recover original vegetation structure and ecosystem function. The importance of secondary habitats to biodiversity conservation has increasing recognition [7], although investigation of the ecosystem health and diversity of secondary savannahs are few (see [61]). In any case, LUCC analyses of the Cerrado would benefit from improved assessment of both the processes driving encroachment and its extent, which usually results from fire suppression and has raised serious conservation concerns [48].…”

Tropical grassy biomes: linking ecology, human use and conservation

“…While true in some locations [40], in the majority of areas this perspective is misplaced [34]. A wealth of new information including dated phylogenetic analyses demonstrates the antiquity of both tree and grass species (and lineages) specialized to these biomes [25,27,28].…”

“…For the first time, analyses are presented on the biogeography and potential distributions of Asian savannahs [39]. Other significant steps forward in our understanding include: methods for characterizing ancient versus derived grassy biomes [40,41], comparative data on the species diversity of TGB regions across the globe [42], an improved understanding of the complex ecology of herbivory and fire [16,43] and the context-dependent response of vegetation to global change [44], and finally, tools to examine trade-offs in biodiversity, carbon and agriculture to aid land-use planning and policy [10].…”

“…Here, Veldman [40] and Zaloumis & Bond [41], examining the Neotropics and South Africa respectively, differentiate ancient and secondary systems, noting differences in species composition with the former particularly rich in forbs, many with well-developed underground storage organs that facilitate survival in seasonally dry climates with frequent fire. The challenge is to test the generality of these compositional characteristics across TGB regions.…”