The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

Abstract

Africa has the most extensive C4 grassy biomes of any continent. They are highly flammable accounting for greater than 70% of the world's burnt area. Much of Africa's savannas and grasslands occur in climates warm enough and wet enough to support closed forests. The combination of open grassy systems and the frequent fires they support have long been interpreted as anthropogenic artefacts caused by humans igniting frequent fires. True grasslands, it was believed, would be restricted to climates too dry or too cold to support closed woody vegetation. The idea that higher-rainfall savannas are anthropogenic and that fires are of human origin has led to initiatives to 'reforest' Africa's open grassy systems paid for by carbon credits under the assumption that the net effect of converting these system to forests would sequester carbon, reduce greenhouse gases and mitigate global warming. This paper reviews evidence for the antiquity of African grassy ecosystems and for the fires that they sustain. Africa's grassy biomes and the fires that maintain them are ancient and there is no support for the idea that humans caused large-scale deforestation. Indicators of old-growth grasslands are described. These can help distinguish secondary grasslands suitable for reforestation from ancient grasslands that should not be afforested.This article is part of the themed issue 'The interaction of fire and mankind'.

Global trends in wildfire and its impacts: perceptions versus realities in a changing world

“…When considering some of the extreme landscape fires as a form of natural disaster, the number of deaths is actually relatively low compared with other natural disaster types. For example, data by the Emergency Events Database (EM-DAT) 2 Fire has long been a natural factor in many ecosystems around the world, from boreal forests to tropical savannas [76,77]. In these systems, fire is a necessary perturbation to preserve ecosystem health and stimulate rejuvenation [78,79].…”

Tropical grassy biomes: linking ecology, human use and conservation

“…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]. The presence of endemic plant lineages and species, as well as species with unique life histories, functional traits and architectures, including forbs with large underground storage organs, are strong indicators of the antiquity of TGBs [35,41].…”

“…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.…”

“…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].…”

Scrubbing Up: Multi-Scale Investigation of Woody Encroachment in a Southern African Savannah

“…Savannahs with mean annual precipitation levels of around 650mm or below are reasonably stable, but tend to change towards forest above this level (unless other disturbance processes allow a mix of grass and trees to be maintained) [1,14]. Other potential drivers include fire [16][17][18][19][20][21][22][23], disturbance by megaherbivores, particularly elephant (Loxodonta africana) [4,19,[24][25][26][27], grazing by domestic herbivores [28], scale-dependent interactions between climate, fire and herbivory [17,[29][30][31][32], and increasing CO 2 fertilization [5,9,10,17,[33][34][35][36][37][38][39][40][41][42][43]. The vulnerability of a region to changes in the tree-grass balance of savannas will likely depend on the prevailing land use history, the environmental setting, and the functional traits of woody plants that govern their responsiveness to both local and global drivers [12].…”