Water availability is a principal driver of large-scale land cover spatial heterogeneity in sub-Saharan savannahs

Christopher G Marston,David M Wilkinson,Hannah J O’Regan,Julien Louys,Sally C Reynolds

doi:10.1007/s10980-018-0750-9

Abstract

ContextThe heterogeneous mosaic nature of African savannah vegetation is a key aspect of its ecology. This study evaluates mosaic distributions and characteristics across sub-Saharan Africa, investigating the environmental drivers of mosaic formation.ObjectivesThis study was designed to determine: (1) on a continental scale, how frequent are mosaics in savannahs? and (2) what are the key environmental drivers in the formation of mosaics?MethodsLandsat ETM+ satellite imagery was used to generate land-cover maps for 39 sample areas across sub-Saharan Africa. The spatial complexity of land-cover mosaics at 4628 savannah sub-sites was quantified, and modelled using random forests to identify the relative importance of environmental variables driving mosaic presence.ResultsOnly six sub-sites constituted a single land-cover class, illustrating that mosaic habitats are abundant at the scale analysed (19.6 km2), although mosaic characteristics varied considerably. Results indicate precipitation is most important in influencing mosaic complexity, followed by evapotranspiration, temperature, lithology and distance to rivers. Fire and ecosystem engineer presence are of lesser importance at this study scale.ConclusionsMosaics are ubiquitous in the African savannahs studied, their presence influenced by multiple environmental drivers, with water being key. The lower importance of fire and large mammal disturbance is likely resulting from these highly individualistic site-based process varying between sites, resulting in no single, coherent, across-Africa disturbance signal, and/or lack of detail in available data at this scale. Therefore, large-scale determinants of savannah mosaics appear climate-driven. Under future global warming scenarios, African savannahs are likely to become more homogenous.

Highlights

Savannahs are one of the most important terrestrial biomes, covering around 20% of the global land surface (Shorrocks and Bates 2015)
Mosaics are ubiquitous in the African savannahs studied, their presence influenced by multiple environmental drivers, with water being key
The lower importance of fire and large mammal disturbance is likely resulting from these highly individualistic site-based process varying between sites, resulting in no single, coherent, across-Africa disturbance signal, and/or lack of detail in available data at

Summary

Introduction

Savannahs are one of the most important terrestrial biomes, covering around 20% of the global land surface (Shorrocks and Bates 2015). The exact definition of savannah is confusingly variable, the key aspect of almost all definitions is that it is a biome composed of a mix of trees and grass—usually C4 grass— tree-free grasslands are occasionally defined as savannah (Archibold 1995; Torello-Raventos et al 2013; Shorrocks and Bates 2015). Analysis of 854 sites across Africa suggested that at lower rainfall levels savannah may be a stable system, but at higher rainfalls ([ 784 mm mean annual precipitation) savannah may require periodic disturbance events to prevent succession to forest (Sankaran et al 2005). Key disturbance mechanisms include fire and grazing by large herbivores such as elephants (Loxodonta africana) (Laws 1970; Dublin et al 1990; Midgley et al 2010; Daskin et al 2016; Marston et al 2017)

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