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

Abstract

Context

The  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.

Objectives

This  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?

Methods

Landsat  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.

Results

Only  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.

Conclusions

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 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.