Abrupt shifts in African savanna tree cover along a climatic gradient

Abstract

Aim.To describe patterns of tree cover in savannas over a climatic gradient and a range of spatial scales and test if there are identifiable climate-related mean structures, if tree cover always increases with water availability and if there is a continuous trend or a stepwise trend in tree cover. Location. Central Tropical Africa. Methods. We compared a new analysis of satellite tree cover data with botanical, phytogeographical and environmental data. Results. Along the climatic transect, six vegetation structures were distinguished according to their average tree cover, which can co-occur as mosaics. The resulting abrupt shifts in tree cover were not correlated to any shifts in either environmental variables or in tree species distributions. Main conclusions. A strong contrast appears between fine-scale variability in tree cover and coarse-scale structural states that are stable over several degrees of latitude. While climate parameters and species pools display a continuous evolution along the climatic gradient, these stable structural states have discontinuous transitions, resulting in regions containing mosaics of alternative stable states. Soils appear to have little effect inside the climatic stable state domains but a strong action on the location of the transitions. This indicates that savannas are patch dynamics systems, prone to feedbacks stabilizing their coarse-scale structure over wide ranges of environmental conditions. (Résumé d'auteur

Estimation of Tree Cover in an Agricultural Parkland of Senegal Using Rule-Based Regression Tree Modeling

“…Whereas errors might be highest in absolute terms in the high tree cover areas ( Figure 7 and Table 1), in relative terms they are more significant where tree cover is lowest. This confirms findings by Hansen [17] that low tree cover is difficult to accurately estimate, as the soil background dominates over the tree signal. Figure 7.…”

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Tree–grass interaction dynamics and pulsed fires: Mathematical and numerical studies

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Analysis of stable states in global savannas – a response to Staver and Hansen

“…We applaud the efforts of Staver et al 2011, Hirota et al 2011, Favier et al 2012 and others to detect these patterns at continental and global scales. We remain convinced, however, that the potential statistical artefacts in the VCF tree-cover product make it unsuitable for this particular purpose.…”