Grass and tree cover responses to intra-seasonal rainfall variability vary along a rainfall gradient in African tropical grassy biomes

Luke Sweeney,Donatella D’Onofrio,Jost Von Hardenberg,Mara Baudena

doi:10.1038/s41598-019-38933-9

Luke Sweeney, Donatella D’Onofrio + Show 2 more

Open Access

https://doi.org/10.1038/s41598-019-38933-9

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Abstract

Although it is well known that mean annual rainfall (MAR) and rainfall seasonality have a key role in influencing the distribution of tree and grass cover in African tropical grassy biomes (TGBs), the impact of intra-seasonal rainfall variability on these distributions is less agreed upon. Since the prevalent mechanisms determining biome occurrence and distribution change with MAR, this research investigates the role of intra-seasonal rainfall variability for three different MAR ranges, assessing satellite data on grass and tree cover, rainfall and fire intervals at a sub-continental scale in sub-Saharan Africa. For MAR below 630 mm y−1, rainfall frequency had a positive relationship with grass cover; this relationship however became mostly negative at intermediate MAR (630–1200 mm y−1), where tree cover correspondingly mostly increased with rainfall frequency. In humid TGBs, tree cover decreased with rainfall intensity. Overall, intra-seasonal rainfall variability plays a role in determining vegetation cover, especially in mesic TGBs, where the relative dominance of trees and grasses has previously been largely unexplained. Importantly, the direction of the effect of intra-seasonal variability changes with MAR. Given the predicted increases in rainfall intensity in Africa as a consequence of climate change, the effects on TGBs are thus likely to vary depending on the MAR levels.

Highlights

Grasslands and savannas, known together as tropical grassy biomes (TGBs), are estimated to cover ca. a third of Africa and a fifth of the world’s land area[1,2]
T had a positive trend with mean annual rainfall (MAR) (R2 = 0.22), which is the most important factor among those we considered in explaining tree cover variations in this range
Our results appear to indicate that intra-seasonal rainfall variability has a role to play in determining the relative predominance of grasses and trees in TGBs in sub-Saharan Africa, with that role and importance dependent on mean annual rainfall (MAR) levels

Summary

Introduction

Grasslands and savannas, known together as tropical grassy biomes (TGBs), are estimated to cover ca. a third of Africa and a fifth of the world’s land area[1,2]. Characterised by continuous C4 grassland, and possible coexistence with trees[2], they support a growing proportion of the world’s population, are home to the majority of the world’s remaining megafauna[2] and are a critical store of biodiversity[3] Despite their importance, TGBs have garnered little public attention or conservation effort in comparison to tropical forests[1,4], and the role of grass in characterizing these biomes has only recently been studied at sub-continental level[5]. Whereas tree and grass cover is limited primarily by water availability in dry TGBs, in mesic areas the high fire frequency, fostered by high grass cover, helps to limit tree cover[5,7] At these higher MAR levels, both tropical forests and savannas exist[10,11,12], with savannas mostly observed in areas with highly seasonal rainfall regimes, but partly under overlapping climatic conditions with forests[5,11,13]. Changes in rainfall variability have been closely associated with anthropogenic climate change[34], and, with rainfall intensity in Africa expected to increase in the future[35], changes in vegetation patterns may result

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