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

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.

Distinct response of gross primary productivity in five terrestrial biomes to precipitation variability

“…This is confirmed in our study where temperate biomes that are neither water nor energy-limited (warm-wet grasslands, warm forests) present a significant negative response to unevenly distributed rainfall. Indeed, it has been shown that rain frequency plays an important role in the spatial distribution of these biomes 29 . The primary benefit of evenly distributed rainfall is that it mitigates two stressful situations: floods 30 (waterlogged/hypoxic soils and loss of available precipitation through runoff) and droughts 31 (waterstress eventually leading to downregulation of photosynthesis and plant senescence).…”

Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

“…The latter depends in part on MAP but appears much more strongly constrained by edaphic factors. It seems more likely that intra‐annual rainfall variability rather than MAP would play an important role on determining tree cover under these conditions (Case & Staver, ; D’Onofrio et al, ). If the extraction constraints proposed by Fensham et al () explain the patterns we observe, a shift to small but frequent rainfall events may favour trees (D’Onofrio et al, ) by maintaining ψ above the point of stomatal closure over a longer portion of the growing season.…”

“…Climate generally varies over large spatial scales, but substantial variation in tree cover (from open grassland to forest) can occur at much finer scales (0.01-0.1 km), distances over which climate is essentially invariant. At these scales, the effects of soils on soil moisture availability, interacting with intraannual variation in precipitation (Case & Staver, 2018;D'Onofrio, Sweeney, von Hardenberg, & Baudena, 2019), likely play a larger role. In particular, there is a well-known association between tree cover and soil texture in savannas, with coarse-textured soils tending to favour trees over grasses (Case & Staver, 2018;Fensham, Butler, & Foley, 2015;Knoop & Walker, 1985;Noy-Meir, 1973;Sankaran, Ratnam, & Hanan, 2008;Staver, Botha, & Hedin, 2017), implying a texture-driven hydrological mechanism.…”

Linking Vegetation-Climate-Fire Relationships in Sub-Saharan Africa to Key Ecological Processes in Two Dynamic Global Vegetation Models

“…The issue of scale and upscaling in ecology is not resolved (e.g., Staver, 2018) and, thus, may have led to a mismatch between the ecological scale of the fire processes and the spatial resolution of both models, especially in JSBACH. Furthermore, the weak or not significant relationships might also indicate that there are other discarded factors explaining the tree and grass cover variability, such as intra-seasonal rainfall variability (Good and Caylor, 2011;Xu et al, 2018;D'Onofrio et al, 2019) related also to soil texture (Case and Staver, 2018) or herbivores (both livestock and wildlife), which are common in Africa and can have an effect on vegetation comparable to fire and can themselves negatively affect fire occurrence (Hempson et al, 2017).…”