Abstract
Abstract. Vegetation structure in water-limited systems is to a large degree controlled by ecohydrological processes, including mean annual precipitation (MAP) modulated by the characteristics of precipitation and geomorphology that collectively determine how rainfall is distributed vertically into soils or horizontally in the landscape. We anticipate that woody canopy cover, crown density, crown size, and the level of spatial aggregation among woody plants in the landscape will vary across environmental gradients. A high level of woody plant aggregation is most distinct in periodic vegetation patterns (PVPs), which emerge as a result of ecohydrological processes such as runoff generation and increased infiltration close to plants. Similar, albeit weaker, forces may influence the spatial distribution of woody plants elsewhere in savannas. Exploring these trends can extend our knowledge of how semi-arid vegetation structure is constrained by rainfall regime, soil type, topography, and disturbance processes such as fire. Using high-spatial-resolution imagery, a flexible classification framework, and a crown delineation method, we extracted woody vegetation properties from 876 sites spread over African savannas. At each site, we estimated woody cover, mean crown size, crown density, and the degree of aggregation among woody plants. This enabled us to elucidate the effects of rainfall regimes (MAP and seasonality), soil texture, slope, and fire frequency on woody vegetation properties. We found that previously documented increases in woody cover with rainfall is more consistently a result of increasing crown size than increasing density of woody plants. Along a gradient of mean annual precipitation from the driest (< 200 mm yr−1) to the wettest (1200–1400 mm yr−1) end, mean estimates of crown size, crown density, and woody cover increased by 233, 73, and 491 % respectively. We also found a unimodal relationship between mean crown size and sand content suggesting that maximal savanna tree sizes do not occur in either coarse sands or heavy clays. When examining the occurrence of PVPs, we found that the same factors that contribute to the formation of PVPs also correlate with higher levels of woody plant aggregation elsewhere in savannas and that rainfall seasonality plays a key role for the underlying processes.
Highlights
African savannas are complex tree–grass systems controlled by combinations of climate, soil, and disturbance processes such as fire and herbivory (Sankaran et al, 2008)
Boxplots with woody properties divided into mean annual precipitation (MAP) bins (Fig. 6) show that woody cover and crown sizes increased more sharply with increasing rainfall than crown densities
If we focus on sites with rainfall seasonality above 0.8, there is a more linear relationship with lower crown density and cover in areas with high rainfall seasonality which could be associated with the long periods of high water stress in more seasonal systems
Summary
African savannas are complex tree–grass systems controlled by combinations of climate, soil, and disturbance processes such as fire and herbivory (Sankaran et al, 2008). Water availability determines the establishment, growth and survival of plants and competitive plant traits are often of a water-saving nature (Chesson et al, 2004; Pillay and Ward, 2014). Abiotic environmental factors, such as the rainfall regime, soil type, and topography, impact ecohydrological processes by controlling infiltration rates, runoff generation, and available water capacity, which in turn impact the growth and survival of woody plants in the landscape (Ludwig et al, 2005). A thorough understanding of the underlying processes that influence savanna vegetation structure is key to assessing the future resilience and productivity of these ecosystems
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