Articles | Volume 14, issue 13
Biogeosciences, 14, 3239–3252, 2017
Biogeosciences, 14, 3239–3252, 2017

Research article 10 Jul 2017

Research article | 10 Jul 2017

Patterns in woody vegetation structure across African savannas

Christoffer R. Axelsson1 and Niall P. Hanan2 Christoffer R. Axelsson and Niall P. Hanan
  • 1Geospatial Sciences Center of Excellence, South Dakota State University, Brookings, SD, USA
  • 2Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM, USA

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.

Short summary
We used high-resolution satellite images to analyze woody vegetation (trees and shrubs) across African savannas. By delineating individual tree crowns we estimated the number of trees, sizes of crowns, and the spatial pattern of woody vegetation in the landscape. We find several interesting relationships for how vegetation properties (including the emergence of highly organized periodic vegetation patterns) are influenced by the local environment.
Final-revised paper