Patterns in woody vegetation structure across African savannas

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.

Constraints on shrub cover and shrub–shrub competition in a U.S. southwest desert

“…) is driven primarily by increases in mean patch size, rather than increases in patch density. This is consistent with a recent study (Axelsson and Hanan ) that found increases in woody cover across an African savanna rainfall gradient were more closely related to crown size than density of shrubs and trees.…”

“…, Good and Caylor , Lehmann et al. , Axelsson and Hanan , ) and temperate semi‐arid grasslands (Scholtz et al. ).…”

Rates of woody encroachment in African savannas reflect water constraints and fire disturbance

“…We used the same dataset, image preprocessing steps, and classification framework as in a recent study on patterns in African woody vegetation structure (Axelsson & Hanan, ) and refer the reader to this paper for a more thorough description of the methodology. Our sampling frame was sub‐Saharan African savannas with a minimum of anthropogenic disturbances.…”

Alternative Vegetation States in Tropical Forests and Savannas: The Search for Consistent Signals in Diverse Remote Sensing Data

“…Global distributions of vegetation structure, species composition, and functional composition are governed primarily by climate and phylogenetic processes [1][2][3][4][5]. In the seasonal tropics, precipitation patterns and ecohydrological interactions are the principal drivers of large scale vegetation patterns [6][7][8], with increasing precipitation generally corresponding to increasing tree density, canopy cover, and tree height [8][9][10]. While continental-scale average tree density, cover, and height may increase with precipitation, at fine spatial scales, local factors including soil type and hydrological interactions, fire and herbivory, and agriculture and wood harvest lead to considerable spatial heterogeneity in vegetation structure [3,7,[11][12][13][14][15]).…”