One savanna, many shapes: How bush control affects the woody layer in the southern Kalahari

Abstract

Bush thickening (shrub encroachment) is a major ecological and economic threat in southern African savannas. Different types and intensities of bush control (BC) are applied to counteract and mitigate bush thickening. However, woody vegetation response to BC and possible ecological side effects can be manifold. Land users therefore require solid knowledge about the associated structural and compositional changes, helping to apply an informed BC strategy that preserves multiple ecosystem services and functions. The present study addresses this need in a South African thornbush-type savanna. We sampled 41 rangeland sites with a known history of BC in comparison to benchmark conditions. The BC treatments included a selective and non-selective arboricide application and a selective stem burning. We identified each woody species in belt transects and measured the size and canopy of all individuals. The data were used to calculate measures of diversity, cover and density and to describe the population structure of key species. Marked differences in woody species composition and abundance patterns as well as significant alterations of the horizontal and vertical woody vegetation structure clearly reflected the intensity and level of selectivity with which BC had been conducted. While population trends displayed regeneration of key species, important structural elements (large mature trees) were often at risk of being lost from the savanna ecosystem. Results suggest varying degrees of ecosystem functional integrity following the BC treatments. It is concluded that selective BC treatments should be preferred as they allow to create open and well-structured savannas in benefit of the stability and biodiversity of the system. Yet, for improved predictions further research is needed into species-specific population dynamics and trait-related plant–plant interactions. In addition, there is still uncertainty about possible long-term ecological effects associated with the use of arboricides in this type of savanna, which should receive special attention in future research.