Rooting Strategies Of Savanna Shrubs In The Kalahari Basin: Implications For The Coexistence Of Woody And Herbaceous Plants And Shrub Encroachment In The African Savannas

Abstract

The savanna biomes are characterised by a coexistence of two antagonist – woody plants and herbaceous plants in defiance of competing theories. Scientific efforts to understand this unique coexistence are still largely inconclusive; various theories have been proposed, but no unanimous theoretical framework exists to date. Among these theories, the root niche-partitioning model offers the most popular, yet the most controversially debated viewpoint. It argues that this coexistence is a result of vertical root niche-partitioning, a natural mechanism by which woody plants develop deeper root systems to avoid competition with herbaceous plants. Despite its prominence and subsequent integration into models of species coexistence and arid eco-hydrology, several shortcomings of this model are evident. For example, it overlooks the critical issue of root plasticity. This study was thus designed to investigate the root systems of various savanna shrubs across a rainfall gradient in the Kalahari to test the aforementioned model. The overall aim was to investigate, compare, and contrast the root system architecture (RSA) of encroaching shrubs and those of non-encroaching shrubs within the proximate environmental setting. Using a direct excavation method, 183 shrubs were sampled, had their roots exposed and were subjected to morphometric measurements. Shrub encroachers were randomly selected and four non-encroaching shrubs surrounding each of the sampled encroacher plant were systematically chosen, using the nearest-neighbour approach. Results indicated that shrubs in the Kalahari develop diverse root system architecture which exhibits significant inter- and intra-species plasticity. In particular, three bush encroaching shrubs, Terminalia sericea, Senegalia mellifera, and Dichrostachys cinerea, tend to develop root systems essentially composed significantly (p < .001) of lateral roots deployed within shallow soil sub-surfaces; and partly without taproots, ii more especially in the drier part of the Kalahari. Overall, the architecture of the savanna shrubs’ root system can be classified into three major architecture groups: i) a fibrous or lateral root system, ii) a dual root system, and iii) a taproot system. These architecture groups are not necessarily unique to any species or environment, which suggests that plants develop their root systems plastically in response to prevailing environmental conditions. These findings are not consistent with the premise that the savanna shrubs are largely deep-rooted. This oversight has major implications for our current understanding of the savanna biomes. Whereas deeper-rooted shrubs may allow for root niche-partitioning with grasses, shallow-rooted shrubs are potentially in direct competition with grasses, suggesting that shrub encroachment is a probable manifestation of this competition. These findings may also explain why the phenomenon of shrub encroachment is largely attributed to shallow-rooted shrubs such as T. sericea.