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Coexistence and bush encroachment in African savannas: the role of the regeneration niche

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posted on 03.02.2021, 08:00 by Joel Lewis, G. Anthony Verboom, Edmund February
Data accompanying the following publication;
J.Lewis, G.A.Verboom and E.C.February (2020). Niche separation, coexistence and bush encroachment in African acacias.

Abstract
Globally there is increasing concern over the biodiversity and economic losses associated with bush encroachment. The dominant species responsible for bush encroachment in Africa belong to the mimosoid genera Senegalia and Vachellia, with previous research identifying Vachellia as the more aggressive invader. We asked whether these genera have evolved different traits to enable germination and establishment among competitively superior grasses. We hypothesize that these traits enable coexistence at local scales and underpin differences in invasiveness.
We conducted a large greenhouse pot experiment to quantify functional trait differences between the two genera during seedling establishment both in the presence and absence of grasses. We also examined differences in seed morphology and tested whether co-occurrence of the two genera at the plot scale is greater than expected by chance.
Our results show that Vachellia grows faster and taller than Senegalia in the absence of competition from grasses. In the presence of grasses, however, Senegalia increases root tissue density while Vachellia does not. The seed coats of Vachellia species are thicker than those of Senegalia, and their seeds are spherical while those of Senegalia are discoid. The two genera coexist at a local (plot) scale.
We speculate that the thicker seed coat and spherical seed shape of Vachellia reflects its primarily endozoochorous dispersal mode, while the thinner seed coat and more discoid seeds of Senegalia reflects it use of wind dispersal. Since animal dispersal is directed, promoting the movement of seed into sites frequented by animals, and in which competition from grasses and fire has been removed, animal dispersal may contribute to the greater invasiveness of Vachellia. Once established in such sites, low density root tissue facilitates faster growth and more rapid resource acquisition in Vachellia than in Senegalia. Nonetheless, Senegalia is still able to persist, with evidence of greater-than-expected plot-scale co-occurrence of the two genera.
Where previous research has focused on environmental filtering, competition, predation or facilitation as explanations of plot-scale phylogenetic overdispersion, our work suggests a potential role for regeneration niche in structuring local co-occurrence.

Funding

The research was funded by the Andrew W. Mellon Foundation (#30600716). JRL was funded by the National Research Foundation of South Africa and the University of Cape Town.

History

Department/Unit

Biological Sciences