Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants

Authors

  • Jana Schleicher,

    1. Institute of Ecology, University of Jena, Dornburger Str. 159, D-07743 Jena, Germany
    2. Department of Bioinformatics, University of Jena, Ernst-Abbe-Platz 2, D-07743 Jena, Germany
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  • Katrin M. Meyer,

    1. Institute of Ecology, University of Jena, Dornburger Str. 159, D-07743 Jena, Germany
    2. Ecosystem Modelling, Faculty of Forest Sciences and Forest Ecology, University of Göttingen, Büsgenweg 4, D-37077 Göttingen, Germany
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  • Kerstin Wiegand,

    1. Institute of Ecology, University of Jena, Dornburger Str. 159, D-07743 Jena, Germany
    2. Ecosystem Modelling, Faculty of Forest Sciences and Forest Ecology, University of Göttingen, Büsgenweg 4, D-37077 Göttingen, Germany
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  • Frank M. Schurr,

    1. Plant Ecology and Nature Conservation, Institute of Biochemistry and Biology, University of Potsdam, Maulbeerallee 2, D-14469 Potsdam, Germany
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  • David Ward

    1. School of Biological and Conservation Sciences, University of KwaZulu-Natal, Scottsville 3209, South Africa
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  • Schleicher, J. (corresponding author, jana.foerster@uni-jena.de); Meyer, K.M. (Katrin.Meyer@forst.uni-goettingen.de) & Wiegand, K. (mail@kerstin-wiegand.de): Institute of Ecology, University of Jena, Dornburger Str. 159, D-07743 Jena, Germany
    Schurr, F.M. (schurr@uni-potsdam.de): Plant Ecology and Nature Conservation, Institute of Biochemistry and Biology, University of Potsdam, Maulbeerallee 2, D-14469 Potsdam, Germany
    Ward, D. (ward@ukzn.ac.za): School of Biological and Conservation Sciences, University of KwaZulu-Natal, Scottsville 3209, South Africa
    Schleicher, J.: Current address: Department of Bioinformatics, University of Jena, Ernst-Abbe-Platz 2, D-07743 Jena, Germany
    Meyer, K.M. & Wiegand, K.: Current address: Ecosystem Modelling, Faculty of Forest Sciences and Forest Ecology, University of Göttingen, Büsgenweg 4, D-37077 Göttingen, Germany.

  • Co-ordinating Editor: Beverly Collins

Abstract

Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species?

Location: Semi-arid savanna in the Kimberley Thorn Bushveld, South Africa.

Methods: We developed a two-step protocol for the statistical differentiation of association-promoting mechanisms in plants based on the Acacia erioloba–Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non-spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non-spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter.

Results: We first showed a significant positive spatial association of the two species. Thereafter, the non-spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred.

Conclusions: Our two-step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).

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