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Competitive interactions between grass and succulent shrubs at the ecotone between an arid grassland and succulent shrubland in the Karoo

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Abstract

Nearest-neighbour analysis was used to examine the competitive interactions between Stipagrostis brevifolia, a C4 perennial grass, and two leaf succulent shrubs, Ruschia robusta and Leipoldtia pauciflora, at the ecotone between semi-arid grassland and succulent shrubland in the Karoo. The root distribution in the soil was also compared to assess the degree of overlap in the potential use of soil resources. Regressions between the combined sizes of interspecific, nearest-neighbour species and the distance between them showed significant positive correlations for S. brevifolia and R. robusta, which suggest the presence of competition. We infer from individual species regressions that the grass exerted a stronger competitive force on the shrub R. robusta than R. robusta on the grass. There was also evidence for strong intraspecific competitive relationships within S. brevifolia and R. robusta. There was no evidence of competition between S. brevifolia and L. pauciflora or among L. pauciflora individuals. S. brevifolia had the deepest root system, and was recorded at depths of 70 cm. Most of this root mass occurred between 10 and 40 cm. Ruschia robusta roots were recorded as deep as 55 cm, but more than 90% was found in the top 20 cm of the soil, creating a degree of overlap with the vertical root distribution of S. brevifolia. A clear separation in rooting depths occurred between S. brevifolia, and L. pauciflora which had only 3% of the total root mass below 10 cm. The partial overlap in the vertical root distribution between S. brevifolia and R. robusta may account for the observed competitive relationship, but each species dominates in a different layer, potentially minimising the net competition between S. brevifolia and R. robusta. Our findings demonstrate the possibility of a two-layer water-obtaining strategy in a semi-desert ecosystem, where the succulent shrubs seem to be playing the typical “grass” role described in most models of water partitioning between grass and woody plants. The stronger competitive effect of S. brevifolia on R. robusta at all the sites is of significance to species dynamics, and might be related to winter/summer rainfall dynamics at the climatic transition.

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Acknowledgments

This study was funded under the umbrella of the BIOTA Southern Africa project (www.biotaafrica.org), by the German Federal Ministry of Education and Research under promotion number 01 LC 0024A. We thank the Mazda Wildlife Vehicle Fund for the use of a courtesy vehicle during the course of the study. Rainfall data for Goegap Nature Reserve and Vaalputs Radioactive Waste Disposal Facility were supplied by their respective offices, while Burdensputs and De Riet rainfall data were obtained from the landowners. We gratefully acknowledge the landowners in Namaqualand for kindness and permission to work on their land, the management of Vaalputs Radioactive Waste Disposal Facility for permission to work on Vaalputs farm, and the Northern Cape Department of Nature Conservation for permission to undertake research on Goegap Nature Reserve. Special thanks to Mariana Lot, the para-ecologist at Paulshoek, for field assistance. The following people are thanked for assistance in one way or the other during field trips, Lee Simons, Azieb Woldensae, Sebataolo Rahlao, Elizabeth Claassen and Vonkie Claassen.

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Author notes

  1. Ndafuda Shiponeni

    Present address: Department of Biological Sciences, University of Namibia, Private Bag 13301, Windhoek, Namibia

  2. Nicky Allsopp

    Present address: South African Environmental Observation Network, SANBI, Private Bag X7, Claremont, 7735, South Africa

Authors and Affiliations

  1. Plant Conservation Unit, Botany Department, University of Cape Town, PO Box X3, Rondebosch, 7701, South Africa

    Ndafuda Shiponeni, Peter J. Carrick & M. Timm Hoffman

  2. ARC-Livestock Business Division, Private Bag X17, Bellville, 7535, South Africa

    Nicky Allsopp

Authors
  1. Ndafuda Shiponeni
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  3. Peter J. Carrick
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  4. M. Timm Hoffman
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Correspondence to Ndafuda Shiponeni.

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Shiponeni, N., Allsopp, N., Carrick, P.J. et al. Competitive interactions between grass and succulent shrubs at the ecotone between an arid grassland and succulent shrubland in the Karoo. Plant Ecol 212, 795–808 (2011). https://doi.org/10.1007/s11258-010-9864-0

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