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Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

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Abstract

Understanding the factors controlling tree seedling recruitment is an integral research priority for savanna ecosystems, particularly for the management of woody plant encroachment. We investigated the effects of increased nutrients, fire and grazing on the survival, growth and biomass of Acacia sieberiana using a completely randomized experimental design in the greenhouse. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, with seedlings of A. sieberiana. The trees and grasses were allowed to establish for 12 weeks. Thereafter, treatments of fertilizer (100 kg N/ha), simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits of the trees (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to the grass only, increasing their total biomass. While, the direct removal of grass biomass by grazing in our study revealed no main (statistical) effect on tree seedling growth, we observed decreased survival of tree seedlings with higher grass biomass. Fire increased the tree seedlings’ investment in shoot growth and stem diameter. However, in the absence of grass competition, there were no differences in the post-fire allocation of the resprouting tree seedlings. Higher grass biomass was found to result in decreased specific leaf area of tree seedlings. We conclude that grass competition is important for controlling the encroachment of A. sieberiana.

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Acknowledgements

We would like to thank the National Research Foundation Grant No. 90448 for supporting this study, and the technical staff (Welcome Ncgobo and Alison Young) at the University of KwaZulu-Natal for help with plant harvesting and data collection. We are also grateful to Admore Mureva and Desale Y. Okubamichael for their valuable contributions.

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  1. Tiffany Pillay

    Present address: Department of Botany, Rhodes University, Lucas Avenue, Grahamstown, 6139, South Africa

Authors and Affiliations

  1. School of Life Sciences, University of KwaZulu-Natal, P. Bag X01, Scottsville, 3209, South Africa

    Tiffany Pillay

  2. Department of Biological Sciences, Kent State University, Kent, OH, 44242, USA

    David Ward

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  1. Tiffany Pillay
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Correspondence to David Ward.

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Communicated by Marjan Jongen.

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Pillay, T., Ward, D. Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings. Plant Ecol 222, 149–158 (2021). https://doi.org/10.1007/s11258-020-01094-1

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