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Belowground competitive suppression of seedling growth by grass in an African savanna

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Abstract

Coexistence of N2-fixing legumes and non-legume trees with grasses in African savannas results in intense competition between these life-forms. We hypothesised that belowground competition might induce different nutritional constraints in N2- versus non-N2-fixing species. A field (Hluhluwe-imFolozi nature reserve, South Africa) competition experiment with two N2-fixing legume species (Acacia burkei and Acacia karroo) and two non-N2-fixing species (Schotia brachypetala and Spirostachys africana) both with clipped grass and without grass was established. Plants were supplied with no fertilizer, or generous amounts of fertilizer (200 kg N ha−1, 100 kg P2O5 ha−1, 7.1 kg K2O ha−1) supplied as either 28–10 (N–K), P or a combination of these fertilizers (NPK). Regularly clipped grass suppressed growth (by more than 90 %) of both N2- and non-N2-fixing seedlings equally. Biomass accumulation of seedlings grown with grass and the grasses themselves responded positively to NK and/or NPK, but not P, although P-fertilization did have effects on foliar [N] and δ15N values of trees and grasses showing that plants accessed the fertilizer. Tree δ15N values and foliar [N] were also modified by NPK, demonstrating access to fertilizer. However, the ameliorative effects of NPK on grass competition-induced biomass suppression were only partial. This may be due to ‘non-resource competition’ (i.e. root gaps) imposed by dense grass roots. The fact that nutrients were able to partially ameliorate the effects of grass competition, however, indicates that such ‘non-resource competition’ may be partially overcome by even more generous supply of fertilizers.

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Acknowledgments

We thank the field staff of the Zululand Tree Project for field assistance and Ian Newton (Department Archeometry, University of Cape Town) for mass spectrometer analysis. Ezemvelo KZN Wildlife staff are thanked for field assistance and the anonymous reviewers are thanked for their helpful contributions. We are grateful for funding from the Mellon Foundation and National Research Foundation.

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  1. Department of Botany, University of Cape Town, Private Bag X1, Rondebosch, 7701, South Africa

    Michael D. Cramer, Julia L. Wakeling & William J. Bond

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  1. Michael D. Cramer
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  2. Julia L. Wakeling
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  3. William J. Bond
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Correspondence to Michael D. Cramer.

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Cramer, M.D., Wakeling, J.L. & Bond, W.J. Belowground competitive suppression of seedling growth by grass in an African savanna. Plant Ecol 213, 1655–1666 (2012). https://doi.org/10.1007/s11258-012-0120-7

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