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Defence strategies in African savanna trees

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Abstract

Southern African savannas are commonly polarised into two broad types based on plant functional types and defences; infertile savannas dominated by broad-leaved trees typically defended by nitrogen-free secondary compounds and fertile savannas dominated by fine-leaved trees defended by structural defences. In this study, we use trait and other data from 15 wooded savanna sites in Southern Africa and ask if broad-leaved and fine-leaved species dominate on nutrient-poor and nutrient-rich soils, respectively. We then test if there is there any evidence for trade-offs in chemical (i.e., condensed tannins and total polyphenols) vs. structural defences on different soil types. We did not find strong evidence for a general divide in fine- vs. broad-leaved savannas according to soil fertility, nor for a simple trade-off between chemical and structural defences. Instead, we found savanna species to cluster into three broad defence strategies: species were high in leaf N and either (A) highly defended by spines and chemicals or (B) only structurally defended, or (C) low in leaf N and chemically defended. Finally, we tested for differences in browser utilisation between soil types and among plant defence strategies and found that browsing by meso-herbivores was higher on nutrient-rich soils and targeted species from groups A and B and avoided C, while browsing by elephants was mostly not affected by soil type or defence strategy. We propose a framework that can be used as a basis for asking strategic questions that will help improve our understanding of plant defences in savannas.

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Acknowledgements

This work was funded through a BDI-PED grant from the CNRS as well as the Andrew W Mellon Foundation and the National Centre for Biological Sciences. We are hugely grateful to SANParks, Ezemvelo KZN Wildlife, the Zimbabwe Parks and Wildlife Authority and several anonymous private landowners for their support. We are grateful for the thoughtful and thorough suggestions provided by the anonymous reviewers which substantially improved the manuscript.

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Authors and Affiliations

  1. National Centre for Biological Sciences, Tata Institute of Fundamental Research, GKVK, Bellary Road, Bangalore, 560 065, India

    Benjamin J. Wigley

  2. School of Natural Resource Management, Nelson Mandela University, George Campus, Madiba Drive, Private Bag X6531, George, 6530, South Africa

    Benjamin J. Wigley & Corli Coetsee

  3. UMR CNRS 5558, LBBE, Université Claude Bernard Lyon 1, Bât. Grégor Mendel 43 bd du 11 novembre 1918, 69622, Villeurbanne cedex, France

    Hervé Fritz

  4. Sustainability Research Unit, Nelson Mandela University, George Campus, Madiba Drive, Private Bag X6531, George, 6530, South Africa

    Hervé Fritz

  5. Scientific Services, Kruger National Park, Private Bag X402, Skukuza, 1350, South Africa

    Corli Coetsee

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  1. Benjamin J. Wigley
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  2. Hervé Fritz
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  3. Corli Coetsee
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Contributions

BJW, CC, and HF designed the research, wrote, and revised the manuscript. BJW collected the database and performed the analyses.

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Correspondence to Benjamin J. Wigley.

Additional information

Communicated by Ian Kaplan.

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Wigley, B.J., Fritz, H. & Coetsee, C. Defence strategies in African savanna trees. Oecologia 187, 797–809 (2018). https://doi.org/10.1007/s00442-018-4165-8

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