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Secondary metabolites and nutrients of woody plants in relation to browsing intensity in African savannas

Abstract

Carbon-based secondary metabolites (CBSMs) are assumed to function as defences that contribute to herbivore-avoidance strategies of woody plants. Severe browsing has been reported to reduce concentrations of CBSMs and increase N concentrations in individual plants, causing heavily browsed plants to be characterised by N-rich/C-poor tissues. We hypothesised that concentrations of condensed tannins (CT) and total polyphenols (TP) should decrease, or N increase, in relation to increasing intensity of browsing, rendering severely browsed plants potentially more palatable (increased N:CT) and less N-limited (increased N:P) than lightly browsed ones. We sampled naturally browsed trees (taller than 2 m) of four abundant species in southern Kruger National Park, South Africa. Species-specific relationships between N:CT, CT, TP and P concentrations and increasing browsing intensity were detected, but N and N:P were consistently invariable. We developed a conceptual post-hoc model to explain diverse species-specific CBSM responses on the basis of relative allocation of C to total C-based defence traits (e.g. spines/thorns, tough/evergreen leaves, phenolic compounds). The model suggests that species with low allocation of C to C-based defence traits become C-limited (potentially more palatable) at higher browsing intensity than species with high allocation of C to C-based defences. The model also suggests that when N availability is high, plants become C-limited at higher browsing intensity than when N availability is low.

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Acknowledgments

The Zululand/Sweden Kruger Browse Project was funded by the National Research Foundation, University of Zululand, Agricultural Research Council, Swedish Research Council and Swedish International Development Agency. Scientific Services, Kruger Park, provided critical support for fieldwork. Tuulikki Rooke, Dawood Hattas, Luthando Dziba and Alpheus Zobolo contributed enthusiastically. Thandeka Mamashela, Patricia Shabangu, Ntuthuko Mkhize, Julius Tjelele, Basanda Nondlazi, Gilbert Pule, Frederik Engdahl, and Elin Gunve helped in the field and lab. Comments from Rina Grant and two anonymous reviewers improved the manuscript.

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Correspondence to Peter F. Scogings.

Additional information

Communicated by Tim Seastedt.

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Scogings, P.F., Hjältén, J. & Skarpe, C. Secondary metabolites and nutrients of woody plants in relation to browsing intensity in African savannas. Oecologia 167, 1063–1073 (2011). https://doi.org/10.1007/s00442-011-2042-9

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Keywords

  • Functional trait
  • Growth-defence
  • Herbivore
  • Photosynthesis
  • Source–sink