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Grass, rainfall and herbivores as determinants of Acacia erioloba (Meyer) recruitment in an African savanna

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Abstract

Acacia species in arid environments are thought to only establish in years of above-average rainfall, so should exhibit cohorted or pulsed recruitment. I studied population demography of Acaciaerioloba Meyer in semi-arid savanna in the Kimberley area (mean annual precipitation = 425 ± 132 mm), South Africa, to establish whether they recruit episodically. This species was found to have a sapling bank at the sites, indicating that even though cohorted recruitment probably occurs, it is not the primary factor limiting recruitment to larger size classes. A. erioloba saplings given supplementary water and protection from herbivory showed significantly less height growth than saplings given water but not protected from herbivores, and grass within exclusion plots was taller and denser than outside of exclusion plots. The generalized linear model, although finding a significant difference between watered and protected and watered and unprotected saplings, explained only 16% of variation in growth, demonstrating the importance of factors other than competition from grass when rainfall is above-average. Average height increase across all treatments for a growing season was small (43.1 ± 30.81 mm), suggesting most investment is belowground. A. erioloba sapling below surface stem diameter correlated positively with aboveground growth, indicating larger, and presumably older, individuals grow faster. When grass was removed around saplings, growth rates were not significantly greater than for saplings surrounded by grass, over a growing season. Thus, in these study sites, pulsed events allow seedlings to establish, and then saplings slowly accumulate over many growing seasons within the grass sward, owing to slow aboveground growth combined with competition from grass in absence of grazers. Release events, e.g., heavy grazing combined with good rainfall, may allow mass release, giving the impression of cohorted or pulsed recruitment.

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Acknowledgments

Jeremy Midgley, Richard Dean, Ruan Veldtman, Onno Huyser, Sue Milton, Niels Blaum and anonymous reviewers commented and improved the manuscript considerably. Richard Allcorn, Rachel Wiseman, Stuart Priestley, Melissa Stander, Susan Harris, Michelle Taylor, Wayne Dawson, Onno Huyser and Eddie Riddell were all great help in the field, finding Acacia saplings and setting up exclusion plots. De Beers Diamond Mines Farms and Ecology Division kindly allowed us access to their land; in particular, Peter Gibbs was very helpful in finding study sites. Financial support was provided by the German Ministerium für Bildung und Forschung (BMBF) under project number 01LC0024 (BIOTA—southern Africa); the National Research Foundation of South Africa under Grant number 2053674, and the Sprigg Scholarship of the FitzPatrick Institute at the University of Cape Town.

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  1. Colleen L. Seymour

    Present address: Conservation Science Programme, Kirstenbosch Research Centre, South African National Biodiversity Institute, Private Bag X7, Claremont, 7735, South Africa

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  1. Percy FitzPatrick Institute, DST/NRF Centre of Excellence, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa

    Colleen L. Seymour

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Correspondence to Colleen L. Seymour.

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Seymour, C.L. Grass, rainfall and herbivores as determinants of Acacia erioloba (Meyer) recruitment in an African savanna. Plant Ecol 197, 131–138 (2008). https://doi.org/10.1007/s11258-007-9366-x

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