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Positive and negative effects of grass, cattle, and wild herbivores on Acacia saplings in an East African savanna

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Abstract

Plant–plant interactions can be a complex mixture of positive and negative interactions, with the net outcome depending on abiotic and community contexts. In savanna systems, the effects of large herbivores on tree–grass interactions have rarely been studied experimentally, though these herbivores are major players in these systems. In African savannas, trees often become more abundant under heavy cattle grazing but less abundant in wildlife preserves. Woody encroachment where cattle have replaced wild herbivores may be caused by a shift in the competitive balance between trees and grasses. Here we report the results of an experiment designed to quantify the positive, negative, and net effects of grasses, wild herbivores, and cattle on Acacia saplings in a Kenyan savanna. Acacia drepanolobium saplings under four long-term herbivore regimes (wild herbivores, cattle, cattle + wild herbivores, and no large herbivores) were cleared of surrounding grass or left with the surrounding grass intact. After two years, grass-removal saplings exhibited 86% more browse damage than control saplings, suggesting that grass benefited saplings by protecting them from herbivory. However, the negative effect of grass on saplings was far greater; grass-removal trees accrued more than twice the total stem length of control trees. Where wild herbivores were present, saplings were browsed more and produced more new stem growth. Thus, the net effect of wild herbivores was positive, possibly due to the indirect effects of lower competitor tree density in areas accessible to elephants. Additionally, colonization of saplings by symbiotic ants tracked growth patterns, and colonized saplings experienced lower rates of browse damage. These results suggest that savanna tree growth and woody encroachment cannot be predicted by grass cover or herbivore type alone. Rather, tree growth appears to depend on a variety of factors that may be acting together or antagonistically at different stages of the tree’s life cycle.

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Acknowledgments

We would like to thank Frederick Erii, John Lochikuya, Jackson Ekadeli, and Patrick Etelej for their invaluable assistance in the field. We also thank the Mpala Research Centre staff for their logistical support. We are grateful to Maureen Stanton and Kevin Rice for productive conversations, and to Jacob Goheen, Kari Veblen, Todd Palmer, Niall Hanan, Alan Knapp, and one anonymous reviewer for their comments on earlier versions of this manuscript. The exclosure plots were built and maintained by grants from the James Smithson Fund of the Smithsonian Institution (to A.P. Smith), The National Geographic Society (4691–91), The National Science Foundation (LTREB BSR-97-07477 and 03-16402) and the African Elephant Program of the U.S. Fish and Wildlife Service (98210-0-G563) (to T.P. Young). This research was additionally funded by a National Science Foundation Graduate Research Fellowship and a Jastro-Shields fellowship to C. Riginos. All research was carried out in accordance with the laws of the government of Kenya.

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Authors and Affiliations

  1. Department of Plant Sciences, University of California, Mail Stop 1, One Shields Avenue, Davis, CA, 95616, USA

    Corinna Riginos & Truman P. Young

  2. Mpala Research Centre, P.O. Box 555, Nanyuki, 10400, Kenya

    Corinna Riginos & Truman P. Young

  3. Center for Population Biology, University of California, Davis, CA, 95616, USA

    Truman P. Young

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  1. Corinna Riginos
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  2. Truman P. Young
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Correspondence to Corinna Riginos.

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Communicated by Alan Knapp.

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Riginos, C., Young, T.P. Positive and negative effects of grass, cattle, and wild herbivores on Acacia saplings in an East African savanna. Oecologia 153, 985–995 (2007). https://doi.org/10.1007/s00442-007-0799-7

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