Browsing lawns? Responses of Acacia nigrescens to ungulate browsing in an African savanna

D. A. Fornara, J. T. Du Toit

    Research output: Contribution to journalArticle

    87 Citations (Scopus)

    Abstract

    We measured browsing-induced responses of Acacia trees to investigate ``browsing lawns'' as an analogy to grazing lawns in a semiarid eutrophic African savanna. During the two-year field study, we measured plant tolerance, resistance, and phenological traits, while comparing variation in leaf nitrogen and specific leaf area ( SLA) across stands of Acacia nigrescens, Miller, that had experienced markedly different histories of attack from large herbivores. Trees in heavily browsed stands developed ( 1) tolerance traits such as high regrowth abilities in shoots and leaves, high annual branch growth rates, extensive tree branching and evidence of internal N translocation, and ( 2) resistance traits such as close thorn spacing. However, phenological ``escape'' responses were weak even in heavily browsed stands. Overall, browsing strongly affected plant morpho-functional traits and decreased both the number of trees carrying pods and the number of pods per tree in heavily browsed stands. Hence, there is experimental evidence that tolerance and resistance traits may occur simultaneously at heavily browsed sites, but this comes at the expense of reproductive success. Such tolerance and resistance traits may coexist if browsers trigger and maintain a positive feedback loop in which trees are continually investing in regrowth ( tolerance), and if the plant's physical defenses ( resistance) are not nutritionally costly and are long-lived. Our results confirm that chronic browsing by ungulates can maintain A. nigrescens trees in a hedged state that is analogous to a grazing lawn. Further research is needed to fully understand the long-term effects of chronic browsing on reproduction within such tree populations, as well as the overall effects on nutrient cycling at the ecosystem level.
    LanguageEnglish
    Pages200-209
    JournalEcology
    Volume88
    Issue number1
    Publication statusPublished - Jan 2007

    Fingerprint

    browsing
    ungulates
    savannas
    regrowth
    pods
    grazing
    plant spines
    Senegalia nigrescens
    Acacia
    biogeochemical cycles
    long term effects
    branching
    leaves
    herbivores
    leaf area
    spatial distribution
    shoots
    ecosystems
    nitrogen

    Cite this

    Fornara, D. A. ; Du Toit, J. T. / Browsing lawns? Responses of Acacia nigrescens to ungulate browsing in an African savanna. In: Ecology. 2007 ; Vol. 88, No. 1. pp. 200-209.
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    abstract = "We measured browsing-induced responses of Acacia trees to investigate ``browsing lawns'' as an analogy to grazing lawns in a semiarid eutrophic African savanna. During the two-year field study, we measured plant tolerance, resistance, and phenological traits, while comparing variation in leaf nitrogen and specific leaf area ( SLA) across stands of Acacia nigrescens, Miller, that had experienced markedly different histories of attack from large herbivores. Trees in heavily browsed stands developed ( 1) tolerance traits such as high regrowth abilities in shoots and leaves, high annual branch growth rates, extensive tree branching and evidence of internal N translocation, and ( 2) resistance traits such as close thorn spacing. However, phenological ``escape'' responses were weak even in heavily browsed stands. Overall, browsing strongly affected plant morpho-functional traits and decreased both the number of trees carrying pods and the number of pods per tree in heavily browsed stands. Hence, there is experimental evidence that tolerance and resistance traits may occur simultaneously at heavily browsed sites, but this comes at the expense of reproductive success. Such tolerance and resistance traits may coexist if browsers trigger and maintain a positive feedback loop in which trees are continually investing in regrowth ( tolerance), and if the plant's physical defenses ( resistance) are not nutritionally costly and are long-lived. Our results confirm that chronic browsing by ungulates can maintain A. nigrescens trees in a hedged state that is analogous to a grazing lawn. Further research is needed to fully understand the long-term effects of chronic browsing on reproduction within such tree populations, as well as the overall effects on nutrient cycling at the ecosystem level.",
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    Fornara, DA & Du Toit, JT 2007, 'Browsing lawns? Responses of Acacia nigrescens to ungulate browsing in an African savanna', Ecology, vol. 88, no. 1, pp. 200-209.

    Browsing lawns? Responses of Acacia nigrescens to ungulate browsing in an African savanna. / Fornara, D. A.; Du Toit, J. T.

    In: Ecology, Vol. 88, No. 1, 01.2007, p. 200-209.

    Research output: Contribution to journalArticle

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    AB - We measured browsing-induced responses of Acacia trees to investigate ``browsing lawns'' as an analogy to grazing lawns in a semiarid eutrophic African savanna. During the two-year field study, we measured plant tolerance, resistance, and phenological traits, while comparing variation in leaf nitrogen and specific leaf area ( SLA) across stands of Acacia nigrescens, Miller, that had experienced markedly different histories of attack from large herbivores. Trees in heavily browsed stands developed ( 1) tolerance traits such as high regrowth abilities in shoots and leaves, high annual branch growth rates, extensive tree branching and evidence of internal N translocation, and ( 2) resistance traits such as close thorn spacing. However, phenological ``escape'' responses were weak even in heavily browsed stands. Overall, browsing strongly affected plant morpho-functional traits and decreased both the number of trees carrying pods and the number of pods per tree in heavily browsed stands. Hence, there is experimental evidence that tolerance and resistance traits may occur simultaneously at heavily browsed sites, but this comes at the expense of reproductive success. Such tolerance and resistance traits may coexist if browsers trigger and maintain a positive feedback loop in which trees are continually investing in regrowth ( tolerance), and if the plant's physical defenses ( resistance) are not nutritionally costly and are long-lived. Our results confirm that chronic browsing by ungulates can maintain A. nigrescens trees in a hedged state that is analogous to a grazing lawn. Further research is needed to fully understand the long-term effects of chronic browsing on reproduction within such tree populations, as well as the overall effects on nutrient cycling at the ecosystem level.

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