BROWSING LAWNS? RESPONSES OF ACACIA NIGRESCENS TO UNGULATE BROWSING IN AN AFRICAN SAVANNA

Author(s)D. A. FORNARA AND J. T. DU TOIT
Year Published2007
JournalEcology
Page Numbers200-209
Size267.67 KB
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.

Key words: Acacia nigrescens; compensatory growth; grazing lawns; herbivory; Kruger National Park, South Africa; plant defensive traits; resistance; savanna; spinescence; tolerance

Authors: D. A. FORNARA AND J. T. DU TOIT

Journal: Ecology 


FileAction
2007 Fornara.pdfDownload 
Terms and Conditions: Any PDF files provided by the GRC are for personal use only and may not be reproduced. The files reflect the holdings of the GRC library and only contain pages relevant to giraffe study, and may not be complete. Users are obliged to follow all copyright restrictions.