Browsing by Author "Beukman, Ruth Pam"

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    Intraspecific competition in Leucadendron xanthoconus (O.Knutze) K. Shun
    (1988) Beukman, Ruth Pam; Bond, William J
    Sexual dimorphism, characteristic of a number of dioecious genera has inspired workers to generate hypotheses as regards intersexual and intrasexual competition. The Proteaceae genus Leucadendron is an example. In the Highlands State forest near Grabouw (Cape Province, South Africa) a number of Leucadendron xanthoconus populations exist. Here, an investigation for the detection of competition between and within the sexes was carried out, following the methods of Pielou (1960). The importance and the intensity of competition was estimated adopting the criteria pointed out by Welden and Slauson (1986). Based on the hypothesis that maternal investment is far greater than paternal investment at reproduction, female resource requirements were presumed to be larger than males. Consequently, females are the stronger competitors. Evidence for intersexual and intrasexual competition was found, the intensity of each type varied however with stand age. Intense negative interference by females on males was evidenced, supporting the idea that females were the stronger competitors. It was apparent that this caused male reproductive fitness to decline. Except for the 15-year-old plants there was no evidence to suggest niche partitioning between the sexes. Though competition was detected, its importance in relation to other factors in the community appeared to be low. Further studies are required to determine the processes operative in such dioecious populations. Depending on the degree of sexual dimorphism, does the importance and intensity of intraspecific competition vary? Does the pollination biology of dioecious, sexually dimorphic species influence niche partitioning?
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    The role of nurse plants in the vegetation dynamics of the succulent Karoo
    (1991) Beukman, Ruth Pam; Bond, William J
    Little is known about the vegetation dynamics of the Karoo region of South Africa. The aim of this study was to focus on the role of the "availability of suitable space" in the regeneration of Succulent Karoo vegetation. By undertaking a series of manipulative experimental transplants and pattern analyses, it was shown not only that shrubs in the Karoo facilitate other species by provision of 'sui table space' beneath their canopies, but also that certain shrub species are more effective "nurses" I than others . The plants dependent on nurse plants are referred to as "patient" plants. Not only did the 'main patient species Tylecodon wallichi require nurse plants generally for successful establishment, but this species was also most successful (in terms of reaching maturity) beneath a specific nurse Pteronia pallens. Thus the patient was "nurse specific". In time however, the initial commensal relationship between the nurse and the patient (whereby the patient benefits and the nurse is unaffected) changes to a competitive one, since nurse vigour declines as the patient grows larger. Despite this area being arid, a manipulative experimental investigation revealed that competition between the nurse and the patient was not for water. An examination of the rooting patterns of the two species showed that their root systems were separated in vertical and horizontal space. Though the mechanism of competition between Pteronia and Tylecodon was not resolved in this study, the patient Tylecodon did appear to reduce nurse vigour and ultimately replace the nurse Pteronia pallens. Since specific species associations do exist (i.e. some patient species are nurse specific), the replacement of one species by another (the nurse by its patient) is predictable and so 'succession' at the scale of the individual plant can be said to occur in the Karoo.