Browsing by Author "van Blerk, Justin"

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    Sexual dimorphism in the genus Leucadendron : Morphology and plant hydraulics
    (2013) van Blerk, Justin; West, Adam G
    The genus, Leucadendron, of the Cape Proteaceae family, is made up of over 70 dioecious species that vary in their degree of sexual dimorphism. Males are generally more highly ramified (branched) with smaller leaves compared to corresponding females. It has been hypothesised that sexual dimorphism in Leucadendrons is linked to serotiny (a fire-adapted reproductive strategy), where highly serotinous females may incur extra resource costs in order to keep their transpiring cones alive between fires. This hypothesis predicts that the female morphology might be associated with more efficient resource acquisition compared to males in order to support their greater resource requirements. Another hypothesis suggests that selection for greater floral display in males has lead to a higher degree of ramification as male cones are borne terminally on branches. This highly branched morphology may be associated with subsequent physiological costs. The idea that different male and female morphologies might be associated with different physiological costs or benefits was tested in this experiment with a focus on plant hydraulics. Hydraulic supply is known to affect photosynthetic capacity and maximum assimilation rate. Using a specially designed vacuum chamber, leaf-specific and xylem-specific hydraulic conductance was measured in males and females of the highly dimorphic Leucadendron rubrum and non/marginally dimorphic Leucadendron daphnoides. Using microscopic imagery, xylem anatomy was analysed in an attempt to explain the hydraulic conductance results.
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    A simulated history of Aloe dichotoma recruitment and its link to rainfall : insights from an isolated population near Kenhardt
    (2013) van Blerk, Justin; Hoffman, Timm; Jack, Sam
    Aloe dichotoma populations differ in their rates of recruitment and mortality across the species distribution in South Western Africa. Little is known about how recruitment and mortality rates are affected by local climate and ecological factors. Using repeat photography and a statistical recruitment model, the recruitment history of an emerging population of over 200 A. dichotoma individuals near Kenhardt was simulated. Historical rainfall data was used in order to assess the possible link between rainfall and recruitment. Recruitment patterns were found to be a combination of episodic events of varying length and magnitude superimposed over a background of low-rate continuous recruitment. The largest, most extended episodic recruitment period peaked in the mid-1970s and led to the establishment of most of the individuals in the Kenhardt population. These trees were generally between 150 and 250 cm in height (2013). Newly discovered juveniles below 100 cm in height were predominantly the result of relatively small and brief episodic recruitment events that occurred in the mid and late 2000s. Between episodic recruitment events, evidence for low-rate continuous recruitment is present, particularly in more recent years. Episodic periods of high-rate recruitment were found to be linked to high rainfall. The largest, most extended recruitment period coincided with the exceptionally high rainfall of the 1970s. The more recent episodic recruitment events also coincided with high rainfall spikes during the 2000s. While correlations between rainfall variables and recruitment were significant, many other factors may affect the strength of this relationship.