Browsing by Author "Jack, Sam"

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    Changing land use / land cover around an urban estuary : implications for ecosystem functioning
    (2006) Jack, Sam; Hoffmann, Timm
    The changing spatial and temporal patterns in land use/land cover surrounding Zandvlei estuary were investigated over the period 1944 to 2005. Changes in the extent of four terrestrial and two aquatic land use/land cover categories were mapped from high quality aerial photographs using ArcView GIS. Basic spatial analyses were performed to quantify changes in area, 'edge-effects' and relative dominance through time. Semi-natural and seasonally inundated classes accounted for over 70% of land cover in 1944, but declined steadily as urban land use and permanently inundated land cover expanded to a present-day extent of 42% and 19% respectively. The following major drivers of change were identified: 1) the construction of the railway embankment bisecting Westlake wetland and Zandvlei estuary, which led to sedimentation and a change in plant species composition of the wetland, but decreased nutrient inflows into the vlei; 2) agricultural practices within the catchment at the start of the 20th century which increased sediment and nutrient inflows; 3) elevated water levels due to dredging operations between 1947 and 1961, which resulted in a significant loss of seasonally inundated land cover with concomitant changes in species composition and nutrient dynamics; and 4) urban development surrounding the vlei (with particular reference to Marina da Gama), which has expanded at the expense of semi-natural areas and significantly increased effluent and litter inputs into the vlei. A socially and ecologically balanced management policy governing the entire catchment is required to mitigate future impacts.
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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.
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    Stable isotope analysis as an indicator of tree and grass rooting depth in a mesic African savanna
    (2006) Jack, Sam; February, Edmund C
    This exclosure-based study in a mesic southern African savanna investigated the validity of the Walter Hypothesis, which states that trees and grasses root at different depths and subsequently exploit different soil moisture layers. Root samples were extracted from vertical soil profiles in open and under canopy cover and analysed by means of carbon nitrogen isotopes. While δ¹⁵N showed little pattern due to complexities in the fractionation process, δ¹³C revealed that tree and grass rooting depths did not adhere to separate niches as proposed by Walter (1971). Root material from both growth forms occurred throughout the profile but concentrated within the upper 20-30 cm. Significantly higher densities of tree fine roots were found under canopy cover than in the open. Grass rooting followed similar patterns in both open and canopy sites, only dominating in the upper 30 cm of the open sites. Contrary to what was expected, tree fine root density increased in the top 20 cm of the open treatment, possibly due to the lack of constraining factors such as herbivory and fire within the exclosure. It is postulated that woody cover may become increasingly dominant over time due to the relatively high rainfall in the region.