Browsing by Author "Jack , Sam"

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    Analysis of long-term changes in populations of the Clanwiliam Cedar (Widdringtonia cedarbergensis) using repeat photography
    (2013) White, Joseph Douglas Mandla; Hoffman, Timm; Jack , Sam; February, Edmund C
    Repeat photography photosets were used to analyse 20th century changes in populations of Widdringtonia cedarbergensis from four sites in the Cederberg Mountains, South Africa. Systematic evaluation of 115 photosets was combined with field observation and environmental data to determine the age class and degree of foliage cover, as well as fire frequency, rockiness of habitat, altitude, aspect of slope, annual mean temperature and annual precipitation for each tree viewed in photosets. Mortality and recruitment events were documented in each of the photosets. A total of 1315 trees were recorded in historical photographs (1931- 1982), with 968 of these shown to have died in the repeat photographs (2007-2013), indicating 74% mortality. With only 45 (3.4%) recruits being recorded, the total current living population of W. cedarbergensis in the repeat photographs was 392. There was no significant difference found in mortality or recruitment across the sampled sites. From the historical to repeat photosets there was an 8% increase in the proportion of mature adults and a 13% increase in the proportion of W. cedarbergensis individuals with sparse foliage cover. A generalized linear model was used to determine the effects of environmental factors on W. cedarbergensis mortality in natural populations.
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    Open Access
    Understanding changes in plant productivity using EVI satellite data in Tswalu Kalahari Reserve
    (2016) Tokura, Wataru; Hoffman, Timm; Jack , Sam; Anderson, Tania
    In the arid African savanna, the limited availability of water strongly affects plant productivity, but other key drivers of vegetation dynamics, such as herbivory and fire, are usually considered to have a relatively minor impact. The main purpose of this study was to characterise the spatial and temporal pattern in plant productivity in the 100 000 hectare Tswalu Kalahari Reserve (TKR) in the semi-arid Northern Cape and relate the observed changes to potential drivers using medium spatial resolution of MODIS Enhanced Vegetation Index (EVI) time series data (16 day, 250 m) from 2000 to 2015. The time series of EVI for the past 16 years in TKR presented a highly seasonal pattern which fluctuated between years. A composite of annual small integrated value of EVI images highlighted spatial and temporal heterogeneity of plant productivity in the area. The EVI value was mainly influenced by rainfall and effect of fire and herbivory was considered to be minor. These observations confirmed the extreme variability of plant productivity in the drylands in the summer rainfall region of South Africa. Additionally, most of the values concerning the phenometrics of EVI differ significantly among vegetation types. This suggests that the structure and function of the vegetation determine plant productivity as well as their being a possible effect of soil property and reflectance. The trend in plant productivity computed by residual trend analysis (RESTREND) detected a significant positive trend in plant productivity in the east and south west of TKR, which overlapped with shrub-dominated vegetation, providing evidence for possible ongoing bush encroachment in these areas. On the other hand, a negative trend was detected in some locations in the west. The data generated from MODIS EVI and the small integrated value of EVI using TIMESAT produced biologically interpretable results. However, the correlative relationship between the EVI derived from Landsat Operational Land Imager (OLI) and plant cover estimated in the field was poor or not significant and needs to be examined further.