Susan Ringrose

The use of integrated remotely sensed and GIS data to determine causes of vegetation cover change in southern Botswana

Abstract The characteristics and dynamics of dry savanna vegetation cover are receiving considerable attention from the perspectives of both global change and range degradation studies. Problems include the establishment of major savanna determinants and the floristic response of vegetation cover to given stimuli. Basic work on determinants is required to assess the nature and causes of natural resource depletion, particularly in the Kalahari region. Use of image processing techniques involving the association of pixel values and field data have resulted in the development of a vegetation map indicating floristic content and structure. Results indicate that a clear distinction can be made between classes containing high proportions of taller woodland species and those that contain mainly woody weeds. Degraded areas with sparse vegetation cover and large areas of bare soil were also identified. The GIS technique of buffer analysis was applied to determine the extent to which herbivory (livestock) and the gathering of bush products by the local population were directly involved in the spatial distribution of savanna types. Results indicate that most of the degraded areas are within 2 km of villages and boreholes. Most of the woody weed areas fall within a 2–4-km zone around boreholes. Spatial association indicates that uncontrolled bush product harvesting and goat grazing are primarily responsible for village-centred degradation, while cattle grazing around numerous boreholes is a primary cause of woody weed development. These kinds of savanna adaptive responses are difficult to reverse in rural Botswana because of increasing population pressure and concomitant poverty.

Spectral assessment of indicators of range degradation in the Botswana hardveld environment

Abstract The literature suggests that two different approaches have been applied to problems of rangeland monitoring using MSS data. These are referred to as the near infrared over red ratio which has been successfully applied to areas of relatively dense vegetation in the humid zone, and the darkening effect which is applicable in the sparsely vegetated semiarid zone. Data from Botswana suggests that neither of these is singularly appropriate in the savanna woodland zone of southern Africa. In the Botswana hardveld the measured vegetation cover consists of green vegetation which generally occupies less than 60% of the cover in a given area. The soil component is dominant. This, in addition to other vegetation components which produce a darkening effect, results in high reflectance values in the red and infrared parts of the spectrum for areas with a low vegetation cover and low reflectance values in both wavebands for areas with a high vegetation cover. In savanna woodland environments which contains elements of both the near infrared to red ratio and the darkening approach, the most suitable indicators of range condition and degrees of desertification can be obtained by directly applying spectral ranges from the red band. The range of values used is heavily ecosystem, therefore soil-type-dependent, and is referred to as the savanna woodland model.

The darkening effect in drought affected savanna woodland environments relative to soil reflectance in Landsat and SPOT wavebands

Abstract Results of soil tests and correlation analyses on 94 soil samples showed that calcium carbonate content was relatively significant in influencing soil reflectance in the Botswana semiarid savanna woodland environment. Low vegetation density reflectance (Acacia and Terminalia species) was measured in situ using Landsat and SPOT wavebands against a background of typical Kalahari soils. The soils were grouped according to Munsell Color notation. Results showed that combined soil plus vegetation reflectance was dependent on the reflectance of the soil background in the case of moderately dark to light soils. Low density vegetation cover lowered the combined reflectance entirely in red wavebands. Reflectance in the near infrared was also lowered by Acacia species when the vegetation cover was lower than Leaf Area Index = 2.5. This lowering of reflectance is referred to as “darkening.” Analysis of darkening variables in the case of a moderately dark soil showed that green leaf cover contributed 16.3% (red band) and 11.8% (near infrared band) to the overall darkening effect. Savanna woodland shrubs induce this effect by internal and external drought adaptive mechanisms which decrease near infrared scatter and enhance through shadow the efficacy of red absorption as a predictor of green vegetation cover. The rate of red absorption varies as a function of soil reflectance (relative soil brightness). This is shown on twelve predictive regression curves which were developed for both Acacia and Terminalia species.

Environmental change in the mid-Boteti area of north-central Botswana: Biophysical processes and human perceptions

Increased interest in environmental change issues has led researchers to consider more integrated approaches to change dynamics. This paper examines change in terms of land degradation in north-central Botswana from both biophysical and human perspectives. Although seasonal and periodic droughts were prevalent, analysis of rainfall data over the past 70 years revealed no downward trend. However, indicators of declining productivity such as soil erosion, loss of vegetation cover, and a declining groundwater table were amply evident. The GIS analysis of remotely sensed data has shown that complete vegetation recovery after drought is not taking place, particularly in the south-central part of the study area. These areas contained the highest human and livestock population densities. The local people acknowledged facing increasing resource depletion and indicated drought as the main cause. Pressures on available resources, particularly during drought periods, appeared to have impeded the regenerative capacity of the natural vegetation cover, thereby inducing land degradation. This situation may not easily be rectified because of widespread poverty and inappropriate local perceptions of the solutions. Both of these hinder the adoption of sustainable land management.

Assessment of vegetation indexes useful for browse (forage) prediction in semi-arid rangelands

Considerable significance is placed on the mapping and monitoring of degraded areas in semi-arid regions of the world, including Botswana. Degraded areas include those suffering from bush encroachment, believed to result from heavy cattle grazing over a number of years. However, certain bush encroachment species have been found to be relatively nutrient-rich. The present work considers the extent to which a series of quantified layers through mainly bush encroachment canopies can be identified using conventional and newly derived vegetation indexes and transforms based on Thematic Mapper (TM) imagery. Field work involved the stratification of green biomass into firstly the herbaceous cover layer; secondly the 0.3-1.5 m browse layer; then the 1.5-2.5 m browse layer; and finally the >2.5 m browse layer. Biomass measurements from these layers were statistically associated with conventional vegetation indexes and transforms such as the Normalized Difference Vegetation Index (NDVI), brightness and greenness values, and relatively newly derived darkening indexes involving the mid-infrared bands. When green biomass and transformed pixel data were averaged per classified vegetation unit, weak negative correlations emerged between grass biomass and the transformed pixel data and no significant correlations developed with the woody biomass (browse) layers. However, when point data were used in the analyses, results showed that most indexes and the brightness transform were significantly correlated with the lower browse layer. Only the darkening indexes and brightness function were sensitive to the browse layers individually and the browse plus grass layers. This work shows the limitations of conventional indexes such as the NDVI in terms of browse and herbaceous layer assessment. New indexes for forage assessment based on relationships between the mid-infrared bands, such as those found in the new MODIS TERRA platform, are urgently required for semi-arid areas.

A Landsat analysis of range conditions in the Botswana Kalahari drought

Abstract An analysis was undertaken of range conditions in the Botswana Kalahari during the 1980-1987 drought. Information from Landsat MSS print and tape data was field-checked both aerially and on the ground through two wet seasons. Comparison of 327 field and digitized aerial photograph sites with spectral data on MSS4, 5 and 7 in relation to hues on Landsat colour composite print data revealed three main spectral-vegetation types. Type 1 darkened areas comprise green tree and shrub vegetation, shown as varying tones of blue-green hues. The rate of darkening varied with the soil substrate but regression analysis showed that higher green vegetation densities occurred at lower MSS4 and 5 reflectance values. Type 2 areas combine darkened areas with areas of high near-infrared reflectance. Vegetation indices do not predict green leaf cover in type 1 or type 2 areas. This is best achieved through density slicing on MSS5. Type 3 areas show high near-infrared and low red reflectance. Green leaf cover can be p...

Mapping ecological conditions in the Okavango delta, Botswana using fine and coarse resolution systems including simulated SPOT vegetation imagery

Aspects of wetland monitoring in southern Africa are significant in the light of the Ramsar Convention, which has recently been signed by Botswana. The present work initially indicated that most ecological units in the Okavango delta can be identified and mapped using fine resolution satellite imagery and aerial photography. A second step involved assessing which of these units could also be identified using coarse resolution imagery. Contrasting results were obtained from a relatively high quality solid state sensor system (simulated SPOT 4-VEGETATION) and a relatively low quality system (NOAA-AVHRR). The extent of the water surface in the flooded areas could not be accurately mapped using NOAA-AVHRR data because of pixel saturation effects. However the presence of water was generally located using AVHRR data. The ecologically significant riparian woodlands were found to saturate pixel response on Thematic Mapper (TM) imagery. As a result of detailed comparisons, it was found that simulated SPOT VEGETATION data could be used for mapping and monitoring major changes in grassed floodplains and wooded peripheral drylands which are not spectrally separable using NOAA-AVHRR data. Most ecological change in the Okavango delta which involves such small-scale transformations as thicket development (shrub growth) and minor land-use changes related to tourism, requires the use of finer resolution systems for instance high resolution false colour aerial photography in conjunction with fieldwork. In terms of overall needs for mapping and monitoring wetland conditions in semi-arid areas it appears that major changes in flood extent and related floodplain drying vegetation changes are best detected accurately using fine resolution (TM) satellite data. Large-scale ecological changes can be semi-quantified using high quality imagery data at coarse resolutions similar to SPOT VEGETATION data.

Use of image processing and GIS techniques to determine the extent and possible causes of land management/fenceline induced degradation problems in the Okavango area, northern Botswana

Abstract Attention worldwide has been focused on the need to assess the appropriateness of land management strategies especially where these occur near sensitive areas of wildlife habitat. This work considers the use of mainly Thematic Mapper data in providing an assessment of the relative impact of different land management strategies on the natural vegetation cover in part of the sensitive Okavango area in Botswana. Supervised classification (maximum likelihood) techniques when used on six-band TM imagery showed that differential degradation was prevalent in land management areas, especially where these are separated by fencelines with an overall accuracy 72 per cent. Marginally more degradation is evident in a controlled hunting area adjacent to the Game Reserve, relative to a communal grazing area. Band transform analyses indicate that distinctive changes in cover type and density frequently take place over boundaries or fencelines separating land management areas. Some degradation in the controlled h...

Evaluating Ecohydrological Theories of Woody Root Distribution in the Kalahari

The contribution of savannas to global carbon storage is poorly understood, in part due to lack of knowledge of the amount of belowground biomass. In these ecosystems, the coexistence of woody and herbaceous life forms is often explained on the basis of belowground interactions among roots. However, the distribution of root biomass in savannas has seldom been investigated, and the dependence of root biomass on rainfall regime remains unclear, particularly for woody plants. Here we investigate patterns of belowground woody biomass along a rainfall gradient in the Kalahari of southern Africa, a region with consistent sandy soils. We test the hypotheses that (1) the root depth increases with mean annual precipitation (root optimality and plant hydrotropism hypothesis), and (2) the root-to-shoot ratio increases with decreasing mean annual rainfall (functional equilibrium hypothesis). Both hypotheses have been previously assessed for herbaceous vegetation using global root data sets. Our data do not support these hypotheses for the case of woody plants in savannas. We find that in the Kalahari, the root profiles of woody plants do not become deeper with increasing mean annual precipitation, whereas the root-to-shoot ratios decrease along a gradient of increasing aridity.

Results from the EO‐1 experiment—A comparative study of Earth Observing‐1 Advanced Land Imager (ALI) and Landsat ETM+ data for land cover mapping in the Okavango Delta, Botswana

The Earth Observing‐1 (EO‐1) satellite acquired a sequence of data in 2001 and 2002 that highlighted the annual flooding of the lower Okavango Delta. The data were collected as part of the calibration/validation programme for the Advanced Land Imager (ALI) sensor on the NASA EO‐1 satellite. The primary purpose of this study was to compare the capability of ALI to that of Landsat ETM+ for large‐scale mapping applications in the Okavango Delta. While the extent and inaccessibility of many areas of the Delta make application of remote sensing attractive, the availability of data with adequate spatial and spectral resolution has limited the characterization of the complex patterns of land cover and geomorphology in the Delta. Initial analysis of the ALI data via supervised classification clearly showed macro‐flood features, delineation of downstream channel flow areas, and lateral‐downstream inundation of the floodplain. These patterns and the proportions of flooding of the channel compared to that of the floodplain (impoundment) varied annually, from the wetter seasonal swamps through the drier seasonal and occasional swamps. Consistently higher classification accuracies achieved using ALI data relative to ETM+ data are attributed to the higher signal‐to‐noise ratio and the increased dynamic range of the ALI data.