Trends in vegetation productivity and seasonality for Namaqualand, South Africa between 1986 and 2011: an approach combining remote sensing and repeat photography

This thesis presents an assessment of vegetation change and its drivers across a subset of Namaqualand, South Africa. Namaqualand forms part of the Succulent Karoo biome, which is characterised by exceptionally high species biodiversity but which has undergone severe transformation since the arrival of pastoral colonists. Vegetation productivity in Namaqualand is of great importance since there is a high dependence on natural resources, livestock and agriculture for both subsistence and income. However, there is considerable debate on the relative contribution of land-use change and climate change to vegetation change and land degradation in Namaqualand. Early studies based on bioclimatic envelop models suggest that an increase in temperature and more arid conditions could result in the vegetation cover of the Succulent Karoo being significantly reduced. On the other hand, more recent studies show that less extreme changes in rainfall could result in the vegetation of the biome remaining fairly stable with possible increases in the spatial extent by 2050. Furthermore, field observations and repeat photography, suggest that the change in vegetation in the region over the course of the 20 century generally portrays an increase in cover largely as a result of changes in land-use. By combining repeat photography and satellite data from NOAA-AVHRR and TERRA-MODIS sensors as well as baseline climatology data from the CRU TS 3.2 data set this study aimed to:  Determine the critical pathways of inter-annual and intra-seasonal vegetation change in the Namaqualand  Investigate the role of land-use and climate variability as key drivers of vegetation change in Namaqualand. This study presents a unique approach in the sense that it employs multi-source and multitemporal data to assess vegetation change over time. A key component of this research was to determine whether historical repeat photography could be successfully used in combination with the remotely sensed vegetation indices possibly serving as a means of validation. The first component of the research investigated the trends in local climate for Namaqualand. This provided a basis for the interpretation of the trend in vegetation derived Un ive rsi ty of Ca pe To wn DVSCLA007 Page | v from remotely sensed information as well as the repeat photographs. There is good evidence, based on the analysis of the CRU TS 3.2 data set (1901-2009), to suggest that temperatures in the region have been increasing over the last century and that the rate of warming has been increasing. There was no clear evidence, however, for a significant change in mean annual rainfall and the rainfall time series remains dominated by oscillating wet and dry periods. The second component of the study qualitatively assessed the repeat photographs in order to determine the extent of vegetation change within the different land tenure systems (commercial or communal) and land-use practises (livestock grazing or cultivation) in the region. The repeat photographs demonstrated that perennial species have been able to recolonize previously cultivated fields. This improvement is, however, limited in the communal areas where sites have been subjected to heavy grazing pressure over a long period of time. The repeat photographs also provided evidence of an increase of Acacia karroo along rivers and small tributaries. The third component of the research investigated the extent, nature and rate of recent changes in vegetation productivity and phenology using remotely sensed vegetation indices. Both the normalised vegetation index (NDVI) and the enhanced vegetation index (EVI) derived from the AVHRR and MODIS satellite sensors were used as proxies of vegetation in this research. The remote sensing data was analysed using a combination of spatial and nonspatial trend analysis techniques including the Earth Trends Modeler in Idrisi Taiga, Rain-use efficiency (RUE), trend in the standardised residuals (RESTREND), TIMESAT software as well as linear and piece-wise regressions. Phenological and productivity metrics derived from the AVHRR NDVI and MODIS NDVI and EVI time series data included the start, end and length of the growing season, time of peak NDVI, maximum NDVI, cumulative NDVI of the growing season, and minimum NDVI. The results indicate that the vegetation in Namaqualand is considerably variable over space and time and responds to changes in rainfall and altitudinal gradients. Increases in vegetation productivity were noted over the escarpment and Kamiesberg regions of Namaqualand whereas decreases were observed over Bushmanland. There were no distinct changes in the start, end or peak of the growing season but a significant decreasing trend decrease in the seasonal amplitude supported the finding from the repeat photographs that there has been a shift in dominance from annual to perennial plants in the study area. In terms of the sensor datasets used in this study, there is a clear trade-off between the longer time series of the older AVHRR sensor with a lower resolution and higher noise and the MODIS sensor with the shorter time series but higher resolution and less noise. This study was able to demonstrate that climate variability and changing land-use patterns have a considerable influence on vegetation change in the region and are important filters through which to interpret vegetation change in Namaqualand. Overall, a trend towards improved vegetation cover and composition is evident in the study area. The findings of this Un ive rsi ty of Ca pe To wn DVSCLA007 Page | vi study have created a baseline or reference conditions for Namaqualand against which future change can be assessed. This study offers new insights into the spatial and temporal trends in vegetation productivity and seasonality for Namaqualand that could not have been accomplished without the combined use of both long-term satellite time series data and repeat photography. The repeat photographs were able to add value the analysis of NDVI and EVI by providing information on species composition changes as well as changes in landuse practises. The photograph pairs, however, only represent two, sometimes 3 snapshots in time and are thus not representative of the average cover experienced at a particular site. Furthermore, the start and end dates of the repeat photographs do not match those of the satellite time series. Despite these limitations, repeat photographs could be a promising alternative ancillary data source that could be used in satellite based vegetation monitoring studies in semi-arid regions.