Abstract
Mapping woody cover over large areas can only be effectively achieved using remote sensing data and techniques. The longest continuously operating Earth-observation program, the Landsat series, is now freely-available as an atmospherically corrected, cloud masked surface reflectance product. The availability and length of the Landsat archive is thus an unparalleled Earth-observation resource, particularly for long-term change detection and monitoring. Here, we map and monitor woody vegetation cover in the Northwest Province of South Africa, an area of more than 100,000 km<sup>2</sup> covered by 11 Landsat scenes. We employ a multi-temporal approach with dry-season data from 7 epochs between 1990 to 2015. We use 0.5 m-pixel colour aerial photography to collect > 15,000 point samples for training and validating Random Forest classifications of (i) woody vegetation cover, (ii) other vegetation types (including grasses and agricultural land), and (iii) non-vegetated areas (i.e. urban areas and bare land). Overall accuracies for all years are around 80 % and overall kappa between 0.45 and 0.66. Woody vegetation covers a quarter of the Province and is the most accurately mapped class (balanced accuracies between 0.74-0.84 for the 7 epochs). There is a steady increase in woody vegetation cover over the 25-year-long period of study in the expense of the other vegetation types. We identify potential woody vegetation encroachment 'hot-spots' where mitigation measures might be required and thus provide a management tool for the prioritisation of such measures in degraded and food-insecure areas.
Highlights
Drought, land degradation and desertification (DLDD) are serious global threats to humans and the environment
Woody vegetation covers a quarter of the area of the Northwest Province
Woody vegetation encroachment in southern African savannas causes an imbalance in the grass-woody ratio and a significant decrease of agricultural production. 10 to 20 million ha of rangelands in South Africa have seen a decrease in carrying capacity due to bush encroachment (Ward, 2005)
Summary
Land degradation and desertification (DLDD) are serious global threats to humans and the environment. 10-20% of drylands and 24% of the world’s productive lands are potentially degraded, which affects 1.5 billion people and reduces GDP by €3.4 billion. In Africa, DLDD processes affect up to a third of savannahs, leading to a decline in the ecosystem services provided to some of the continent’s poorest and most vulnerable communities. DLDD can be monitored using relevant indicators. The encroachment of woody plants into grasslands, and the subsequent conversion of savannahs and open woodlands into shrublands, has attracted a lot of attention over the last decades and has been identified as an indicator of DLDD. Bush encroachment has rendered 1.1 million ha of South African savanna unusable, threatens another 27 million ha (~17% of the country), and has reduced the grazing capacity throughout the region by up to 50%
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