Abstract
Savannahs are heterogeneous environments with an important role in supporting biodiversity and providing essential ecosystem services. Due to extensive land use/cover changes and subsequent land degradation, the provision of ecosystems services from savannahs has increasingly declined over recent years. Mapping the extent and the composition of savannah environments is challenging but essential in order to improve monitoring capabilities, prevent biodiversity loss and ensure the provision of ecosystem services. Here, we tested combinations of Sentinel-1 and Sentinel-2 data from three different seasons to optimise land cover mapping, focusing in the Ngorongoro Conservation Area (NCA) in Tanzania. The NCA has a bimodal rainfall pattern and is composed of a combination savannah and woodland landscapes. The best performing model achieved an overall accuracy of 86.3 ± 1.5% and included a combination of Sentinel-1 and 2 from the dry and short-dry seasons. Our results show that the optical models outperform their radar counterparts, the combination of multisensor data improves the overall accuracy in all scenarios and this is particularly advantageous in single-season models. Regarding the effect of season, models that included the short-dry season outperform the dry and wet season models, as this season is able to provide cloud free data and is wet enough to allow for the distinction between woody and herbaceous vegetation. Additionally, the combination of more than one season is beneficial for the classification, specifically if it includes the dry or the short-dry season. Combining several seasons is, overall, more beneficial for single-sensor data; however, the accuracies varied with land cover. In summary, the combination of several seasons and sensors provides a more accurate classification, but the target vegetation types should be taken into consideration.
Highlights
Savannahs are heterogeneous landscapes combining grassland, open canopy trees and shrubs
To investigate the reliability of mapping savannahs using Sentinel imagery that comes with an undisputed spatial resolution advantage compared to Landsat or MODIS, we examined the role of different sensor and season combinations on mapping accuracies
Our results show that the combination of Sentinel-2 and Sentinel-1 data achieves higher overall accuracies when compared to single sensor models
Summary
Savannahs are heterogeneous landscapes combining grassland, open canopy trees and shrubs. These ecosystems occur in tropical and subtropical climate zones, mainly in the Americas and Australia, as well as in Africa, where they cover half of the land surface [1]. Savannah ecosystems are important for biodiversity and the global carbon cycle and provide essential ecosystem services for some of the world’s poorest communities [2,3,4,5,6]. The provision of ecosystems services from savannahs has increasingly declined due to extensive land use/cover changes and subsequent land degradation [5,6]. Management policies (e.g., fire management), herbivore pressure, and invasive plant species directly impact savannah dynamics [8,9,14]
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