Mapping areas invaded by Prosopis juliflora in Somaliland on Landsat 8 imagery

Abstract

Prosopis juliflora is a fast growing tree species originating from South and Central America with a high invasion potential in semi-arid areas around the globe. It was introduced to East Africa for the stabilization of dune systems and for providing fuel wood after prolonged droughts and deforestation in the 1970s and 1980s. In many dry lands in East Africa the species has expanded rapidly and has become challenging to control. The species generally starts its colonization on deep soils with high water availability while in later stages or on poorer soils, its thorny thickets expand into drier grasslands and rangelands. Abandoned or low input farmland is also highly susceptible for invasion as P. juliflora has competitive advantages to native species and is extremely drought tolerant.In this work we describe a rapid approach to detect and map P. juliflora invasion at country level for the whole of Somaliland. Field observations were used to delineate training sites for a supervised classification of Landsat 8 imagery collected during the driest period of the year (i.e., from late February to early April). The choice of such a period allowed to maximise the spectral differences between P. juliflora and other species present in the area, as P. juliflora tends to maintain a higher vigour and canopy water content than native vegetation, when exposed to water stress.The results of our classification map the current status of invasion of Prosopis in Somaliland showing where the plant is invading natural vegetation or agricultural areas. These results have been verified for two spatial subsets of the whole study area with very high resolution (VHR) imagery, proving that Landsat 8 imagery is highly adequate to map P. juliflora. The produced map represents a baseline for understanding spatial distribution of P. juliflora across Somaliland but also for change detection and monitoring of long term dynamics in support to P. juliflora management and control activities.

Assessing the Potential of Sentinel-2 and Pléiades Data for the Detection of Prosopis and Vachellia spp. in Kenya

“…Sentinel-2 provided the best results for the Mixed classes. The overall accuracies (Pléiades: 0.88-0.95; Sentinel-2: 0.79-0.96) are comparable to other studies [26][27][28] and the class specific results are in line and outperform the accuracies (Landsat 8: OOB: 0.71) shown in Ng et al [28]. Both maps display significant differences in terms of land cover.…”

“…The mapped spatial distribution of Prosopis and Vachellia (Figure 7) matches literature [10,26,31] and in situ observations. Prosopis is mainly found at the lowest and wettest areas of the ROI, collecting the majority of runoff water and containing highest population density.…”

“…As there is lower population pressure and disturbance, the established native plant communities prevent Prosopis to invade these areas [15]. As mentioned above, Prosopis is mainly found concentrated along the waterways as described by literature [26,27,31]. Some higher elevated areas along the river beds (west of ROI) surveyed during the fieldwork have no Prosopis.…”

“…Second, Europe's ambitious Currently, Copernicus program provides free-of-charge Sentinel-2 imagery at high spatial resolution (ten meter). Compared to Landsat satellites, the increased spatial resolution of Sentinel-2, as well as commercial VHR satellites, is expected to provide more accurate results [26] and offering the capacity to distinguish between different species in mixed stands. The discrimination between different species in mixed stands is an essential component of the detection of the Prosopis invasion and classification between Vachellia and Prosopis.…”

“…Moreover, it allows assessing areas that are difficult to access. In the past, various attempts have been made to map Prosopis using remote sensing approaches [26][27][28]. Based on the increasing problems caused by Prosopis invasion as well as reported observation that the plant has a distinct spectral response compared to surrounding native vegetation, several mappings have been carried out.…”