Prosopis spp. are a fast growing invasive tree originating from the American dry zones, introduced to Kenya in the 1970s for the restoration of degraded pastoral lands after prolonged droughts and overgrazing. Its deep rooting system is capable of tapping into the ground water table reducing its dependency on rain water and increasing its drought tolerance. It is believed that the Prosopis invasion was eased by a hybridization process, described as the Prosopis Juliflora – Prosopis Pallida complex, suggesting that introduced Prosopis spp. evolved into a hybrid, specifically adapted to the environmental conditions, rendering it a superior and aggressive competitor to endemic species. In many dry lands in Kenya Prosopis has expanded rapidly and has become challenging to control. On the other hand, in some cases, an economic use seems possible. In both cases, detailed and accurate maps are necessary to support stakeholders and design management strategies. The aim of this study is to map the distribution of Prosopis spp. in a selected area in north-west Turkana (Kenya), covering a section of the Tarach water basin. The study is funded by the European Union through the National Drought Management Authority (NDMA) in Kenya, and the main purpose is to assess the potential production of Prosopis pods, which can be used to manufacture emergency livestock feeds to support animals during drought events. The classification was performed using novel Sentinel-2 data through a non-parametric Random Forest classifier. A selection of reference sites was visited in the field and used to train the classifier. Very high classification accuracies were obtained.
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.
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