## 1 Introduction  
The Horn of Africa has been affected by its location in the tropics under the influence of the Sahara and Arabian Desert, the Red Sea, and the Indian Ocean. Climate variability in the region is certain [1], and there is a high rate of vulnerability to the impacts of climate change in the plant species and the people who depend on the rain-fed farming [2]. Climate variability across East Africa from historical records show that temperatures have increased by 1.5-2 \({}^{\circ}\)C on average and there are large rainfall variations in the direction and magnitude of changes over the past 50 years [3]. The countries in the Horn of Africa have been exposed to series of drought, and the region at large is exposed to the effects of drought, and the causes are associated with climate change and climate variability [4]. Land surface temperature shows an increasing pattern in many countries of the Horn of Africa, for instance, [PERSON] et al. [5] identified an increase of the annual average land surface temperature as high as 1.8 \({}^{\circ}\)C in Ethiopia. Consequently, the overall impact of climate variability and climate change on vegetation dynamics and pressure on ecosystem services has been immense especially during the last two decades. The biodiversity of the Horn of Africa is also threatened due to land cover and land use changes mainly caused by deforestation. A recent study of land use/land covers changes in the Horn of Africa confirms that forested areas were decreased and there was an expansion in built-up areas (2000-2015) with large impacts on water yield and latent effect to ecosystem services [6]. Specific biodiversity analysis in tropical Africa indicates that 33% of the plant species are potentially threatened with extinction, while Ethiopia is one of the four highlighted regions with a high percentage of potentially threatened species [7]. Similarly, a recent study on land use/land cover change on ecosystem services in the central highlands of Ethiopia [8], concluded that forest cover was reduced by 54.2% from 1973 to 2015 with a very large loss in ecosystem service values. Loss of forest cover or even bushlands lead to decreased carbon sequestration by the woody vegetation [9] and increased land surface temperature and evapotranspiration [10] as shown in Southern Kenya, thus causing climate change first in a small area and, when it happens through the region, then largely. Ecosystem change detections are limited by uncertainties in climate change predictions and their driving forces [11]. The assessment of plant conditions and vegetation dynamics with the impacts of drought and climate variability can be essential to understand the changes in the terrestrial ecosystem and ecosystem resilience to different disturbances. Vegetation dynamics may cause variations in the phenology and changes in the performance of plant species at different time scales [12]. Studies on vegetation dynamics and their quantification are numerous, and accurate measurement at larger areas may increase human understanding of the dynamics in the terrestrial ecosystem, carbon exchange system, and climate-biosphere interactions in the environment [13; 14]. The dynamics of vegetation for small areas can be assessed and monitored with field-based measurements [15], however, as the spatial coverage and temporal variability increases, the complexity of the vegetation and types in structure and phenology also changes and varies essentially. The use of satellite imaging has increased and become a key tool for ecological scientists to monitor seasonal and inter-annual vegetation dynamics [16]. Hence, the use of remote sensing products such as normalized difference vegetation index (NDVI), leaf area index (LAI), gross primary productivity (GPP), and evapotranspiration (ET) become feasible to analyze the trend and productivity of vegetation as well as to evaluate the ecosystem water use efficiency (eWUE) and ecosystem resilience.