# NDVI or PPI: A (Quick) Comparison for Vegetation Dynamics Monitoring in Mountainous Area  
[PERSON]  
1 EnviroSPACE Laboratory, Institute for Environmental Sciences, University of Geneva, Bd. [PERSON] 66, 1205 Geneva, Switzerland 1, [PERSON]  
2 Climate Change Impacts and Risks in the Anthropocene, Institute for Environmental Sciences, University of Geneva, Bd. [PERSON] 66, 1205 Geneva, Switzerland; [PERSON]DDRESS] 23 Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany 3  
[PERSON]  
1 EnviroSPACE Laboratory, Institute for Environmental Sciences, University of Geneva, Bd. [PERSON] 66, 1205 Geneva, Switzerland 1,4 CRID-Geneva, Institute for Environmental Sciences, University of Geneva, Bd. changes in vegetation cover, species composition, and ecosystem structure over time can help identify emerging threats and guide effective land management strategies, conservation measures, and restoration efforts to maintain the ecological integrity and functioning of mountain landscapes [7]. In the Alps, the vegetation seasonal phenological cycle occurs during the snow-free period [20]. Vegetation phenology refers to the study of recurring events in the life cycle of plants (i.e., budding, flowering, senescence) reflecting the dynamics of terrestrial ecosystems [21]. Vegetation phenology is particularly sensitive to climate change as well as land-cover and land-use change [21], inducing various consequences such as phenological shifts (e.g., change in start, duration, end of season) or change in vegetation productivity [6; 8; 21; 22], strongly affecting mountainous ecosystems [12; 23]. As mentioned by [24], the length of the growing period is one of the main determining factors of productivity. In addition, a global vegetation greening trend has been identified since the 1980s [25; 26]. In the cold areas, the greening is consistent with the recent temperature increase [26]. Although this trend is well-documented in the Arctic [4; 27], there are only a few studies in the European Alps [8; 11; 28]. However, there is an incomplete understanding of the mechanistic links between climate change and these changes in vegetation dynamics [27; 29]. Recent technological advancements, in particular remote sensing, offer new possibilities for LUC change [30; 31; 32; 33] and vegetation dynamics monitoring, particularly in wide and complex ecosystems [21; 34]. Remote sensing and derived vegetation indices offer the opportunity to observe vegetation dynamics over large areas and long periods [5; 34; 35]. Depending on their spatial, temporal, and spectral resolutions, different sensors may provide various opportunities for vegetation monitoring [35]. For global, continental, and national mapping, the MODerate resolution Imaging Spectroradiometer (MODIS) is extensively used [8; 36; 37] due to its 1 to 2 days temporal resolution and 250 to 1000 m spatial resolution (depending on the spectral band). Landsat satellites are widely used on a regional to national scale with a spatial resolution of 30 m and a temporal resolution of 16 days [4; 11; 38]. More recently, Sentinel-2 satellite constellations and their multispectral products offer new observation possibilities for continental to local mapping with a spatial resolution of 10 m and a revisit time of 5 days [39], providing new capabilities for more detailed ecosystems monitoring, such as mountain vegetation dynamics or phenological metrics retrieval at intra- and inter-annual time steps [34]. Among the different satellite-derived vegetation indices, such as the Enhanced Vegetation Index (EVI), the Green Normalized Difference Vegetation Index (GNDVI) the kernel Normalized Difference Vegetation Index (kNDVI), the Chlorophyll Vegetation Index (CVI), or the Plant Phenology Index (PPI), the Normalized Difference Vegetation Index (NDVI) is the most widely used index for assessing vegetation dynamics, specifically in mountain environments [8; 11; 35; 39; 40; 41].