## 1 Introduction  
Phenology is an integrative environmental science embracing biometeorology, ecology, and evolutionary biology [1]; it has become a major concern in current global change research due to its ability to monitor, understand, and predict the periodicity of biological events related to climate [2], from intra- to inter-specific level to global scale. As discussed by [PERSON] and [PERSON] [3], historically, because of its concrete application in agricultural cultivations, much phenological research has focused on issues, such as pest management, agro-meteorology, and horticulture, overlooking purely phenological topics. Even ecology-based studies with a marked phenological focus did not refer to phenology as a science per se [3]. In the last two decades, the growing attention on the impacts of climate change has exponentially enhanced the interest towards phenology. The recent launch of the USA National Phenology Network (USA-NPN; [[https://www.usanpn.org/](https://www.usanpn.org/)]([https://www.usanpn.org/](https://www.usanpn.org/))), the Pan European Phenology Project (PEP725; [[http://www.pep725.eu/](http://www.pep725.eu/)]([http://www.pep725.eu/](http://www.pep725.eu/))), the Phenological Eyes Network in Japan (PEN; [[http://www.pheno-eye.org](http://www.pheno-eye.org)]([http://www.pheno-eye.org](http://www.pheno-eye.org))), the GLOBE phenology project ([[https://www.globe.gov/web/phenology-and-climate](https://www.globe.gov/web/phenology-and-climate)]([https://www.globe.gov/web/phenology-and-climate](https://www.globe.gov/web/phenology-and-climate))) confirm the current necessity of understanding phenological dynamics under a changing environment and at multiple geographical scales [4; 5]. Plant phenology has been studied at a range of spatial and temporal scales and by employing a variety of tools [6]. Phenological studies can be led through small scale, ground-based studies [7], and large-scale investigations using proxy approaches through remote sensing (the so-called land surface phenology, LSP) [8; 9]. Significant limitations exist in monitoring phenology at the ground level, mainly due to the difficulty of (i) harmonizing data records over plant species and phenological events; (ii) using such data at global or regional scales due to the time and efforts consuming nature of field observations; (iii) integrating field data with observations of climatic variables, which have a very coarse spatial resolution; and (iv) extending the phenological events identified from individual species to communities [10]. Remotely sensed phenology (rs+pheno), or LSP, has the potentiality to overcome some of the field observation limits, filling the gap between traditional phenological (field) observations and the large-scale view of global models [11], enabling the mapping and monitoring of phenology at the ecosystem level, and providing an integrative framework at the landscape scale [12]. Satellite observations constitute a spatially aggregated signal from heterogeneous surface conditions that may not be representative of any single plant species response (pixel size from meters--e.g., Sentinel-2, to km--e.g., SPOT (Satellite Probatoire d'Observation de la Terre) Vegetation) [9]. Rs+pheno is defined as the seasonal pattern of variation of vegetation indices (i.e., greening, senescence, dormancy, etc.) in vegetated land surfaces observed from satellite remote sensing [13]. Accordingly, the length of the time series (from 5 years of Sentinel-2 to 30 years of Landsat), the high temporal frequency (from twice daily of MODIS (Moderate Resolution Imaging Spectroradiometer) to 5 days of Sentinel-2), internal consistency, and continuous availability of the satellite measurements are fundamental requirements when dealing with ecosystem responses to climate change dynamics [14]. Large to small scale remote sensing techniques are based on deriving vegetation indices (VIs), leaf area index (LAI), a fraction of absorbed photosynthetically active radiation (fAPAR) from satellite-based sensors [15].