## 1 Introduction  
Growing consensus on severity of the ecological impacts posed by recent global warming and its amplification in the high latitudes of the Northern Hemisphere (NH) increases the importance of understanding the interaction between the climatic changes and terrestrial ecosystems [1; 2; 3; 4; 5]. Plant cyclical biological events (plant phenology) are strong indicators of the seasonality of the environment, revealing the implications of rising temperatures on vegetation functioning [6; 7; 8; 9]. Depending on the vegetation cover and climate zone, plant phenology responds differently to climatic factors, such as air temperature, precipitation and photoperiod [10]. However, altered temperature conditions have been observed to have a dominating influence on vegetative activity in the Northern Hemisphere, which makes changes in plant phenology a significant indicator of biological responses to increasing temperatures [1; 11; 12; 13; 14]. Not only is plant phenology crucial for understanding the impacts of climate change, but phenology is also controlling various processes in the terrestrial ecosystems, such as net primary productivity and seasonal exchanges of CO\({}_{2}\), water and energy between the land surface and the atmosphere [15; 16; 17; 18]. It has been observed that increasing temperatures in the NH are associated with advanced onset of growing season, which may possibly result in longer growing season and consequently alteration of the annual carbon (C) cycle [19; 20]. Boreal forests--the dominating biome of the NH, are representing 32% (272 \(\pm\) 23 PgC) of the global forest \(C\) stock, hence playing a crucial role in the global \(C\) cycle, either as a sink or source [21; 22]. Investigating phenology of boreal forests is therefore important for understanding the impacts of climate change and associated changes in global \(C\) balance [23]. Satellite-based observations are becoming essential for boreal region plant phenology retrieval, since it allows for remotely capturing phenological variations both at small and large scales, while ground observations of the Northern mid and high latitude plant phenology are limited and represent local conditions [7; 16; 23; 24; 25; 26]. Existing satellite-based research in plant phenology over the boreal zone of the NH has been mainly carried out by employing spectral vegetation indices (VI), calculated from the reflectance acquired from optical remote sensing using satellites [27; 28]. The most widely applied VI that combines visible and near infrared light reflected by vegetation is the Normalized Difference Vegetation Index (NDVI) [29; 30]. NDVI is considered to a proxy of plant phenology, since it observes vegetation canopy greenness at the same time as minimizing clouds, changing sun angles, topography and atmospheric contamination effects. However, it has been reported to be influenced by canopy background, atmospheric conditions and reduced sensitivity to variations in high biomass regions [31; 32]. In order to improve upon these drawbacks, an Enhanced Vegetation Index (EVI) was developed by incorporating soil adjustment and atmospheric resistance factors that minimize soil brightness influences, increase sensitivity to variations in dense canopies and further reduce atmospheric influences [33]. However, existing studies based on NDVI and EVI reveal that existing methods for retrieval of boreal region plant phenology often involve uncertainties [34; 35; 36; 37], leading to inconsistency among findings of satellite-based boreal forest phenology studies in relation to ground observations that have shown a continuous advance in the onset of the growing season both in North America and Eurasia [6; 38; 39]. The sources of uncertainty are mainly attributed to insufficient methods for overcoming the presence of snow that significantly affects VI signal [16; 40; 41; 42; 43], as well as difficulties in capturing subtle seasonal variation in canopy greenness of evergreen forests [15; 44; 45; 46; 47].