## 5 Discussion
### VI SOS Temporal Patterns  
Temporal trends in observed SOS dates agree for the different VIs at the overall level (Figure 4) despite the significantly better performance of PPI in SOS detection as compared to EVI and PPI. However, the analysis for different land cover types and boreal regions reveals distinct differences between trends of VIs. Evergreen needleleaf forest is characterized by a markedly negative trend in PPI (0.25 days/year), suggesting a change towards earlier SOS dates, whereas NDVI shows the opposite (weak) trend towards later SOS dates (0.04 days/year) (Table 2). Some of the major boreal regions do also differ; e.g., the Boreal Shield, Boreal Plains and Hudson Plains do all show negative PPI trends towards earlier SOS dates (\(-\)0.26, \(-\)0.03 and \(-\)0.26 days/year), whereas NDVI shows positive trends (0.04, 0.16 and 0.21 days/year). These patterns possibly indicate a later snowmelt, but an earlier start of the growing season. For the West Siberian Taiga, the pattern is reversed, with PPI showing a trend towards later SOS (0.13 days/year) and NDVI shows an advance in SOS (\(-\)0.15 days/year). The delay over the West Siberian Taiga is likely to be associated with its geographical setting being the world's largest continuous lowland, with more than a half of the area <100 m above sea level and densely covered by wetlands and floodplains [81]. The delay in SOS can therefore be caused by high sensitivity to altered hydrological conditions of the wetlands as was also reported for the East Siberian Taiga [82]. The delay in SOS observed over the Boreal Plains in Canada for areas dominated by croplands is likely related to changes in cropping practices. Trends for areas under substantial anthropogenic influence are generally expected to be decoupled from the patterns of trends of natural forested ecosystems dominated by changes in climate. The observed patterns of trends (Figure 4) showed good agreement with existing studies [7; 35]; however, we find fewer areas of significant trends in the current study. This is likely due to a short time-period of investigation (as compared to Global Inventory Modeling and mapping Studies (GIMMS) based studies starting in 1982), reduced statistical significance for phenology trends in recent decades [24], outliers in the noisy VI time-series of the high latitudes, and per-pixel based trend estimation method [48; 70; 83]. A better estimation of significant trends could have been achieved by applying panel analysis [84] instead of pixel-based trend analysis, which is reported to give non or minor significant trends in the high latitudes [85; 86]. To avoid areas of abrupt phenology changes caused by fires or deforestation, a dataset of forest loss during 2000-2014 [55] was used to exclude such areas. However, it takes several decades for regrowth of a forest, and it is likely that pixels affected by forest fire or logging activities prior to 2000 are still present in the analysis. Forest fires have a negative effect on satellite-based phenology retrieval, since spectral properties of burnt scar-vegetation signal persist up to several years and may cause observations diverging from the overall trend of the landscape [85]. The overall lower spatial variability in trends for PPI as compared to EVI and NDVI (Table 2) suggest that PPI is less prone to many of the abovementioned confounding factors when determining SOS as compared to the conventional indices. The standard deviations of the trends reported (Table 2), however, also indicate that trends in SOS (days/year) for many International Geosphere-Biosphere Programme (IGBP) land cover classes and boreal regions are highly varying, and, in many cases, the standard deviations are higher than the trends reported. High latitude ecosystems are highly responsive to the interannual variation in the drivers of phenology, and even if the results presented here overall support the earlier onset of growth in the Northern Hemisphere boreal zone, the spatial variability reveals considerable differences for the period of analysis.