## 3 Results
### SOS and POS Trend Analysis (2001-2021)  
The procedures selected during the methodological analysis (ELM\(\times\)TRS\({}_{0.3}\) and ELM\(\times\)GU, respectively, for SOS and POS) were used to investigate possible changes in the phenological timing of European grasslands. Given that from the results obtained in 3.1 (Table 3), the end of the season (EOS) was not well identified by MODIS-kNDVI when compared to EOS extracted from GPP, the analysis considered exclusively the start (SOS) and peak (POS) dates of the growing season. Satellite-derived SOS showed a clear trend over the time span considered. As depicted in Table 4 and Figure 4, 26 sites (out of 31, i.e., 83.9% of the total) evidenced a negative correlation between SOS and years, indicating a progressive advance in the start of the growing season from 2001 onwards. From the SOS-years regression analysis, 17 grasslands sites showed a level of significance (F) \(<\) 0.1 in SOS anticipation. Specifically, 3 sites reported F values between 0.05 and 0.1 (AUS1, POL, RUS1), 5 between 0.01 and 0.05 (GEO, GER2, RUS2, SLV, TUR) and 9 \(<\) 0.01 (AUS2, BOS, CZE, DEN, FRA2, ITA3, HUN, SLK, SWI1). The advance in SOS was subsequently quantified (Figure 4) by applying the equation identified from the specific SOS-years linear regression. From 2001 to 2021, the average advance of the growing season at significant sites (F \(\leq\) 0.1) was 15.92 days (0.76 days year\({}^{-1}\)). However, it should be noted that the significant sites analysed showed different levels of SOS earliness, ranging for example from the 5.6-day advance of the RUS1 site to the 42.0-day advance of the DEN site. As with SOS, the POS dates in the period 2001-2021 showed a trend towards an earlier peak of the grassland growing season. In fact, although in a smaller percentage than in SOS, 21 sites (67.7% of the total) evidenced an advance in POS dates (Table 4 and Figure 5). Figure 4: Changes (days) in SOS during the period 2001–2021. SOS advance is highlighted with red circles, whereas delay is highlighted with green circles. Sites that display a significant SOS-year correlation are marked with * (0.05 \(<\) F \(\leq\) 0.1), ** (0.01 \(<\) F \(\leq\) 0.05) and *** (F \(\leq\) 0.01). From POS-years regression analysis, 13 grassland sites showed a level of significance (F) of \(<\) 0.1 in POS advance during the period 2001-2021. Specifically, 6 sites reported F values between 0.05 and 0.1 (BOS, DEN, FRA2), 3 between 0.01 and 0.05 (AUS2, CZE, SWE) and 4 \(<\) 0.01 (GER2, LAT, RUS2, TUR). In the case of POS, in addition to sites that showed a significant advance in the beginning of the season, two sites highlighted contrasting trends (FRA1 and ITA2), showing a clear delay in the peak of the season (0.01 \(<\) F \(<\) 0.05). As for SOS, changes in phenological dates during the period 2001-2021 were quantified for each site (Figure 5). Considering all the significant sites (F \(\leq\) 0.1), including those that showed a delay, the average advance in the peak of the growing season was 11.66 days (0.56 days year\({}^{-1}\)). In order to carry out an assessment of temperature influence in determining the phenological dates of European grasslands over the 2001-2021 period, statistical analyses were performed to assess the temperature trend over the years considered and the impacts of thermal factors on SOS and POS. From the results of the linear regressions performed between temperatures and years (Table 5), the mean winter temperatures showed an increasing trend in 21 of the 23 sites (sites with no or little weather data were excluded),  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**SOS**} & \multicolumn{4}{c}{**POS**} \\ \hline
**ID** & **Mean** & **R** & **Days** & **Sign. ** & **Pp** & **Mean** & **R** & **Days** & **Sign.