## 2 Materials and Methods
### SOS, POS, EOS Analysis in the 2001-2021 Period
#### 2.2.2 Procedure and Trend Analysis  
In order to proceed with the extraction of phenological dates (i.e., SOS, POS and EOS), an area of 250 \(\times\) 250 m cleared of any interference (bare soil, rocks, shrubs, trees) was identified within each grassland site on the WEkEO portal. From the centroid of this area, coordinates were subsequently extracted and used to identify the MODIS pixel for the processing of the kNDVI vegetation index. For each pixel, annual patterns of kNDVI for the period 2001-2021 were then recreated. From these trends, dates of SOS, POS and EOS were extracted according to the results obtained by the different approaches tested during the preliminary analysis. Specifically, the choice of methods took into account the need to use a single fitting model in order to extract SOS, POS and EOS from the same curve. Instead, the extraction methods were selected in accordance with the best results obtained in the preliminary analysis for each phenological date (i.e., SOS, POS and EOS) in order to achieve better accuracy. The selected FM\(\times\)EM approaches (same FM, different EM for phenological date) were then applied to all sites and years to estimate SOS, POS and EOS. For each site, values of SOS, POS and EOS depicting a difference with the mean value higher than twice the value of deviance were discarded so as to exclude outliers from the analysis. Furthermore, in order to smooth out short-term fluctuations, a moving \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Site ID** & **Country** & **Latitude** & **Longitude** & **Altitude** & **Meteo Station** & **Meteo Years** \\ \hline GER1 & Germany & 50.7660\({}^{\circ}\)N & 10.7831\({}^{\circ}\)E & 518 & Erfurt & 2005–2021 \\ GER2 & Germany & 51.7090\({}^{\circ}\)N & 10.5377\({}^{\circ}\)E & 697 & Fritzlar & 2001–2021 \\ \hline HUN & Hungary & 47.4892\({}^{\circ}\)N & 20.9926\({}^{\circ}\)E & 276 & Debrecen & 2001–2021 \\ \hline ITA1 & Italy & 45.1806\({}^{\circ}\)N & 7.2695\({}^{\circ}\)E & 1975 & Bousson & 2006–2021 \\ \hline ITA2 & Italy & 45.6908\({}^{\circ}\)N & 11.0631\({}^{\circ}\)E & 1576 & Paganella Mountain & 2001–2021 \\ \hline ITA3 & Italy & 42.4005\({}^{\circ}\)N & 13.6756\({}^{\circ}\)E & 1623 & No station & \\ \hline ITA4 & Italy & 40.0129\({}^{\circ}\)N & 9.3181\({}^{\circ}\)E & 1464 & Perdasedefogu & 2006–2021 \\ \hline LAT & Latvia & 57.5046\({}^{\circ}\)N & 27.3139\({}^{\circ}\)E & 581 & Aluksne & 2004–2020 \\ \hline POL & Poland & 50.4299\({}^{\circ}\)N & 16.3272\({}^{\circ}\)E & 630 & Klodzko & 2001–2021 \\ \hline ROM & Romania & 46.8456\({}^{\circ}\)N & 25.1027\({}^{\circ}\)E & 1192 & Batos & 2014–2021 \\ \hline RUS1 & Russia & 53.6425\({}^{\circ}\)N & 35.5488\({}^{\circ}\)E & 165 & Bryansk & 2001–2021 \\ \hline RUS2 & Russia & 43.2756\({}^{\circ}\)N & 41.6877\({}^{\circ}\)E & 2544 & Teberda & 2013–2020 \\ \hline SCO & Scotland & 56.5194\({}^{\circ}\)N & 4.2276\({}^{\circ}\)W & 559 & Glen Ogle & 2001–2021 \\ \hline SLK & Slovakia & 49.1370\({}^{\circ}\)N & 20.2007\({}^{\circ}\)E & 1357 & No station & \\ \hline SLV & Slovenia & 46.4887\({}^{\circ}\)N & 14.0553\({}^{\circ}\)E & 1238 & Ratece & 2013–2021 \\ \hline SPA1 & Spain & 43.0329\({}^{\circ}\)N & 1.147\({}^{\circ}\)W & 992 & Pamplona & 2001–2021 \\ \hline SPA2 & Spain & 42.5973\({}^{\circ}\)N & 0.0713\({}^{\circ}\)E & 1759 & No station & \\ \hline SWE & Sweden & 64.9977\({}^{\circ}\)N & 14.5455\({}^{\circ}\)E & 854 & Stekenjokk & 2002–2021 \\ \hline SWI1 & Switzerland & 46.9242\({}^{\circ}\)N & 6.7304\({}^{\circ}\)E & 1219 & Bullet La Fretaz & 2002–2021 \\ \hline SWI2 & Switzerland & 46.9014\({}^{\circ}\)N & 8.9249\({}^{\circ}\)E & 1749 & Disentis Sedrun & 2001–2021 \\ \hline TUR & Turkey & 41.2610\({}^{\circ}\)N & 42.5550\({}^{\circ}\)E & 2524 & Ardahan & 2009–2021 \\ \hline \end{tabular} Coordinates were reported in WGS84.  
\end{table} 
Table 2: _Cont._average with a three-year window was applied on phenological dates during the time period considered.