## 4 Discussion
This anticipation is in agreement with what was observed in different time frames in both grassland [61; 62; 63; 37; 64] and non-grassland biomes in Europe [36; 38]. Specifically, in our study, SOS and POS advance in significant sites (Table 4) was 0.76 days year\({}^{-1}\) and 0.56 days year\({}^{-1}\), respectively for the 2001-2021 period. In some cases, however, an opposite trend was observed, i.e., a slight tendency to delay in the SOS and/or POS dates. This could be partially explained for SOS and, consequently POS, by an insufficient cooling effect due to warming conditions in late autumn or winter [61]. On the other hand, as regards the POS only, the anticipation of SOS in response to increasing temperatures may shift the following SOS-POS duration to colder environmental conditions (e.g., Figure 9), which in turn may lead in some cases to a progressive lengthening of SOS-POS durations. This is in agreement with the modelling exercise of [PERSON] and [PERSON] [65], who suggested that an earlier flowering in wheat due to temperature increase during the period 1971-2000 may have determined shifts in post-flowering development at lower temperatures, neutralising the trend of increasing temperatures and leaving post-flowering phase duration unchanged. The same was observed in the field of grapevine [66]. This could explain part of our results regarding SOS-POS duration, particularly for those sites showing a lengthening. However, the trend is not evident in all the grassland sites analysed in this study, as a certain number of sites (i.e., 13) showed a shortening of the SOS-POS length. These outcomes result from a not too marked SOS anticipation (e.g., LAT, SWE, SWI2) and a consequent relapse of SOS into a time range (i.e., DOY) similar to the ones observed at the turn of the year 2001. In addition, the generally higher temperatures (i.e., global warming) and, probably, less water availability occurring after each specific SOS probably caused an advance in the vegetation peak [67; 68], inducing a shortening in SOS-POS duration, as the SOS remains unchanged and the POS is advanced. The presence of a spurious bias in SOS-POS length deriving from the different extraction methods for SOS and POS in estimating SOS-POS length is possible. However, investigating SOS-POS length per se was not our final goal, since our attention was mainly focused on trends. In fact, if a bias was present, this error did not influence the trend analysis, as it was present in the same way in all years from which SOS and POS were extracted. The study of seasonal average temperatures (i.e., winter and spring) showed a general rise in temperatures across Europe for the period 2001-2021, resulting in a negative correlation, significant in some cases (Table 5), with SOS and POS dates. Summer and autumn temperatures were not considered since EOS, the phenological date that occurs after these seasons, was excluded from the trend analysis since satellite estimations were not sufficiently reliable. The effect of temperature was found to be a decisive factor in the change of phenological dates of SOS and POS, in agreement with [PERSON] et al. [69] and [PERSON] et al. [63], also influencing phenology spatially [70]. Consequently, the rise in temperatures recorded in the 2001-2021 time frame could also have had an indirect effect on the advancement of the SOS date by causing an advancement in the snowmelt dates recorded in recent years [71], without the risk of increasing frost exposure [72]. Snow melt and snow cover are indeed decisive in determining the length of the growing season and the phenological development of high-altitude and high-latitude grasslands [73; 74], also influencing water availability or thermal conditions by soil insulation [75]. However, according to [PERSON] et al. [76], in a specific study on the European Alps, spring temperature was the predominant factor in SOS advancement, while snow cover and snow melt, although important, played a secondary role.