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This study aims to analyse the phenological dynamics of tree species in response to changes in climatic conditions over the last two climate-normal periods (CLINO 1961−1990 and 1991−2020). We focused on the main climax tree species (Quercus species, Fagus sylvatica L., Abies alba Mill., Picea abies [L.] Karst., and Pinus mugo Turra) dominating eight altitudinal forest vegetation zones (FVZ) and alluvial forests of Western Carpathians and adjacent lowlands in Central Europe. The phenological phases analysed in this study were first leaf (BBCH11) and general leaf colouring (BBCH94) for deciduous species, and the onset of new shoots (BBCH10) for evergreen conifers. The results of partial correlations confirmed that temperature is the main driving factor explaining the shifted onset of phenological phases for the species considered. Furthermore, deciduous species growing in the lower (and warmer) FVZ showed a certain level of drought sensitivity related to the earlier BBCH94. The identified trends in phenology were species-specific and differed between the individual FVZs. The most pronounced shifts toward the earlier BBCH10 and BBCH11 were found in the upper FVZ of the vertical distribution range of species. The presented results will support our understanding of the mechanisms underlying environmental control of tree phenology. This is crucial for predicting how the growing season of trees will be constrained by climate change-induced conditions in individual FVZ.

[Display omitted] • The probability of occurrence of late frosts on at least one day of "The Ice Saints" remains high. • Flowering of hawthorn (Crataegus laevigata) significantly advanced by 3.6–6.2 days per decade from 1987 to 2016. • In the current climate, frosts during "The Ice Saints" are still dangerous to flowering hawthorn at most altitudes. The onset and duration of phenological events are key indicators of the ecological impact of climate change on vegetation. During the flowering time in spring, late frosts are one of the most dangerous meteorological phenomena leading to damage to crops. During 1987–2016, we analyzed the occurrence and intensity of frosts during the period around the weather lore "Frost on the Ice Saints burns all the flowers" in short, "The Ice Saints" and their effects on the flowering phenology of hawthorn (Crataegus laevigata (Poir.) DC.). The late frosts were identified at 0.05 m and 2.0 m above ground from temperature measurement data at five phenological stations located from 160 m to 700 m a.s.l. in Slovakia. Results indicate that recent climate change caused the beginning of flowering (BBCH60) for Crataegus laevigata to start significantly earlier by 4.2, 5.0, 6.2 and 3.6 days per decade at 160, 300, 400 and 500 m a.s.l., respectively. Thus, the risk of late frost damage to flowers during "The Ice Saints" is getting highly probable, particularly at 160, 300 and 500 m a.s.l., where the flowering and frosts occur simultaneously. Decreased risk of frost damage during "The Ice Saints" was found at 400 and 700 m a.s.l. It was because of the rare frost occurrence at 400 m a.s.l. in the last decade (2007–2016) of the studied period, and due to the flowering onset dated mostly after "The Ice Saints" at 700 m a.s.l. in the same period. [ABSTRACT FROM AUTHOR]

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Monitoring plant phenology is one of the means of detecting the response of vegetation to changing environmental conditions. One approach for the study of vegetation phenology from local to global scales is to apply satellite-based indices. We investigated the potential of phenological metrics from moderate resolution remotely sensed data to monitor the altitudinal variations in phenological phases of European beech (Fagus sylvatica L.). Phenological metrics were derived from the NDVI annual trajectories fitted with double sigmoid logistic function. Validation of the satellite-derived phenological metrics was necessary, thus the multiple-year ground observations of phenological phases from twelve beech stands along the altitudinal gradient were employed. In five stands, the validation process was supported with annual (in 2011) phenological observations of the undergrowth and understory vegetation, measurements of the leaf area index (LAI), and with laboratory spectral analyses of forest components reflecting the red and near-infrared radiation. Non-significant differences between the satellite-derived phenological metrics and the in situ observed phenological phases of the beginning of leaf onset (LO_10); end of leaf onset (LO_100); and 80% leaf coloring (LC_80) were detected. Next, the altitude dependent variations of the phenological metrics were investigated in all beech-dominated pixels over the area between latitudes 47°44′ N and 49°37′ N, and longitudes 16°50′ E and 22°34′ E (Slovakia, Central Europe). In all cases, this large-scale regression revealed non-linear relationships. Since spring phenological metrics showed strong dependence on altitude, only a weak relationship was detected between autumn phenological metric and altitude. The effect of altitude was evaluated through differences in local climatic conditions, especially temperature and precipitation. We used normal values from the last 30 years to evaluate the altitude-conditioned differences in the growing season length in 12 study stands. The approach presented in this paper contributes to a more explicit understanding of satellite data-based beech phenology along the altitudinal gradient, and will be useful for determining the optimal distribution range of European beech under changing climate conditions. [ABSTRACT FROM AUTHOR]