This paper analyzes a long-term (1991-2019) flowering period of birch in the Czech Republic. Temporal and spatial evaluation in timing of beginning and end of flowering (Fbegin and Fend) and flowering period (Fperiod) of Betula pendula were investigated in different zones of the Czech Republic. The field observations were carried out at 44 sites of the Czech Hydrometeorological Institute phenological network; the detailed analysis including growing degree days (GDD) evaluation to Fbegin and Fend onsets and time of flowering were made at 9 sites in different altitudes. The trends and Pearson's correlation coefficients between Fbegin (Fend) and GDDs were calculated as well. The timing of both phenological stages showed a significant advance to earlier onsets (e.g., - 7.0 d per decade at Měděnec station) and the time of flowering shortens (e.g., - 3.8 d per decade at Rokytnice station). Nevertheless, the most marked shift was observed for mountain area in the north-western and north-eastern part of the Czech Republic. In contrast, the smallest shift was found in the southern part of the Czech Republic. The shift of the GDD values fluctuates from negative to positive values. Pearson's correlation coefficients calculated for both phenophases and period of flowering of Betula pendula showed the highest values in Fperiod (e.g., 0.846 at Modrava station) and in Fend (e.g., 0.711 at Rokytnice station) as well. Thus, our results indicate due to global warming symptoms that birch pollen allergy may appear earlier in the year but for a shorter period.
Keywords: Air temperature; Flowering; Growing degree day; Phenology; Spring.