Dynamics of the Borders of Phenological Seasons in the East of the Russian Plain (Kirov, Russia)

Using the phenological database covering the period of 1961–2019 for Kirov (58°36' N, 49°40' E), the regional features of changes in the seasonal rhythms were analyzed. Mean values were calculated for 1981–2010. Additional types of seasonal rhythms with parameters deviating from the mean values by more than ±2σ were identified. The normal onset of spring, summer, and autumn was more characteristic of the 2000s (50–70%), while the anomalous seasons (with the onset dates and duration deviating significantly from the norm) prevailed in 1991–2000 and 2010–2019, respectively. The trend of variability in the ratio of phenological seasons (shorter winter and spring, longer summer and autumn) began in the middle of the 20th century and has persisted so far. In the last 30 years, compared with the base period of 1961–1990, winter and spring have begun later, but the onsets of summer and autumn have taken place earlier. The duration of winter and spring has kept decreasing. Summer has lengthened slightly. Autumn has become significantly longer. A negative correlation was established between the durations of autumn and winter ( r = –0.65), autumn and spring ( r = –0.61). The revealed phenological trends of the 21st century (the shift in the onset of the seasons, as well as the change in their duration and the increase in the amplitude of their time parameters) have been consistent with the modern restructuring of the climate regime. The data obtained on the dynamics of the natural seasons are important for assessing the response of natural ecosystems in the region to weather and climate fluctuations.