Abstract A phenological calendar with 24 phenological phases was compiled for three meteorological stations in Estonia for the period 1948–1996. We analysed the length of the vegetation period, the order of the pheno- logical phases, and the variability and possible changes for two incremental climate change scenarios (±2°C), and compared the results with examples of extreme years. The statistically significant linear trends show that the spring and summer-time phenological phases oc- curred earlier and the autumn phases moved later during the study period. The study of extreme (minimum and maximum) years shows that 70% of the earliest dates of the 24 phases studied have occurred during the last 15 years with an absolute maximum in 1990 with 8 extreme phases. The phenological spring has shortened (slope –0.23), the summer period has lengthened (slope 0.04), and the autumn has lengthened too. The length of the growing season, determined by the vegetation of rye, has shortened (slope –0.09), which could be the result of changing agricultural technology. The correlation be- tween the starting dates of the phenological phases with the air temperature of the previous 2–3 months is rela- tively high (0.6–0.8). Studying the +2°C and –2°C sce- narios and values for the extreme years shows that, in the case of short variations of air temperature, the phenolog- ical development remains within the limits of natural variation. Keywords Phenology · Phenological calendar · Estonia · Seasonality · Plant phenology Introduction The occurrence of regularities in the natural seasonal cy- cle, the alternation of hot and cold seasons, is one of the most important climatic factors influencing the plant and animal kingdoms as well as human activity in mid lati- tudes. Phenology is the study of annually recurring phe- nomena in the life cycle of an organism or the physical environment (Schultz 1981; Kramer 1996). The natural seasonality is described with phenological calendars (Schnelle 1955), which are lists of the annual sequence of phenological phases in the form of their starting dates, their duration and the intervals between the phases. Starting dates are phenological variables marking certain points in the annual cycle. They have a particular se- quence and they are closely correlated with phases of the same season. A recurrent annual cycle consists of a se- quence of regularly interchanging phenological phases described in terms of their starting date, duration and the interval between. Every period distinguished in the annu- al cycle has a duration; intervals are defined as the peri- ods between any two phenological phases. Natural calen- dars describe the seasonality of biological (living) and physical components, for example species, populations, ecosystems, sites or regions. A study of phenological calendars normally allows the homogeneous periods of the seasonal cycle to be defined as (phenological) sea- sons and the phenological phases indicated. In the current study the phenological calendar is de- fined as a list of the starting dates and selected statistics of the phenological phases, their duration and the inter- vals between them. Most phenologists use phenological calendars. Schnelle (1955) introduced the concept meth- odologically; phenological seasons were first determined by Ihne (1895) and developed by Hopkins and Murray (1933). The methods of Schnelle (1955), Davitaja (1964), Lieth (1974), Schultz (1981) and Defila (1992) show that there is no uniform method of composing a phenological calendar, determining seasons or of analys- ing seasonality. There are many methods and for analys- ing the possible impact of climate change and correlating starting dates and climatic parameters (Defila 1992; Schwartz 1999). Only the analysis of phenological time series can indicate climate change or climate organism interactions (Ahas 1999; Menzel and Fabian 1999; Crick and Sparks 1999). Phenologists study correlations with climatic seasons (Jaagus and Ahas 1998) or the diurnal R. Ahas ( ✉ ) · J. Jaagus · A. Aasa Institute of Geography, University of Tartu, Vanemuise 46, Tartu 51014, Estonia Int J Biometeorol (2000) 44:159–166 © ISB 2000 ORIGINAL ARTICLE R. Ahas · J. Jaagus · A. Aasa The phenological calendar of Estonia and its correlation with mean air temperature Received: 29 November 1999 / Revised: 15 May 2000 / Accepted: 16 May 2000