To investigate the impact of recent climatic changes on plant development, this study used phenological data of the volunteer networks in Latvia and Lithuania from the 1971–2000 period. The phenological calendar method was applied. Phenological seasons were described using data on 6 phenological phases at 10 stations. The growing season was described using birch Betula pendula as an example. Correlation analysis, linear regression and non-parametric Mann-Kendall trend tests were applied to establish the relationship between phenological phases and meteorological factors (temperature, precipitation) and the North Atlantic Oscillation (NAO). The results indicate a statistically significant trend toward earlier onset of spring and summer phases. The study found that, unlike the majority of trends observed in Europe, on average the onset of phenological autumn in Latvia and Lithuania also started earlier. The observed trends in spring correlated well with temperatures in the preceding months and with the NAO. A strong correlation (in 12 cases out of 20) was found between spring phenophases and precipitation in February.