Many studies on the changes in the rhythm of nature (animate and inanimate) revealed that plants are good indicators of climate change, because they are responsive to air temperature variations. There is a clear trend towards earlier onset of plant development in Poland, especially for spring phenological events as flowering and leafing. The main objective of this study was to identify phenological tendencies of selected plant indicator species in Poland and to explore the statistical relationship between mean phenological onset date against mean monthly air temperature. Phenological observations of 9 plant species: Corylus avellana L., Tussilago farfara L., Padus avium L., Taraxacum officinale F.H. Wiggers coll., Betula pendula Roth, Syringa vulgaris L., Aesculus hippocastanum L., Robinia pseudoacacia L., Tilia cordata Mill. from 25 locations acquired in the period 1951–1990 were used. The results proved that phenological changes were the reaction of plants to air temperature variations. For spring and summer phenophases, the mean correlation coefficient was negative — higher air temperature was related to earlier onset dates. The advance of spring and summer phenology was estimated at -2.2 days per 1°C. Majority of phenological events were the most significantly correlated with mean monthly air temperature of the preceding months.
Phenological trends observed in Poland were mainly negative (73% of analysed stations), significant (22%) but diversified between the selected species. Linear regression analysis of the onset dates against year (including regression coefficient) showed the highest values for Corylus avellana flowering: -0.24 days per year (20% significant) and Betula pendula leaf unfolding: -0.18 days per year (33% significant). Robinia pseudoacacia showed significant advances (72% of negative trends were significant) in a summer season with an average rate of -0.3 days per year. Whereas in case of Tilia cordata no tendencies to advance has been proved.