The analysis of long-term time series of spring phenology for different deciduous trees species has shown that leaf unfolding for all the investigated species is the most sensitive to temperatures in March and April and illustrates that forcing temperature is the main driver of the advancement of leaf unfolding. Available chilling amount has increased by 22.5 % over the last 90 years, indicating that in the investigated geographical region there is no threat of chilling shortage. The projection of climatic parameters for Central Lithuania on the basis of three global circulation models has shown that under the optimistic climate change scenario (RCP 2.6) the mean temperature tends to increase by 1.28 °C and under the pessimistic scenario (RCP 8.5) by 5.03 °C until the end of the current century. Recently, different statistical models are used not only to analyze but also to project the changes in spring phenology. Our study has shown that when the data of long-term phenological observations are available, multiple regression models are suitable for the projection of the advancement of leaf unfolding under the changing climate. According to the RCP 8.5 scenario, the projected advancement in leaf unfolding for early-season species birch consists of almost 15 days as an average of all three used GSMs. Markedly less response to the projected far future (2071-2100), climate change is foreseen for other investigated climax species: -9 days for lime, 10 days for oak, and 11 days for maple.

The aim of the study was to investigate impact of climate warming on timing of deciduous tree species spring (leaf unfolding) and autumn (leaf falling) phenology and to compare seasonal development and growing season changes of native (Quercus robur L.) and non-native (Quercus rubra L.) tree species. Long-term dataset of phenological observations from Kaunas Botanical Garden of Vytautas Magnus University (central Lithuania) were used for this study. Increased temperature was detected to be a strong driver of spring phenology for both species. It was detected that red oak had a greater response in leaf unfolding 9.5 days comparing with common oak which displayed advance by 8 days during the investigated period. Leaf fall was delayed for common oak by 13.5 days, for red oak -by 1.9 days. An advance of leaf unfolding and delay of leaf fall extended the growing season of investigated tree species. Native tree species responded more than non-native species in response of changes in temperature and the growing season for Q. robur extended by 21.6 days, while for Q. rubra -11.4 days.