Altitudinal Variation in Flowering Time of Lilac ( Syringa vulgaris L.) in the Alps in Relation to Temperatures

Abstract

The flowering time of lilac is a frequently used bioindicator for temperature driven developmental processes in spring time. In the present study the influence of altitude on the flowering dates of lilac was investigated for 24 sites in the Northern and Central Alps, and 10 sites in the Southern Alps. The onset of flowering dates in these areas of the Alps are closely related to the mean air temperatures in April. A temperature decrease by 1 C postpones the onset of flowering by 6.6 days. This results in a gradient of lilac flowering times of 4.2-4.6 days per 100 m in the Central Alps between 600 and 1500 m a.s.l. and a gradient of 3.7 days per 100 m a.s.l. in the Southern Alps between 90 and 1100 m a.s.l. Over the 20th century a deviation from the mean onset of flowering dates of about AE12 days was seen due to climatic fluctuations in the Northern and Central Alps. On the Southern rim of the Alps the thermal conditions are most favourable for the development of lilac and therefore, there are no remarkable time shifts to be seen.