Agricultural and Forest Meteorology 108 (2001) 101–112 Response of tree phenology to climate change across Europe Frank-M. Chmielewski ∗ , Thomas Rötzer Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Institute of Crop Sciences, Subdivision of Agricultural Meteorology, Albrecht-Thaer-Weg 5, D-14195 Berlin-Dahlem, Germany Received 25 October 2000; received in revised form 5 February 2001; accepted 19 February 2001 Abstract To investigate the impact of recent climatic changes on the plant development in Europe, this study uses phenological data of the International Phenological Gardens for the period 1969–1998. For this study, the leafing dates of four tree species (Betula pubescens, Prunus avium, Sorbus aucuparia and Ribes alpinum) were combined in an annual leaf unfolding index to define the beginning of growing season. The end of growing season was defined using the average leaf fall of B. pubescens, P. avium, Salix smithiana and R. alpinum. A nearly Europe-wide warming in the early spring (February–April) over the last 30 years (1969–1998) led to an earlier beginning of growing season by 8 days. The observed trends in the onset of spring corresponded well with changes in air temperature and circulation ( North Atlantic Oscillation Index (NAO-index)) across Europe. In late winter and early spring, the positive phase of NAO increased clearly, leading to prevailing westerly winds and thus to higher temperatures in the period February–April. Since the end of the 1980s the changes in circulation, air temperature and the beginning of spring time were striking. The investigation showed that a warming in the early spring (February–April) by 1 ◦ C causes an advance in the beginning of growing season of 7 days. The observed extension of growing season was mainly the result of an earlier onset of spring. An increase of mean annual air temperature by 1 ◦ C led to an extension of 5 days. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Phenology; Growing season; Climate change; NAO; Temperature 1. Introduction Phenological observations are some of the most sensitive data in identifying how plant species respond to regional climate conditions and to climatic changes. Therefore, phenology has emerged recently as an important focus for ecological research (Schwartz, 1999). In mid-latitudes the seasonal timing of spring events such as budding, leafing or flowering of plants depends highly on air temperature. With increasing ∗ Corresponding author. Tel.: +49-30-314-71210; fax: +49-30-314-71211. E-mail address: chmielew@agrar.hu-berlin.de (F.-M. Chmielewski). temperatures plant development in spring starts earlier within a year. A lot of recent phenological studies were reported on earlier spring events in recent decades. Depending on the species and the investigated period, the results vary to a certain extent. Beaubien and Freeland (2000) reported on a long- term trend (1900–1997) in timing of first bloom of Populus tremuloides (aspen poplar) of -2.7 days/ decade at Edmonton/Alberta (Canada). Mainly since 1973 the negative deviations from the long-term mean prevail. The spring flowering index — mean of the first flowering dates of P. tremuloides, Amelanchier alnifo- lia (saskatoon) and Prunus virginiana (chokecherry) — had also advanced by 1.3 days/decade in the 0168-1923/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0168-1923(01)00233-7