Phenological series in Bologna (Northern Italy): temporal trends and spatial pattern of greening

In Europe, in the last thirty years, the onset of spring has advanced (Estrella et al. 2009 Clim Res. 39). However the phenological shifts vary from one country to another and long time series are scanty in Southern Europe (Peňuelas et. al. 2002, Glob. Change Biol.; Črepinšek et Kajfež 2003, Ann.Ser.Hist.Nat.13): for this reason we analyzed the data on leafing of some woody species recorded during the last 3 decades (1977-2009) in the city of Bologna (Emilia-Romagna, Northern Italy) and in several hillside stations in the neighbourhood of the town. As BGS index, the average day of “first leaf unfolding” (GFI=V4, BBCH=11) of a group of common woody species was chosen: the BGS days were analysed in relation with time and temperature changes. The BGS happens on average at the end of March. The species observed display a negative trend along the period (about 2 days of advance per decade) and the BGS days show a significant correlation with the mean temperature of the period January-March (about 3 and a half days of earlier start per degree of increasing temperature). Since an increase in winter temperatures has been predicted in the Emilia-Romagna region in the next decades (Tomozeiu et al. 2007, Theor. Appl. Climatol. 90), the growing season in our region is probably going to extend. Phenological spatial patterns of the neighbourhood of the city are also analysed: greening generally starts on southfacing hills between 100 and 300 m asl.