Woody plants in temperate regions, in order to prevent the water in their cells from freezing, interrupt their growth entering into a physiological state called``dormancy''. Trees also have a heat requirement that must be full® lled before actual growth is resumed and pollination occurs. The objective of this study is to ascertain the in¯uence that a climatic parameter such as temperature exerts on Alnus, in order to identify the start of the dormancy period, its duration and the consequent heat requirement that triggers¯owering in three European regions located at the same latitude. In this regard, we chose two areas in Spain (Vigo and Santiago de Compostela) and one in Italy (Perugia), since they have diVerent temperature patterns, ranging from mild in the case of Vigo to cold in that of Perugia, including an intermediate temperature pattern (Santiago). Monitoring in the three stations was carried out by means of a 7-day Lanzoni VPPS 2000 pollen trap. Alnus began its pollen season between the second and third week of January in Spain, while in Italy it began during the ® rst week of February. During the four years under study, in order to overcome the dormancy period, Perugia needed an average of 962 chilling hours (CH), Santiago 622 CH, and Vigo had the smallest chilling accumulation with only 460 CH. In the case of heat accumulation, we found that Perugia had the smallest requirement and Vigo the largest, thus Perugia needed an average of 42 (growth degree days) GDD, Santiago around 50 GDD and Vigo 62 GDD.

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In order to survive periods of adverse cold climatic conditions, plant requirements are satisfied by means of physiological adaptations to prevent cells from freezing. Thus, the growth of woody plants in temperate regions slows down and they enter into a physiological state called dormancy. In order to identify the chilling and heat requirements to overcome the dormancy period of Betula in the south of Europe, a comparative study was carried out with aerobiological pollen data of a 7-year (1995)(1996)(1997)(1998)(1999)(2000)(2001) period in Vigo (Spain) and Perugia (Italy). To satisfy chilling requirements, base temperatures of 7 C and 5.75 C showed a lower standard variation coefficient: 3.94% and 2.36% in Perugia and Vigo respectively. In the case of heat accumulation, the sum of mean temperatures in Perugia and the sum of maximum temperatures in Vigo were the parameters that showed a minor coefficient of variation (11.13% and 14.51% respectively).

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