Although aerobiological data are frequently used as a flowering sign in phenological research, airborne pollen counts are influenced by a number of factors affecting pollen curves. A study was made about the reproductive biology of birch and environmental factors influencing its pollen release and transport, in order to achieve a reliable interpretation of Betula pollen curves. Aerobiological data were recorded in 2002 and 2003 at two sites in NW Spain and phenological observations were carried out on 20 trees from four Betula populations (three Betula alba L. and one B. pendula Roth.). Pollen production was calculated for six Betula alba trees. Chilling and heat requirements for triggering development were calculated. Due to differences in the geographical location, budbreak and flowering started first in Betula pendula. The flowering period lasted from 8-13 days. Reduced pollen output per anther and catkin in individual trees in 2003 prompted a marked decline in overall pollen production. Major differences observed in birch pollen curves were attributed to the influence both of weather conditions and pollen transport from areas where the flowering occurs at a different time. Heat requirements calculated using phenological and aerobiological data were similar when the peak pollen-count date was used.