The relationship between flowering phenology and pollen seasons of Alnus Miller

The dynamics of flowering and pollen release in anemophilous plants and the length of the particular phases depend largely on the geobotanical features of a region, its climate, meteorological factors, biological characteristics of vegetation, and abundance of pollen resources. The aim of the study was to determine the relationship between the flowering phases in eight Alnus taxa and the dynamics of occurrence and abundance of airborne pollen grains as well as the meteorological factors (maximum and minimum temperature, relative air humidity, maximum wind speed, and precipitation). The flowering phenophases and pollen seasons were studied in 2008–2011. Phenological observations of flowering were conducted in the Maria Curie-Skłodowska University Botanical Garden in Lublin and they involved the following taxa: Alnus crispa var. mollis, A. glutinosa, A. incana, A. incana ‘Aurea’, A. incana ‘Pendula’, A. maximowiczii, A. rubra and A. subcordata. Spearman’s r correlation coefficients were calculated in order to determine the relationship between the dynamics of inflorescence development and meteorological conditions. Aerobiological monitoring using the gravimetric method was employed in the determination of Alnus pollen content in the air. The annual phenological cycles in 2008-2011 varied distinctly in terms of the time of onset of successive flowering phases in the Alnus taxa studied, which depended largely on the taxonomic rank and meteorological factors. The following flowering sequence was revealed in the 2008-2011 growing seasons: A. subcordata (December or January), A. incana ‘Pendula’, A. incana, A. maximowiczii, A. rubra, A. glutinosa, A. incana ‘Aurea’ (February or March), and A. crispa var. mollis (April). The study demonstrated that the pollen of the taxa persisted in the air, on average, from mid-December to early May. The mean length of the flowering period, which coincided with various phases of the pollen season, was 17 days. The Alnus pollen season in 2008 started at the end of January and lasted until mid-March. In 2009, 2010, and 2011, the beginning of the pollen season was recorded in the first week of March and the end in the first week of April. The maximum concentration of airborne Alnus pollen was found at the full bloom stage of mainly A. glutinosa and A. rubra. Inflorescence development was most closely related to temperature and relative air humidity; there was a weaker relationship with wind speed and precipitation.