Pollen morphology and variability of native and alien, including invasive, species of the genus Spiraea L. (Rosaceae) in Poland

The pollen morphology was studied in 25 taxa of the genus Spiraea L. The aims of this study were to describe the pollen morphology and variability and to determine whether the pollen features of alien, expansive or invasive Spiraea species differ from those of other taxa. The species of Spiraea were analysed for nine quantitative pollen traits as well as the following qualitative traits: the outline, shape and exine ornamentation. In total, 750 pollen grains were measured. Based on the pollen key exine ornamentation features, then individual Spiraea species were distinguished, while the other species formed groups of usually 2–3, up to 8 species. The most important pollen features included length, width and course of grooves and striae, presence or absence of perforations, as well as their number and diameter. The most variable taxa for all the nine biometric traits jointly were S. ×billardii, S. veitchii, S. nipponica and S. cana. The pollen of the invasive S. tomentosa differed from the other taxa studied, unlike the other invasive species (S. douglasii and S. japonica).