Airborne pollen might be transported over thousands of kilometres, which has important ecological, evolutionary and clinical consequences. The long-distance transport (LDT) of birch (Betula sp.) pollen has been described in detail for northern Europe. However, a comprehensive analysis of this transport from other European regions is lacking. This study focused on the post-seasonal LDT of birch pollen to Poland (central Europe), with special attention paid to determining potential source areas of pollen and describing the causal mechanism favouring LDT episodes. Pollen monitoring (1997-2016) was conducted in Poznań and Rzeszów (500 km away from each other) using volumetric traps. The LDT episodes were characterized by analysing the (1) bi-hourly backward air mass trajectories using the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT); (2) sea level pressure (SLP) and 500 hPa geopotential height (z500) anomalies; and (3) patterns of the Enhanced Vegetation Index to determine the birch flowering time along the moving air mass trajectories. The potential locations of birch populations within broadleaved forests were estimated with GLOBCOVER data. Finally, the movement of pollen emitted from potential source areas was simulated using the HYSPLIT dispersion model. LDT episodes were mainly recorded in the first fortnight of May. The main source areas of pollen to Poland were western Russia, Belarus and to a lesser extent the eastern Baltic republics and the Scandinavian Peninsula. In most cases, a high-pressure centre located over Scandinavia and an elevated z500 over Germany-Denmark-Sweden favoured pollen transport. On average, the post-seasonal LDT episodes of birch pollen to Poland occur almost every year (Poznań) or every second year (Rzeszów). The episodes are highly variable in time; thus, the pollen concentration may unexpectedly cause allergy symptoms in sensitized patients. In some cases, these episodes may be extremely severe, thereby prolonging and strengthening the exposure to birch pollen allergens.
Keywords: Betula; Birch; Enhanced vegetation index; Land surface phenology; Long-distance transport; Sea level pressure gradient.
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