Oak Pollen Seasonality and Severity Across Europe and Modelling the Season Start Using a Generalized Phenological Model

Oak pollen seasons are relatively unexplored in large parts of Europe despite producing allergens and being a common tree in both continental and northern parts. Many studies are concentrated only on the Iberian Peninsula. In this study, the seasonal pattern of oak pollen in Europe was analysed using 10 observation sites, ranging from Spain to Sweden. The magnitude of peaks and annual pollen integral together with season-length were studied and substantially higher pollen levels and longer seasons were found in Spain. Two northern sites in Denmark and Sweden showed high oak pollen peaks together with two sites in Spain and United Kingdom. The study also tested four common definitions of season start and applied a generalized phenological model for computing the start of the pollen season. The most accurate definition for European-wide description of the oak pollen start was the cumulative pollen count 50 grains per cubic meter, daily average. The computation of the start used a thermal time model based on Growing Degree Day (GDD), utilizing daily maximum and minimum temperatures and a generalized approach to identify model parameters applicable to all included sites. GDD values varied between sites and generally followed a decreasing gradient from south to north, with some exceptions. Modelled onsets with base temperatures below 7°C matched well with observed onsets and 76% of the predictions differed ≤4 days compared to observed onsets when using a base temperature of 2°C. Base temperatures above 7°C frequently predicted onsets differing >1week from the observed. This general approach can be extended to a larger area where pollen observations are non-existent. The presented work will increase the understanding of oak pollen variation in Europe and provide knowledge of its phenology, which is a critical aspect both for modelling purposes on large-scale and assessing the human exposure to oak allergens.