Modelling the Start of the First Flowering Oak Pollen Season in Europe, Using a Generalised Phenological Model

Background
Oak pollen seasons are a relatively unexplored field in Europe, despite oak being an abundant tree in large areas, represented by several common native oak species (e.g. Q.ilex, Q,suber, Q.petraea and Q.robur) varying in location between southern and northern climate regions. Additionally, the oak allergen (Que a 1) is listed as one of the eight major Bet v 1 allergens of the Fagales and is considered a moderate cause of pollinosis. The number of studies of oak pollen are generally few and current ones mainly focus on oak in Spain and North America. Our study aims at describing the variation of oak seasons in a large part of Europe, establishing an accurate definition of the onset and a generalized model to predict the onset of oak pollen seasons.

Method
The seasonal pattern of oak pollen was analysed on genus level using 10 observation sites from Spain, France, Netherlands, United Kingdom, Denmark and Sweden between the years 2006-2015. Season characteristics in terms of magnitude of pollen peaks, annual pollen sum together with onset and length of season were calculated. Furthermore, four common definitions of the season onset were tested and a generalized phenological model based on thermal time (growing degree day, GDD) was applied to predict the onset of the pollen season.

Result
The most accurate definition for a European-wide description of the start of the oak pollen season was found to be the cumulative daily pollen concentrations above 50 grains m3. GDD thresholds for the start of the pollen season varied between sites and followed a decreasing gradient from south to north, with some exceptions. 73% of the model calculations for onset of the season showed a difference of four days or less compared to the observed onset of the season when a base temperature of 2°C was used.

High levels of oak pollen were often recorded in Spain (total annual sum: ~5000-17000 grains); this was also the site with the comparatively longest oak pollen seasons (~2.5 months) and several different oak species dominating the area. Northern sites generally presented lower pollen levels (total annual sum: ~ 300-3800 grains) and shorter seasons (~ 0.5-1 month) with only two dominant species (Q.petraea and Q.robur).

Conclusion
The work presented in our study will increase the understanding of oak pollen variation in Europe and provide knowledge of its phenology, which is a critical aspect for modelling purposes. The general model approach used can be implemented in other areas where pollen observations are non-existent.