The start of the birch pollen season in Finnish Lapland:: separating non-local from local birch pollen and the implication for allergy sufferers

Determining the start of the birch pollen season requires the reliable separation of non-local from locally produced birch pollen. The research was undertaken close to the latitudinal birch tree line at the Kevo Subarctic Research Institute (69 degrees 45'N 27 degrees 01'E) in northern Finland. By comparing phenological and aerobiological observations, the proportion of birch pollen present in the air before local anthesis commences can be delimited. We coupled this with data of pollen deposition monitored by means of a modified Tauber trap. The dominant birch species at Kevo is the mountain birch Betula pubescens ssp. czerepanovii, whereas B. pubescens ssp. pubescens is very rare, hence we consider the proportion of the southerly B. pubescens-type pollen deposited in the pollen trap to be non-local in origin. We did not observe any trend towards an earlier start of the phenologically observed mountain birch anthesis at Kevo as predicted from work elsewhere. Moreover, the fixed 2.5% threshold method for determining the birch pollen season proved not to be applicable since in many years this threshold was reached before the end of continuous snow cover. The results indicate that in some years non-local birch pollen contributes considerably to the allergen load in Lapland with up to 57% of the total birch pollen sum being recorded before the day on which local anthesis commenced, and up to 70% of the annual birch pollen deposited being of the southerly birch type.