Trends in mountain birch seed abundance and its relation to temperature and moth abundance: a ten-year study from Swedish Lapland

Abstract: Boreal forest is the world’s largest terrestrial biome. Being one of the most ubiquitous and coldtolerant tree species in Sweden, the mountain birch forms a unique deciduous treeline and marks the forest-tundra ecotone. Its reproduction and ability to spread is therefore important from both a societal and ecological perspective, locally as well as globally. Folivorous larvae of the species autumnal moth and winter moth reach outbreak levels in arctic and alpine areas approximately every 9-10 years and can severely defoliate birch trees. Cold winters kill the eggs of the folivorous insects. Global warming and changed interspecific interactions possibly present new challenges for mountain birch growth and reproductive success. The aim of this study was to investigate trends in mountain birch seed abundance and how seed abundance is related to temperature and abundance of folivorous larvae. Further the aim was to investigate the effects of winter temperature on larval abundance. Seed rain was collected over a time period of ten years in Abisko, northern Sweden, by using seed traps set out from August to June. The seeds were manually sorted, and mountain birch seeds were counted. Larval abundance was counted as average number of larvae per hundred birch shoots. The results showed that seed abundance is neither increasing nor decreasing and that temperature and larval abundance do not significantly affect seed abundance. Larval abundance decreased over time but increased with increasing winter temperature, possibly indicating that larval abundance and distribution will increase with the increasingly warmer climate. Larval abundance and growing season temperature appear to not influence seed abundance and possible future changes in birch seed abundance might then be caused by other factors, such as precipitation, but for now appears to be rather stable.