Seed rain and seed bank along an alpine altitudinal gradient in Swedish Lapland

We studied the seed flux, including seed rain and seed bank (germinable and total), at twelve sites along an altitudinal gradient in the Abisko area in northernmost Swedish Lapland during a period of 3 years with contrasting summer climates. The study sites were evenly spaced in altitude from the timberline at 700 m above sea level to the highest peaks in the area (1560 m). A subalpine birch forest site was included for comparison. Each site was equipped with seed traps, replaced and emptied directly upon snow-melt each summer. Soil samples for seed bank assessment were taken at all sites, and inventories of the vascular plant flora were carried out in the 10 m radius neighborhood of the traps. The results revealed high variation among years with regard to seed rain and its partitioning over various functional types of plants. Even though most of the seed rain could be attributed to species present in the plant community of the trap sites themselves, some more long-distance dispersal takes place every year. A number of extrazonal recoveries are reported, often several hundred m above the distributional limit of the species. Even though seed number and species diversity declined rapidly from seed rain over total seed bank to germinable seed bank, the correlation among all three aspects of the seed pool was high. The dominant species in the seed flux at moderate altitudes, Empetrum hermaphroditum Hagerup, has a persistent seed bank with an average turnover of more than 200 years.