Onset of flowering and climate variability in an alpine landscape: a 10-year study from Swedish Lapland.

Long-term studies on phenology are rarely reported from arctic and alpine areas, but are essential for understanding biotic and abiotic controls on flowering. We monitored first flowering day (FFD) for 144 species in a subarctic-alpine area in Swedish Lapland over a period of 10 yr (1992-2001). Temperature and global radiation were monitored continuously, and snowcover duration was observed. Thawing degree-days and snowcover duration (exposure) were the dominant environmental controls on phenology. We introduce a lability index (LI) to describe the interannual variability in FFD among species. The temporal sequence of species is very constant among years, although a few species are more labile. The species were also classified into the catagories "Functional type," "Raunkiær's life form," and "Sørensen's wintering floral type." The last two reflected the environmental data best, and together with "Exposure" they were combined into a phenology index (PI). The index was subsequently used in a triangular ordination together with FFD. The ordination illustrates whether species flower earlier or later than expected from their preconditions. We hypothesize that species having a delayed flowering respond more readily to global warming than species having an already optimized flowering.