Flowering phenology in the central highland of Iceland and implications for climatic warming in the Arctic.

The cool and short growing season that characterizes Arctic climates puts severe constraints on life cycles and reproduction in the Arctic flora. The timing of flowering is particularly critical and may affect both breeding system and reproductive success through the heavy penalties associated with later flowering. An 11-year study of 75 species in the central highland of Iceland showed that the onset of flowering varies greatly among years. The number of species in flower by the first week of July was closely correlated with air temperature (degree days above zero) in the preceding 5 weeks, but no correlations were found with degree days in May or with total degree days in the previous growing season. Time of snowmelt, which has widely been regarded as the environmental event initiating growth and flowering in alpine and arctic tundra, only had a significant effect when two exceptionally cold and late summers were included. The species studied, most of which have a wide distribution in the Arctic, are predicted to respond quickly to warmer spring and early summer temperatures. Accelerated phenologies may alter patterns of resource allocation, have implications for pollinators and pollinator-competition, and could increase the size, species richness and intraspecific genetic diversity of the soil seed bank.