Reconstructing summer intensity in the High Arctic from stem length growth of a dwarf shrub

Annual stem length growth of the circumarctic dwarf shrub Cassiope tetragona has proven to be a reliable proxy for past and ongoing climate change in the Arctic. This reliability is based on its strong linear relationship with monthly climate parameters. Monthly means are however coarse units for prediction of growth in marginal regions with short growing seasons. Growing degree-days (GDD) are a potential alternative. GDD is a measure for growing season intensity and defined as the cumulative temperature above a given threshold, usually set at 5 oC (GDD5). The aims of this study were to test whether GDD5 is better predictor of growth than monthly mean temperatures over a high to sub arctic climatic gradient, and to reconstruct past high arctic GDD5. For this reconstruction, two shoot length records from Svalbard were used, 154 and 169 years in length. These records are the two longest growth chronologies for this species developed so far. Correlative analysis shows that GDD5 is a better predictor of annual shoot length growth than mean monthly temperatures, both at C. tetragona’s northern and southern distribution limit, as well as at its assumed climatic optimum. Svalbard Airport GDD5 was successfully reconstructed back to 1885. This reconstruction shares 61% of variance with the instrumental record. This opens up the possibility to obtain an arctic network of climate reconstructions with high temporal and spatial resolution through construction of C. tetragona stem length chronologies.