The height-increment record of summer temperature extended over the last millennium in Fennoscandia

New data have allowed us to extend a previous height-increment chronology of Scots pine (Pinus sylvestris L.) at the northern Fennoscandian timberline 817 years backwards in time, from 1561 to 745. Our final transfer model accounts for 31% of the dependent instrumental (mean June—August) temperature variance between 1908 and 2007. According to the 1263 yr long summer temperature proxy, the most severe summers were experienced in 1601, 1790 and 782. Correspondingly, the summers of 1689, 885 and 1123 were the most favourable for growth. Two drastic shifts in temperature variability were also found. The twentieth century experienced a multidecadal change as the cold 1905—1914 period was immediately followed by a warm period from 1915 to 1944. An even more prominent shift occurred in the Middle Ages, as the most severe cold spell during 1135—1164 was preceded by the warmest period only a decade earlier, during 1115—1124. The Fourier spectrum of the reconstruction shows significant concentrations of variance around 33.3, 23.3 and 11 years, and between 2.6 and 3.0 years. The wavelet spectrum was able to date several centres of fluctuating periodicities between 745 and 2007. Furthermore, daily temperature records allowed us to define the major growth forcing climatic factor in more detail than in previous response analyses. The mean temperature during a 53 day season from 14 June to 6 August produced the strongest positive growth response (r ² = 0.36).