Dendroclimatic reconstruction of maximum summer temperatures from upper treeline sites in Interior British Columbia, Canada

Two independent reconstructions of maximum May-August temperatures are developed from a new network of Engelmann spruce tree-ring chronologies at treeline sites across Interior British Columbia (IBC). The IBC reconstruction (ad 1600-1997) uses the longest three ring-width (RW) and two maximum latewood density (MXD) chronologies from the region. The shorter (regional) reconstruction (REG, 1845-1997) is based on an independent, more broadly based network of chronologies (12 RW and 5 MXD) and verifies the regional signal in the parsimoniously sampled IBC reconstruction. Both models explain 53% of the regional temperature variance (1912-1995) and correlate strongly (r = 0.92) over their common period. The IBC reconstruction indicates two prolonged cooler intervals, c. 1620-1710 and 1775- 880, separated by warmer conditions that approached late twentieth-century normals between c. 1710 and 1730. The mean anomaly over the 1600-1900 period is estimated at 0.38degreesC below the 1961-1990 mean with the seventeenth century (1601-1700) being marginally colder than the nineteenth century (-0.53:-0.49degreesC). Both reconstructions model the rise in temperatures from the 1880s to 1940s and indicate that maximum summer temperatures since 1930 have been warmer than at any period since 1600. The IBC record from 1600-1900 is very similar to the mean summer-temperature record reconstructed in the adjacent Canadian Rockies, providing mutual verification for the regional nature of the signal in both reconstructions. This is the first maximum summer-temperature reconstruction from North America. Significant changes are also noted in the relationships between summer mean, maximum and minimum temperatures in this region in the last few decades with a greater absolute rate of increase in mean and minimum temperatures. These changing relationships suggest it is prudent to model tree-ring response to a variety of temperature parameters rather than using mean-temperature values.