A 247-years tree-ring reconstruction of spring temperature and relation to spring flooding in Eastern Boreal Canada

Few spring paleoclimate records are available for boreal Canada and given the warming of spring temperatures in recent decades and its impact on snowmelt and hydrological processes, the search for spring climate proxies is receiving increasing attention. Tree-ring anatomical features and intra-annual widths were used to reconstruct regional mean March-April-May temperature from 1770 to 2016 in eastern boreal Canada. Nested principal component regressions calibrated on 116 years of gridded temperature data were developed from one Fraxinus nigra and ten Pinus banksiana sites. The reconstruction indicated three distinct phases in spring temperature variability since 1770. Ample phases of multi-decadal warm and cold springs persisted until the end of the Little Ice Age (~1875) and were gradually replaced since ~1940 by decadal to interannual variability associated with an increase in the frequency and magnitude of warm springs. Significant correlations with other paleotemperature records, and gridded snow cover extent and runoff support that historical high floods were associated with late, cold springs with heavy snow cover. Most of the high spring floods reconstructed for the nearby Harricana River also coincided with the lowest reconstructed spring temperatures. However, the last 50 years of observed and reconstructed mean spring temperature showed a reduction in the number of extreme cold springs contrasting with the last few decades of extreme flooding in the eastern Canadian boreal. This result indicates that warmer late spring mean temperatures on average may contribute, among other factors, to advance the spring ice break-up and to likely shift the contribution of snow to rain in spring flooding processes.