Boreal temperature variability inferred from latewood maximum density and historical plant phenology records

Plant-based data from southern Finland were used to reconstruct late Holocene warm-season temperature variability on inter-annual to longer scales. Temperature-sensitive records representing maximum latewood density of Pinus sylvestris tree rings (since AD 760) and phenological stages of several plant species (since AD 1750) explained ~60% and ~70% of instrumentally observed temperature variance, respectively. The value of a multi-proxy approach was demonstrated by statistical models including both variables, which explained ~80% of the temperature variance. Temperatures from the CRUTEM5 and Berkeley datasets had slight variations in their correlativity with proxy data, possibly resulting from their differing spatial representativeness over the proxy sites. Temperature history inferred from maximum latewood densities extended over the past millennium and correlated with previously published data from similar proxy records in Fennoscandia and adjacent areas. These data indicate that the region cooled since the Medieval Climate Anomaly and warmed markedly since the Little Ice Age/Maunder Minimum. In the study region, the magnitude of this long-term warming was 2.1 °C and 2.8 °C, calculated between the coldest and warmest 100-year and 30-year intervals, respectively. Collectively, our results display the potential of plant-based data from low-lying and mild boreal sites to extend our understanding of preindustrial and recent climatic changes.