Tree growth and inferred temperature variability at the North American Arctic treeline

We present white spruce (Picea glauca) tree-ring width and maximum latewood density chronologies for two latitudinal treeline sites in northern interior Canada: along the Coppermine River in the Northwest Territories (NWT); and in the Thelon River Sanctuary, Nunavut. These chronologies provide climate and tree growth information for these two remote locations, filling a sizeable gap in spatial coverage of proxy records used to reconstruct temperature variability for the Northern Hemisphere. They represent some of the longest high-resolution proxies available for northern North America, dating as far back as AD 1046 for Coppermine ring widths. These chronologies correlate significantly with hemispheric-scale annual temperature reconstructions for the past millennium. Density records from both sites show a positive relationship with warm-season temperature data since ∼ the mid-20th century, although this link is somewhat weaker in recent decades (since ∼ 1980). Both ring-width chronologies demonstrate even greater loss of temperature sensitivity, and in the Thelon ring-width series a sustained reduction in growth appears linked to increased drought stress in this recent period. Diminishing correlations with temperature are also found when the Thelon ring-width and climate data are prewhitened, indicating that any low frequency uncertainties in the instrumental or tree-ring data (e.g., artifacts from the standardization process) cannot entirely account for this result. Our findings therefore suggest a recent loss of temperature sensitivity at these northern treeline locations that varies with the parameter and site studied. These and other uncertainties in the tree-ring as well as instrumental data will need to be resolved in future efforts to relate northern tree-ring records to temperature variability on a range of spatial scales.