Anomalous 20th century tree growth, Mackenzie delta, Northwest Territories, Canada

A number of contemporary dendroecological studies from northwestern North America have highlighted a divergence in growth trends during recent decades. These studies suggest that warmer temperatures are now exceeding the physiological threshold of some northern tree species, or perhaps are contributing to increased drought stress as current precipitation is insufficient to offset increasing water demands under warmer conditions. Here we document additional evidence of these diverging growth trends from the Mackenzie Delta region of Canada and show they are anomalous to the twentieth century. Using wavelet coherency analyses we demonstrate that our white spruce tree ring chronologies exhibit little divergence from one another during the past four centuries, but coherency of the data-sets rapidly break down after the 1930s.