NOAA/WDS Paleoclimatology - Sniderhan - Inuvik - PCMA - ITRDB CANA612

Northwestern Canada's boreal forest has experienced rapid warming, drying, and changes to permafrost, yet the growth responses and mechanisms driving productivity have been understudied at broad scales. Forest responses are largely driven by black spruce - the region's most widespread and dominant tree. We collected tree-ring samples from four black spruce-dominated sites across 15 degrees of latitude, spanning gradients in climate and permafrost. We investigated (1) differences in growth patterns, (2) variations in climatic drivers of growth, and (3) trends in water use efficiency (WUE) through 13C isotope analysis from 1945 - 2006. We found positive growth trends at all sites except mid-latitude, where rapid permafrost thaw drove declines. Annual growth was lowest at the tree limit site and highest at the treeline. Climatic drivers of these growth patterns varied; positive growth responses at the northerly sites were associated with warmer winters, whereas d13C trends and climate-growth responses at mid-latitude sites indicated growth was limited by moisture availability. d13C signatures indicated increased WUE at the southernmost site, with no significant trends at northern sites. These results suggest that warming will increase growth of trees at black spruce's northern extent, but southerly areas may face drought stress if precipitation does not balance evapotranspiration.