Tree-ring width and stable isotope analyses of Picea sitchensis from Iceland reveal growth potential under predicted climate change

Sitka spruce has been one of the most planted tree species in Iceland since the mid-twentieth century. Here, we use different dendrochronological methods to identify the controlling climatic factors of its growth and possible changes in their influence over time. We develop annually resolved and absolutely dated measurements of tree-ring width (TRW) from 21 trees and oxygen (delta 18O) and carbon (delta 13C) stable isotopes from six trees to evaluate growth trends and climate sensitivity in Iceland's Hallormsstaour National Forest over the past 54 years (1965-2018). Warmer and wetter summers in the last few decades have resulted in significantly increasing radial growth. While TRW and delta 13C reflect strong July-August temperature signals, with the highest correlation for July, delta 18O is mainly controlled by the temperature in March. The occurrence of negative pointer years in TRW decreases with increasing temperature in July but increases with excessive precipitation in August. Our study shows great continued potential for Sitka spruce cultivation in Iceland.