Decoupling of temperature and growth of white spruce at western treeline, Alaska, is unrelated to intrinsic water stress.

Late 20th century warming has weakened the relationship between temperature and growth in northern forests. This loss of sensitivity to temperature, often attributed to drought stress in warmer regions of the boreal forest, has also been observed in cool, mesic environments, where warming-related growth declines are poorly understood. We used annually resolved ring-width and carbon stable isotope (δC) chronologies from white spruce forest and woodland stands in southwest Alaska to examine tree responses to climate during a recent warming period (1970-2011). Trends in ring-width index (RWI) varied by stand type: RWI increased in forests and woodlands in the first half of the record (1970-1990), but only woodlands continued that trend in the latter half (1991-2011). Warmer spring temperatures predicted increased RWI in both stand types, and warmer summer temperatures predicted decreased RWI in forests, but these relationships weakened through time. Carbon isotope discrimination (ΔC), a frequent indicator of stomatal regulation of water loss, showed no coherent trend in either stand type but was positively associated with summer precipitation in forests, suggesting the potential for intermittent water limitation. Our results indicate that RWI and ΔC have responded differently to climate through time and that these responses have been mediated by stand type.