Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress

The extension of growing season at high northern latitudes seems increasing ly clear from satellite observations of vegetation extent and duration(1,2) . This extension is also thought to explain the observed increase in amplit ude of seasonal variations in atmospheric CO2 concentration. Increased plan t respiration and photosynthesis both correlate well with increases in temp erature this century and are therefore the most probable link between the v egetation and CO2 observations(3). From these observations(1,2), it has bee n suggested that increases in temperature have stimulated carbon uptake in high latitudes(1,2) and for the boreal forest system as a whole(4). Here we present multi-proxy tree-ring data (ring width, maximum late-wood density and carbon-isotope composition) from 20 productive stands of white spruce i n the interior of Alaska. The tree-ring records show a strong and consisten t relationship over the past 90 years and indicate that, in contrast with e arlier predictions, radial growth has decreased with increasing temperature . Our data show that temperature-induced drought stress has disproportionat ely affected the most rapidly growing white spruce, suggesting that, under recent climate warming, drought may have been an important factor limiting carbon uptake in a large portion of the North American boreal forest. If th is limitation in growth due to drought stress is sustained, the future capa city of northern latitudes to sequester carbon may be less than currently e xpected.