Varying boreal forest response to Arctic environmental change at the Firth River,Alaska

Abstract

The response of boreal forests to anthropogenic climate change remains uncertain,with potentially significant impacts for the global carbon cycle, albedo, canopyevapotranspiration and feedbacks into further climate change. Here, we focus on tree-ringdata from the Firth River site at treeline in northeastern Alaska, in a tundra–foresttransition region where pronounced warming has already occurred. Both tree-ring width(TRW) and maximum latewood density (MXD) chronologies were developed to identify thenature of tree growth and density responses to climatic and environmental changes in whitespruce (Picea glauca), a dominant Arctic treeline species. Good agreement was foundbetween the interannual fluctuations in the TRW chronology and summer temperaturesfrom 1901 to 1950, whereas no significant relationships were found from 1951 to 2001,supporting evidence of significant divergence between TRW and summer temperature inthe second half of the 20th century. In contrast to this unstable climatic responsein the TRW record, the high frequency July–August temperature signal in theMXD series seems reasonably stable through the 20th century. Wider and denserrings were more frequent during the 20th century, particularly after 1950, thanin previous centuries. Finally, comparison between the tree-ring proxies and asatellite-derived vegetation index suggests that TRW and MXD correlate withvegetation productivity at the landscape level at different times of the growing season.