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