Effective moisture (precipitation minus
evaporation) is extremely important in
subarctic ecosystems. Interior Alaska has a
relatively dry climate with annual precipitation
ranging from 25-45 cm. Records from interior
Alaska lake sediment cores show low lake
levels following the Last Glacial Maximum,
with significant increases at 12,000 and 9,000
carbon-14 years B.P. Using lake-level
reconstructions and models based on
modern hydrologic and meteorologic data, we
infer precipitation of 35-75 per cent less than
modern at 12,000 yr. BP, 25-45 per cent less
than modern at 9,000 yr. BP, and 10-20 per
cent less than modern at 6,000 yr. BP. Trees
were scarce on the interior Alaskan landscape
during the late Pleistocene with birch species
appearing about 12,000 BP and spruce
species approximately 3500 years later. The
correspondence between lake-level and
vegetation changes suggests that moisture
may have been one of the limiting factors in
the establishment of these tree species.

Alaska climate records show a climatic
regime shift in the mid-1970s. Less effective
moisture is available over the past 30 years
because summer temperatures in interior
Alaska have been increasing without a
concurrent increase in precipitation. Radial
growth of white spruce at 20 low elevation
stands in interior Alaska declined
corresponding with this climatic change. The
observation that moisture limits spruce growth
in Alaska today is consistent with our
inference of moisture limitation in the early
Holocene. A 200-year reconstruction was
developed based on two tree ring proxies,
carbon-13 discrimination and maximum
latewood density, which together show
excellent agreement with the recorded
Fairbanks average May through August
temperatures. The first half of the 20th century
is characterized by the coolest summers of
the 200 year period of reconstruction, while
the latter part of the 20th century, particularly
from 1974 onward, is characterized by some
of the warmest summers of the 200 year
period. Mid-19th summer temperatures
reconstruct to be as warm as the latter part of
the 20th century, which is inconsistent with
reconstructions of other regions. It seems
likely, based on current information, that these
inconsistencies may be real and may reflect
regional synoptic conditions unique to interior
Alaska. Distinctive decadal scale regimes
were identified throughout the record.
barber@ims.uaf.edu