Pearl Creek Site (U19)
In 1968 three frost tubes (Rickard and Brown, 1972, Viereck and Lev 1983) were
installed in a Mixed Spruce/Labrador Tea /Feathermoss (Picea glauca-P.mariana/Ledum
groenlandicum/Hylocomium splendens) stand (Viereck et al 1993) about 4 km north of
the University of Alaska Fairbanks (64 o 54’N: 147 o 49’W).. This site is in a flat area at
the bottom of a west-facing slope in the Pearl Creek valley at an elevation of 213 m. The
site was burned in approximately 1914 and developed as a mixed stand of spruce. At the
time of establishment of frost tubes at the site the trees were from 50 to 60 years in age. A
more detailed description of the vegetation can be found in Viereck and Lev 1983.
The soil at the site is a Minto silt loam (Reiger et al 1963). A typical Minto silt loam has
approximately 10% sand, 80% silt and 10% clay. At the Pearl Creek site the overlying
organic layer varies from 24 to 40 cm in thickness and is comprised of an 01(F) layer, 1520 cm in thickness over a 02 (H) humus layer about 5 cm thick. The living moss layer
averages about 5 cm thick, ranging from 2 cm under the spruce trees to 10 cm in the
densest hummocks.
Methods.
The 3 frost tubes and 3 snow stakes were installed in 1968 in 2 parallel lines at 2 meter
spacing. To minimize disturbance at the site, for the first 15 years the depth of thaw was
recorded at irregular intervals, but weekly or bi-weekly during critical periods of thawing
and freezing and time of total freeze. For the maximum active layer thaw the average
maximum depth of thaw of the three tubes was used. In 1981 probing of ten points along
the line of the frost tube using a metal rod was initiated. These points were at one-meter
intervals and included the three sites of the frost tubes. Following this, the average of the
ten probes, done each year at the time of maximum thaw was used to report the annual
maximum active layer thaw.
In 1990 a weather station was established at the site and temperature sensors were
established at 5,10,20,50,100,and 150 cm. A precipitation gage was also installed at this
time. In 1994 an additional sensor was installed at 200 cm. . Since 1990 readings of the
temperatures, precipitation in summer, snowfall in winter, frost tubes and soil
temperatures have been recorded on a weekly basis. Boardwalks were installed to protect
the site from this increased human traffic. In December of 2000 a 7.2 meter deep hole
was bored at the site and thermistors installed. Preliminary results indicate that the
substrate is frozen at least to 7.2 meters with the coldest temperatures being only –0.5 o C
from 4 to 7 m.
Results:
The results are shown in figure XX. Thaw depths at the site have ranged from 51 cm in
1977 to 72 cm in 1995. In the first decade of the study depths of thaw showed no trend,
ranging between 51 and 59 cm. During the period from 1980 until 1995 there was a
regular increase in the thaw with the maximum thaw of 69 cm occurring in 1995. As a
result of a low snowfall through January of 1996 and a moderately cold winter, soil
temperatures were the lowest recorded during the period of study at the site. The
following summer, in spite of an only slightly below normal thawing year (1907 TDD)
the thaw level was only 55 cm, bringing the thaw level back to that of the 1970’s. From
1997 through 2000 the thaw depths have ranged from 55 to 65 cm with no apparent trend.
At this site winter temperatures combined with snow depths seem to have a more
important influence on the depth of thaw that does the summer temperatures: A
comparison between active layer thaw and the square root of the Accumulated Thawing
Degree Days (DDT) shows little to no correlation. As the thaw has increased there may
have been some subsidence as a result of melting of ice in the newly thawed soils.
(Nixon, manuscript) An annual correlation between probing adjacent to the frost tube and
the reading of the frost tube, begun in 1981, has shown an increase from 6 cm in 1981 to
9 cm in 2000. Some of this difference could be due to the accumulation of frost dropping
down the tube and settling in the bottom of the outer tube and thus elevating the inner
tube in relation to its original base.