Frost depth measurement were made several times per winter at ten
sites in Junction Fen (an open peatland), and ten sites in S2 Bog (a
forested bog) since 1985. In these ecosystems, frost measurements are
a direct measure of frozen, saturated soil.
Junction fen is a poor fen with no trees except along the margins. The
surface cover is dominated by ericaceous shrubs (Chamaedaphne
calyculata), Sphagnum mosses, and Polytrichum mosses. The S2 bog is an
ombrotrophic peatland with mature black spruce (Picea mariana),
ericaceous shrubs (Rhododendron groenlandicum, Chamaedaphne
calyculata, Vaccinium angustifolium), and Sphagnum mosses. Both
peatlands have hummock and hollow microtopography.
An initial measurement was made in February, the approximate time of
maximum snow accumulation, until the frost was no longer detected,
usually in late winter or early spring. The depth to frost is also
measured because frost may occur below the surface, especially beneath
hummocks in the hummock/hollow microtopography of the peatlands. For
example, the loose structure of the moss is usually unsaturated and
will not form solid ice, as it does in hollows. Additionally, during
melt, frost melts from the top down and the bottom up. That pattern of
melt creates an increasing depth to frost and a thinning layer of
frost in late winter or early spring.
The measurements are taken in locations where the snow was
undisturbed. Snow insulates and snow compaction and removal causes
deeper frost. Depths were measured in inches and later converted from
inches to centimeters. Depths were rounded to the nearest centimeter.
Depth to frost and thickness of the frost layer are presented as the
average of the 10 points along each transect. The percent of area with
frost is calculated as the percent of points with frozen ground.
From 1985 to February, 2012, frost measurements were made with a Lake
States frost penetrometer (Stoeckeler and Thames 1958), which is a 1.3
cm stainless steel rod about 1 m long with a sharpened and slightly
flared point. The bottom 45 cm were marked in 2.5 cm intervals. The
attached driver, a 5.5 kg weight, is lifted 45 cm and dropped,
whereupon the weight strikes a plate that drives the penetrometer into
the soil. Additional manual force was applied as necessary. When the
flared point broke through the frost movement was perceptibly faster
and easier than in the frost layer. The frost depth was estimated to
the nearest 2.5 cm by subtracting the number of increments remaining
aboveground from the total number of increments). On the rare occasion
when frost was deeper than 45 cm, it was recorded as 45 cm. If the
frost penetrometer hit a buried object in the peat, like a log, it was
pulled up and moved over to another location. A bouncing sensation
with downward progress of the probe was characteristic of impacting a
buried log in peat.
After Feb 16, 2012, depth to frost and frost depth were measured using
a 1.0 cm (3/8 inch) or 1.3 cm (0.5 inch) masonry or spade bit with a
91.4 cm (36 inch) shank driven by a portable drill. Depth to frost was
measured as the distance until resistance to drilling was perceived at
the interface of unfrozen and underlying frozen peat. The depth was
marked along the drill bit, the drill was retracted from the hole, and
the depth was measured with a tape measure to the nearest 2.5 cm (1
inch. Once depth to frost was measured and recorded, drilling downward
continued. The bottom of the frozen layer was determined as the point
at which resistance to drilling eased and downward progress was
unimpeded. The depth was marked along the drill bit, the drill was
retracted from the hole, and the depth was measured with a tape
measure to the nearest 2.5 cm (1 inch).
More information on study sites and methods can be found in:
Sebestyen, S.D., C. Dorrance, D.M. Olson, E.S. Verry, R.K. Kolka, A.E.
Elling, and R. Kyllander (2011). Chapter 2: Long-Term Monitoring Sites
and Trends at the Marcell Experimental Forest. In Randall K. Kolka,
Stephen D. Sebestyen, Elon S. Verry, and Kenneth N. Brooks (Ed.).
Peatland Biogeochemistry and Watershed Hydrology at the Marcell
Experimental Forest (pp 15-71). CRC Press, Boca Raton, FL.
https://www.fs.usda.gov/treesearch/pubs/37979.
Citations:
Stoeckeler, J. H., & Thames, J. L. (1958), The lake states
penetrometer for measuring depth of soil freezing, Soil Science,
85(1), 47-50.
