A Northwoods Almanac for February 21 – March 6, 2014
Weight of Snow
On Sunday, Mary and I
shoveled off our roof in anticipation of the warm days forecast for this week,
and the depth and weight of the snow absolutely exhausted us. We had led a
snowshoe trip the previous morning at Minocqua’s Raven Trail, and we’ve been
out nearly every day skiing or snowshoeing, so we were well aware of how deep
the snow has been this winter. But moving it somewhere else was another story,
one which made me wonder about the weight of snow and how trees are adapted to
withstand the mass. As you might recall, we had a major snowstorm around
Christmas that coated trees with heavy, wet snow, and many trees in our area
still are carrying the burdensome legacy of that storm.
Spruce and balsam trees
evolved the first line of defense against heavy snow and ice storms by
selecting a spire-shaped growth form that acts like a steep, pitched roof,
allowing the snow to slide off. Plus, their pliant softwood allows the branches
to bend with the weight, slough off the snow, and then spring back. Still, snow
does get caught in the foliage. In his book Life
in the Cold, Peter Marchand notes that in a coastal region where snows are
full of moisture, the snow retained in a 40-foot-tall balsam can reach an astounding
weight of 6,600 pounds, but the trees still survive.
As a rule of thumb, saturated snow weighs approximately 20
pounds per cubic foot. However, the moisture content of snow can range from
approximately 1% to 33%, so snow may potentially weigh from 1 pound per cubic
foot to over 21 pounds per cubic foot. That’s a lot of weight on a person’s
shovel, and on a tree’s branches.
Ice, too, can affect tree
growth and survival. Along the surface of the snow, damage from blowing ice
particles can pit or wear away the bark on the windward side of a tree. The ice
particles can also strip needle foliage, increasing water loss and drying. Or
ice storms can increase the weight of branches so dramatically that winds can
break off branches altogether – branches that would have supported new growth
in the spring. The resulting trees are then often lopsided, leaning toward the
heavier foliated side, which increases the chances that they will eventually tip
over due to wind or snow.
Winter Severity Index
The
winter severity index (WSI) is a measurement to help gauge the effects of winter
weather on deer survival. One point is added for each day the temperature falls
below zero and another point for each day the snow depth is greater than 18
inches. Wisconsin’s WSI measurements are recorded annually from Dec. 1 through
April 30 at 34 northern Wisconsin stations (local stations include Eagle River,
Escanaba Lake, Gile, Mercer, Park Falls, Rhinelander, Saxon, Tomahawk, Trout
Lake, and Woodruff). Winter conditions are considered mild if the station
accumulates fewer than 50 points, moderate if between 51 and 80 points, severe
if between 81 and 100, and very severe if over 100. In
February, the WSI only gives a good indication of the potential for a severe
winter. It’s really March when the index numbers tell us how well white-tailed
deer are faring. In very severe winters, up to 30% of the deer herd may be
lost. Our current numbers are above average for northern Wisconsin but not as
high as the severe winter of 1995-96 when Iron County set the state record with
189 points and an estimated 150,000 deer died throughout the northern counties.
The
WSI doesn’t tell the whole story by any means. Last May, we were still waiting
for spring to arrive, and its very late arrival had a significant negative
impact on the deer population. So, timing matters, too, as does an array of
other factors.
Predator Study on Deer
An
article in the Milwaukee Journal recently summarized some preliminary figures from
a four-year research study to determine the causes of mortality in white-tailed
deer in Wisconsin. Deer were radio-collared at two sites: the town of Winter in
the northern forest region of Wisconsin, and Shiocton in the farmland region of
southeastern Wisconsin.
The
rates and causes of mortality in the northern forest from three years of data
for yearling and adult deer of both sexes were:
Human
hunting – 43%
Starvation
– 9%
Coyote
– 7%
Wolf
– 6%
Roadkill
– 6%
If
you add poaching, which was an additional 8%, the human hunt gains even greater
importance. Thus, human hunting causes nearly twice as much deer mortality as
the other four causes combined.
An
important note: On average, 73% of female deer survived the year, compared to
47% of adult bucks.
Fawns
were also tracked, in this case daily through summer, and the eight-week
survival average was 58% in the northern study area. Predators took 79% of the
fawns that died. Black bears killed the most (9 individuals), with coyotes (6)
and bobcats (6) tied for second. Unknown predators took 11 fawns.
In
the eastern farmland area, the leading cause of fawn mortality was starvation
and other natural causes (50%), followed by predators at 36%. Coyotes killed
the most fawns (8 individuals), while black bear, bobcat, and a domestic dog
killed one each. Unknown predators took 4 fawns.
The
article notes that while wolves receive extensive blame for lowered populations
of deer, the study refutes that. Coyotes are far more responsible for taking
more adult deer and fawns in both northern and southern regions.
Final
numbers won’t be available until late 2015 when the study on adult and yearling
deer concludes.
Additional
studies are in the pipeline, which is good news for the continued direction of
science-based deer management, as well as the science-based management of
various predator species.
My take: Predators need prey, and prey
need predators. Each balances the populations of the other. Thus, it’s in
everyone’s best interest to understand what those balances actually are and how
our actions impact them.
Barred
Owls at Feeders
I’ve received several reports, and some
wonderful photos (from Cherie Smith in Lake Tomahawk), of barred owls recently
appearing at people’s bird feeders. While this makes for great photo ops, the
hard truth is that these owls are likely starving. Barred owls are residents of
deep forests, not backyards, and are considered a seminocturnal to nocturnal
hunter, although daytime hunting has been observed on occasion. Barred owls rely on secondary tree cavities for
nests, so they’re most often associated with large trees in old forests. In
fact, barred owls are often used as an indicator species in the management of
old forests.
Since barred owls are essentially
nonmigratory, and usually stay within their home range, they’re not often seen
outside of their forested territories. However, the literature says that during
winters of low prey availability, barred owls may become nomadic in search of
prey. They are a true
generalist predator, consuming a variety of birds up to the size of grouse;
small mammals up to the size of rabbits; and in warm weather amphibians,
reptiles, and invertebrates.
I’d bet that our deep snow has made for
superb protection of small mammals huddling within the snow, so the owls are
now forced out of their home range to be looking for birds. And where are the
best places to find birds? At backyard feeders!
As a side-note, a barred owl’s home-range
is quite large. Radio-tracking studies have found that in Minnesota, 13 birds
had an average home range of 674 acres, while 7 birds in Michigan had an
average home range of 696 acres.
Barred
owls are thought to prefer old forests due to the greater availability
of potential cavity and nest sites, more open understories which facilitate
easier hunting, and closed overhead canopy which provides greater
thermoregulation and protection from mobbing.
Sightings
- Pine Grosbeak and Shrike
Jo Knapp on Rosalind Lake in Presque Isle had a
single male pine grosbeak visit her feeders last week. It then disappeared for
a few days and then reappeared again last Sunday. Jo may be seeing the only
pine grosbeak in this entire area – I know of no one else who has seen one this
winter.
Jim
Mogg had a northern shrike visit his feeders south of Rhinelander, resulting in
the complete disappearance of all of his songbirds until the shrike left.
Celestial
Events
Look
after sunset in the south for Sirius, the first star of the evening, and the
brightest star by far in the night sky. Sirius is “just” 8 ½ light years away,
so is the closest of all stars outside our solar system to the Earth. As the
evening progress, a “winter diamond” forms, with Sirius at the bottom, exceptionally
bright Jupiter at the top, and two bright stars on either side – Procyon to the
left, and Betelgeuse (in Orion) to the right.
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