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ABERT'S Squirrel AND THE PONDEROSA PINE
By Peter D. Goldfinch
Idly gazing from the deck
at the plentiful but precious Ponderosas on our property, it suddenly
crashes into one's awareness that something has nibbled off patches of bark
from several of the trees. Yes, this is the work of tree squirrels who eat
the phloem, or inner layer of bark on upper branches. Indeed! More
specifically, we've become aware of the Abert's, or Tassel-eared squirrel,
whose blackish fur, beady black eyes, and tufted ear tips give it a rather
devilish, perpetrating appearance. Unlike other tree squirrels, Abert's does
not substantially store food for the winter season. Rather, it subsists
through the winter upon phloem from the terminal twigs on the upper crown of
Ponderosas. Of the various species feeding on the Ponderosa, Abert's
squirrel has been considered the most injurious. But, for reasons to be
disclosed shortly, only about 10% of the pines in this population are
targeted by the squirrel for its winter diet.
Our attention was recently
drawn to a 1990 PhD thesis at CU by Marc Alan Snyder, who studied Ponderosas
at three front range sites and identified several factors influencing the
Alert squirrel's dietary choice of certain target trees. He also measured
various impacts of Abert's feeding behavior on the trees' fitness for
survival and reproduction.
Snyder hypothesized that
Abert's squirrels select target trees on the basis of several interactive
chemical factors, some dietarily attractive, some repellent. Over a three
year period he did chemical analyses of the inner bark and of the oleoresin
(pitch) flowing from the twig tips of both target and unaffected trees. It
was easy to identify the target trees, because prior to eating the inner
bark from a twig, the squirrel clips off the terminal needle cluster and
lets it fall to the ground. I've seen many of those needle clusters on the
snow beneath affected trees in past winters.
And what were the findings?
It turned out that the targeted trees had significantly higher levels of
sodium and carbohydrate in the inner bark, both nutritionally desirable to
the squirrels. Trees spared by the squirrels, on the other hand, had
significantly higher levels of substances undesirable to the squirrels, such
as iron and mercury, which can be toxic. Also, the flow rates of oleoresin
and two particular monoterpenes, β-pinene and β-phellandrene, repellent to
squirrels, were higher at statistically significant levels.
If these findings help
explain how Abert's squirrel decides which Ponderosas to have for dinner, we
might next ask what impact Snyder found them to have on the trees they
nibbled. Annual incremental growth, in terms of tree ring width, was
significantly reduced in trees utilized as sources of phloem. Cone
production was reduced in terms of cone weight as well as number and weight
of seeds. And, as if to add insult to injury, the squirrels also harvested
and ate the seeds of more cones from the target trees. Of 180 trees studied,
one died due to the extensive defoliation.
One might wonder what would
happen to a forest if the squirrel population were stable and all the cones,
or in the case of deciduous trees, nuts, were eaten by squirrels each year.
Reproduction would cease. To this threat, trees have apparently evolved a
response, known as masting, wherein every few years a massive crop of cones
or nuts is produced, more than the squirrels could ever eat, thus assuring
the trees' reproduction.
In some years, in the
deciduous forests in the northeast following a mast year, major
overpopulations of gray and fox squirrels have developed, resulting in mass,
lemming-like migrations. The most recent occurred in September of 1968 when
about 20,000,000 gray squirrels migrated in an area extending from Vermont
to Georgia. Fifty- five tons of dead squirrels were removed from one
New York
reservoir which they'd tried to swim across.
But am I digressing? Back to
Abert's. We could hypothesize that these squirrels are agents of Satan, sent
to harass the pious and prosperous people of Pine Brook, by eating their
trees. It's a subject for another PhD thesis, and a very special PhD
candidate.
from The Pine Brook Press |
