Voles and Moose,
Fungi and Spruce
By John Pastor
Have you ever wondered why patch cuts in the North Woods become carpeted with
spruce and fir seedlings within a few years of being cut, while beaver meadows
in the same area stay grassy for decades, or even centuries? So have we, and so
we were thrilled to come across John Pastor’s new book: What Should a Clever
Moose Eat. In this passage Pastor, an ecologist and professor of biology at the
University of Minnesota, sheds some light on the phenomenon.
he upland forests around beaver ponds are often composed
of overstory aspen and understory spruce and balsam fir.
These are quintessential North Woods tree species. The
vertical structure of these forests is readily evident to anyone
sitting in a canoe in a pond, especially this time of year, when
the golden aspen crowns lie between the dark green of the
conifers and the cerulean blue of the sky.
In the fall, just after the blue-winged teal begin to migrate, beavers cut the
aspens and drag the small branches and twigs to their ponds for their winter
food cache. The cut aspen trees sprout again from their roots, but moose
sometimes browse them heavily for their high nutrient content, avoiding the
spruce and fir because their needles are difficult to digest. Freed of competition
from the aspens, the understory spruce and fir grow taller and become the
overstory, all the while casting a dense shade and inhibiting aspen seedlings and
root suckers from growing. Eventually, a dark wall of spruce and fir erects itself
around the pond margin, especially around smaller ponds where beaver can
cut the entire population of aspen. Bereft of their preferred food, the beavers
seek greener aspen pastures elsewhere and abandon the pond.