Voles, Fungi, Spruce, and Abandoned Beaver Meadows

Abstract

The upland forests around beaver ponds are often composed of over- story 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 the pond, especially in the fall, when the golden aspen crowns lie between the dark green of the conifers below and the cerulean blue of the sky overhead. In the fall, just after the blue-winged teal begin to migrate and while the geese are flying, beavers cut the overstory aspens and drag the small branches and twigs to their ponds for their winter food cache next to the lodge. 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 the beaver can cut the entire population of aspen. Bereft of their preferred food, the beavers seek greener aspen pastures elsewhere and abandon the pond.

Voles control whether conifers invade abandoned beaver meadows by dispersing the spores of mycorrhizal fungi that help the spruce seedlings take up nutrients from the soil.