Without the beavers there to repair it, the dam soon fails,
and the pond begins to drain. Just as the aspens in the upland
are replaced by conifers, the flooded pond is replaced by a
wet meadow. As the dam deteriorates during the next several
decades, the water table in the meadow gradually descends.
Sedges and irises and then various grasses invade the meadow,
forming a thick prairie-like cover.
But the conifers remain firmly in place on the edge of the meadow. Few, if any, conifer seedlings can be found in the meadow, even
a few dozen feet from the growing dark wall of spruce and fir. The
seedlings that do establish themselves last but a year or two and
then expire, while inside the forest, conifer seedlings sprout and
grow into saplings and eventually mature, cone-bearing trees.
Why do all conifer seedlings die within a year or two in the
meadow but survive and grow into saplings in the adjacent forest?
It cannot be competition between the conifer seedlings and
the dense grasses, because balsam fir and spruce readily invade
powerline right-of-ways and roadsides, which also usually have
a dense grass cover. It cannot be that the soil in the meadow is
too wet for balsam fir and spruce, as they can grow in peatlands
that are often wetter than the drained meadows.
More than half a century ago, Sergei Wilde, a soil researcher
at the University of Wisconsin, suggested that conifers could not
invade beaver meadows because the meadow soils lack a suite of
fungi known as mycorrhizae. In other settings, these fungi, whose
species number in the thousands, form a tight symbiosis with
plant roots, interpenetrating the root tissues. The fungal myce-
lium extends the root network farther into the soil. The fungi
receive carbohydrates from the tree, and in turn supply the tree
with additional nutrients such as nitrogen and phosphorus, which
the very fine and rootlike fungal hyphae take up from the soil.
This carbohydrate-for-nutrient swap between the mycorrhizal
fungus and the tree sustains both the host tree and the fungus
for the lifetime of the tree. Some fungal species form associations
only with specific tree species, whereas others associate with a
broad range of host tree species. Most trees have associations
with many different fungal species. Without this symbiosis with
mycorrhizal fungi, most seedlings in beaver meadows die within
a year or two.
Wilde found that the mycorrhizal fungi in the meadow are
killed during the long period of time that the beavers actively
maintain the dam and flood the soil behind it. He proposed
that when the pond is abandoned and then drains, spruce and
fir seedlings cannot immediately grow in the soil in the meadow
because the soil is bereft of mycorrhizal fungi.
But as with all good problems in natural history, answering
one question only raises more. Obviously, there must be mycor-
rhizal fungi in the forest immediately adjacent to the meadow
because spruce and fir seedlings grow quite well there. If this is
the case, why don’t these fungi reinvade the meadow even after a
few decades of the meadow being drained? Shouldn’t the spores
produced by their fruiting bodies have dispersed?
The answer to this question involves another symbiotic
relationship – this one between fungi and the red-backed vole.
Many mycorrhizal fungal species have their fruiting bodies
below ground, and the voles burrow in the soil, find the fruiting
bodies, consume them, and void the spores in their fecal pellets.
In a study of the diversity of mycorrhizal spores in red-backed
vole fecal pellets in Minnesota’s North Woods, we identified 15
different species of fungi.
Documenting the broad diversity of mycorrhizal spores in
vole fecal pellets answers one question but prompts several oth-
ers: Are these spores capable of forming symbioses with spruce
or fir seedlings after passage through the vole’s digestive system?
If we inoculate beaver meadow soil with vole fecal pellets con-
taining these spores, will spruce seedlings then be able to grow?
Answering these questions was the objective of a thesis of John
Terwilliger, one of my graduate students.
To do his experiments, John needed a source of both spruce
seedlings and red-backed vole fecal pellets. The first was easy:
He obtained seeds from the Minnesota Department of Natural
Resources and germinated them in sterilized potting soil (the soil
had to be sterilized to ensure that he began with seedlings without
any mycorrhizae). Red-backed vole fecal pellets are not available
from any scientific supply company, so he went into the woods
and caught the voles nightly with live traps. Voles have the conve-
nient habit of voiding when they are first trapped, so every morn-
ing John collected the fecal pellets and, after marking the vole to
determine population density, released it. The vole scurried away,
probably a bit perplexed but unharmed. John then put out a clean
trap for the next night until he had an adequate supply of vole
fecal pellets, perhaps the largest supply of vole feces anyone has
ever collected then or since. He trapped voles in both May (early
spring here in northern Minnesota) and August (the end of sum-
mer) to see whether there was any difference in the amount and
types of fungi in the voles’ diet during the growing season.
Next, he examined the fecal pellets for spores of mycorrhizal
fungi and found many of the same spores that we had found
previously. He then collected soils from three beaver meadows
and three adjacent forests and assigned subsamples from each
of them into one of four groups: meadow soils inoculated with
fecal pellets collected in May, meadow soils inoculated with