fecal pellets collected in August, meadow soils that remained
uninoculated to serve as a control, and the forest soils. He then
grew the spruce seedlings in small pots in soils from each of the
four groups in a greenhouse.
John harvested some of the spruce seedlings after 12 weeks and
the rest after 48 weeks, weighed and dissected them into roots and
shoots, and examined their roots for mycorrhizal fungi hyphae,
which are easily seen under a dissecting microscope. The results
of this experiment were the sort of conclusive results we all hope
for but rarely get. None of the seedlings in beaver meadow soil
without vole fecal pellets had any mycorrhizae. These promptly
died. In contrast, 93 percent of the seedlings grown in forest soil
collected in May and 100 percent of the seedlings grown in forest
soil collected in August had formed symbioses with mycorrhizae.
Almost all of them survived. What’s more, about 30 percent of the
seedlings grown in meadow soils inoculated with vole fecal pellets
formed symbioses with mycorrhizae and survived. Clearly, forest
soils contain sufficient mycorrhizal spores to inoculate spruce
seedlings, and meadow soils contain none, but adding vole fecal
pellets to meadow soils provided a source of mycorrhizae to the
seedlings. In addition, seedlings grown in forest soils weighed
twice as much as those grown in meadow soils without fecal
pellets, and the seedlings grown in meadow soils with fecal pellets weighed somewhere in between. The soils inoculated with
August pellets produced larger seedlings than those inoculated
with May pellets, possibly because the fungi had an entire growing season to produce fruiting bodies.
The fecal pellets could also add nutrients to the soil, and we
had previously found that pellets rapidly release their nitrogen
and phosphorus upon decay. Thus, fecal pellets could also benefit the seedlings by acting as a fertilizer. But the amount of nitrogen and phosphorus in the pellets was much, much less than
that needed to support the growth of seedlings. So the evidence
is in favor of pellets aiding seedling growth mainly by supplying
mycorrhizal fungi rather than by supplying nutrients.
Besides the differences in seedling growth between the
groups, there were also interesting trends in the allocation of
seedling growth to roots (to obtain water and nutrients from
the soil) and shoots (to capture light to drive photosynthesis).
Bereft of any mycorrhizal fungi to extend their root system with
hyphae, the spruce seedlings grown in the uninoculated meadow soils had to produce more of their own roots compared with
seedlings grown in the forest soil or the inoculated meadow
soil. But the seedlings in uninoculated meadow soil were also
smaller than those that formed associations with mycorrhizae
in the forest and inoculated meadow soils. These differences in
seedling growth indicate that it is more effective for a seedling
to supply carbohydrates to mycorrhizal fungi to produce hyphae
than to produce its own roots to capture nutrients.
But don’t the red-backed voles travel from the forest into the
meadow? To answer this question, John placed live traps in the
beaver meadows adjacent to the forests where he captured the
red-backed voles for his supply of fecal pellets. He caught only
one, and it was on a small island in the middle of the meadow
on which eight black spruce trees grew. Clearly, red-backed
voles stay out of beaver meadows, and so the mycorrhizal fungi
population that was killed when beavers flooded the pond could
not be reestablished through dispersal by the red-backed voles.
So why don’t red-backed voles go out into beaver meadows?
Perhaps the dense grasses are not to their liking. Red-backed
voles seem to prefer forests with an abundance of down logs,
whose lower sides provide cover from owls and whose tops
serve as runways, instead of grassy meadows without logs. But
there may be a more interesting reason. The rodent John caught
most often in the beaver meadows was, appropriately enough,
the meadow vole, which he never caught in the forest. Meadow
voles and red-backed voles are aggressive against each other and
each can exclude the other from its respective preferred habitats.
Wars fought between these two species of vole at the boundary
between forest and meadow might prevent red-backed voles
from entering the meadows even if they were inclined to do so.
The obvious next experiment would be to fence and enclose
a large area of meadow extending partly into the adjacent forest
and trap and remove all meadow voles inside. Then, the end in
the forest would be opened so that red-backed voles could enter
it. If the aggression by the meadow voles is the factor keeping
red-backed voles out of the meadow, then eventually we should
begin to trap red-backed voles inside the enclosure once mead-
ow voles are removed. Given enough time for the red-backed
voles to inoculate the meadow soil with fecal pellets, spruce or
fir seedlings should become established in the enclosure. We
have not done this experiment, but here it is, an open invita-
tion for an enterprising graduate student with fence-building
skills. Science is not always done by expensive and complicated
technology such as DNA sequencers and
particle accelerators. Sometimes a simple
fence and a few live traps are enough.
The story was adapted from What Should a Clever Moose
Eat? by John Pastor. Copyright © 2016 John Pastor.
Reproduced by permission of Island Press, Washington,
Ectomycorrhizal mycelium (white) on white spruce roots (brown).