Forest decline, Earthworms, and seedling growth   Lee Frelich
  Sep 11, 2005 12:32 PDT 

Wood-Rill Natural Area is a remnant of old-growth sugar maple, basswood,
red and white oak forest near Minneapolis that has been hit particularly
hard by the forest decline syndrome caused by European earthworms, European
slugs, and overabundance of deer. I did some hiking there this weekend and
found that most of the forest floor is bare soil with no plants and no tree
regeneration, but there are a few spots up to 0.1 acres in size with a
dense carpet of sugar maple regeneration, with very sharp edges, almost as
if the edges were maintained by weeding. These areas that are exceptions
are puzzling, but I was able to come up with a hypothesis that might
explain why most of the forest floor has been affected by the decline, and
why there are places not impacted.

This has been a very hot weekend for this time of year (I predict and hope
it will be the last 90+ degree weekend of the year), and it was obvious
that none of the seedlings were wilted in areas with a dense carpet of
seedlings, whereas the isolated seedlings scattered elsewhere looked pretty
nutrient starved and drought stressed. So, I hypothesize that prior to
removal of the forest floor, the duff held moisture, and the lower bulk
density of the soil in the absence of worms also allowed more water to soak
in, which led to more activity by soil microbes, turning over nutrients
faster, so that the whole forest was relatively moist and rich in
nutrients--moist and rich enough to allow sugar maple seedlings to grow
anywhere. Now, with the earthworms present and the duff gone, the mean
level of moisture and nutrients has declined to the point where it will no
longer support sugar maple seedlings, but because there is variability in
soil conditions, a few of the more moist and nutrient rich spots can still
grow seedlings. Before the earthworms came, the entire distribution and
range of variability in nutrients and water fund throughout the forest
could support seedlings, whereas now my guess if that the top 5% of the
area can support them.

Considering the slugs and deer makes the hypothesis more complex and
interesting. Both eat sugar maple seedlings, and the seedlings must outgrow
them to succeed. This further restricts the area the seedlings can grow on
to even richer spots on the forest floor, that will not only support
growth, but fast growth, perhaps limiting the seedlings to about 1% of the
total area of the forest. This would also explain why the patches of
seedlings have sharp edges, even though the soil changes gradually. At some
point along a soil richness gradient, seedling growth would be just a
little faster than herbivory, allowing success, whereas a slightly slower
growing seedling would die, so that herbivory sharpens the boundary of the

A number of experiments could be done to test this complex hypothesis. This
is how we find out how forests really work. Sounds like a good project for
the next new graduate student.