Lee and Bob,
Lee you spoke of the Kandiyohi Elm Forest:
Lee Frelich, Nov 22, 2009
At the Kandiyohi elm forest, for example, last September I found
what I call the peach-leaf willow super site. Its a stand of peach-leaf
willows 80-100 feet tall with some single trunks up to 40 inches dbh.
Published descriptions of the species say it reaches a maximum height of
40 feet.
Lee Frelich, Nov 22, 2009,
Its easy for that site{Kandiyohi elm forest]: the soils are nutrient-rich
black silt
at least 10 feet deep. The soil has a huge capacity to hold water through
droughts that are common at the prairie-forest border. Super sites are
always about how evenly water and nutrients are supplied to the tree.
The more even an adequate supply (and the fewer episodes of flooding or
drought), the better the site. That's why cobblestone sites at the base
of slopes with trickling water and types of bedrock that weather into
particles with with high nutrient abundance are also super sites.
In this context a Super Site is one for which a particular species grows
exceptionally large - peach leaf willow. That is a reasonable idea and I
can't argue with the logic, but maybe I can suggest a variation of the concept
of a Super Site. When I think of a Super Site I think of one in which the tree
assemblage is pretty much the same as that of the surrounding forest, only the
trees at that site are bigger for trees of the same age range. (generally
old). The question is whether a Super Site as a concept should refer to site
with respect to an individual species or should it reflect the relative size of
general group of species found there in comparison to those in the surrounding
forests? Maybe either one is appropriate.
I am sure that a thick rich soil will tend to promote growth in most settings,
with those areas with better soils tending to grow bigger trees than areas with
poorer soils. I like the idea you seem to be suggesting in that these sites
have a characteristics which helps mitigate some environmental factor that can
limit growth on other sites. In this case the thick soil holds water that
mitigates the drought limitations at the forest-prairie interface. Perhaps a
slightly higher spot in wet areas, say flood plains or hummocks in swamps, may
promote growth for species that are less water tolerant and yield taller trees
relative to in the surrounding wetter areas.
There are examples of disjunct populations of less heat tolerant species
surviving at higher elevations in areas farther south than the main body of the
population. These are certainly taller than the same species in surrounding
areas - because there aren't any. The Kandiyohi forest represents an unusual
assemblage of the dominant tree species because of its soil characteristics.
It isn't really representative of the general forest types found in the region,
so it doesn't have exceptionally large examples of trees of the species found
farther away from the prairie boundary, but is a Super Site for the many of the
individual species that are found there.
What are some examples of Super Sites in which there is basically the same
species assemblage, only the trees are bigger? And why are they bigger? You
gave some examples: 1) rich, thick soils, 2) right balance of water neither
too much nor too little, and 3) rocks that provide good mineral nutrients to
the soil. All of these sound good. Perhaps we should look at other
environmental conditions that are somehow limiting growth in a region and see
if at sites with taller trees, these effects are in some way being mitigated.
If calcium is a limiting factor in the soil, then trees growing in an area with
limestone bedrock should grow better there than elsewhere in the region. If
wind shear is limiting tree heights, then areas such as coves and smaller
valleys may have higher trees because the topography provides some protection
from the wind to these trees. Certain species combinations in certain
proportions might yield taller forests than other combinations - and we could
ask why is this combination growing in this area and not in others.
The small point I am trying to make is that a Super Site for a species or a
forest as a whole is not just a matter of what is great about the site, but
also what is not wrong with it. What limitations are being mitigated by the
specific site conditions? This is sort of a half full or half empty argument,
but it would allow some one to look at the question from two different
approaches and perspectives.- what is great about the site, and what problem is
absent from the site?
Ed
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