Topography and Tree Height: Big Creek   Jess Riddle
  May 25, 2005 20:55 PDT 
A single large ridge comprises most of the Great Smoky Mountains chain.
The tallest mountains in the range are either high points on that ridge,
Clingmans Dome (6643') and Mount Guyot (6621'), or located on short spurs
off the main ridge, Mount LeConte (6593'). Moderately inclined spur
ridges off the main crest force most of the major stream basins in the
range to drain perpendicular to the primary ridge. However, near the
eastern end of the range, the ridge forks, and both segments remain high
for several miles. Draining the unusual region produced between the forks
flows Big Creek. The ridge forks near Mount Guyot, the high point in the
watershed, at Tricorner Knob (6180'). Along the southern boundary of the
drainage, 3.7km from Tricorner Knob the ridge reaches 6234' (Luftee Knob),
and 12.6km distant the ridge remains at 5842' elevation (Mount Sterling).
Similarly, along the northern edge of the drainage, 3.9km from Tricorner
Knob the ridge remains at approximately 5980' (Inadu Knob) and 10.7km away
has descended only to 4980' near Mount Cammerer. The elevation on Big
Creek directly between those two eastern high points is 2640', which lies
about 1.5km upstream of the mouth of Mouse Creek.

Soil surveys of the Great Smoky Mountains National Park have shown the
lower Big Creek area to have very nutrient rich soils, but not more so
than other regions of the park. Hence, some other factor or combination
of factors must account for the observed higher growth rates and absolute
heights, and the unusual topographic setting appears a plausible
explanation. Extrapolating, perhaps more than is wise, from some of Lee
Frelich's comments on how water supply and storms affect maximum tree
height, the topography of the Big Creek basin could positively influence
maximum tree height in at least three ways:

1) Direct sheltering from strong winds. Streams with slopes on both
sides in excess of 30 degrees are common throughout the southern
Appalachians. However, most, if not all, of those creeks have far less
relief between the stream and ridge than does Big Creek. Consequently,
the trees on Big Creek have less exposure to moderate and strong winds.

2) High rainfall for the elevation. Mountains force laterally moving air
masses up thereby cooling them and inducing precipitation; consequently,
elevation and precipitation correlate well. The long steep slopes of the
Big Creek drainage place high elevations adjacent to both sides of low
elevations along the stream. Thus, low elevation areas in the drainage
have an unusually great concentration of high elevation areas in close
proximity to them, and a greater chance of receiving precipitation
associated with the latter areas. The high precipitation would reduce the
probability of low soil moisture and water stress on the trees.

3) Moderated maximum temperature. Valleys provide a pathway for cold,
relatively dense air to drain away from high elevations. Big Creek seems
unusually well situated for cold air drainage given the extremely high
elevations present in the area, and the fact cold air could drain in from
a high proportion of the watershed boundary. Cold air drainage would
reduce maximum summer temperature, which would in turn reduce maximum
transpiration rates. That reduction would decrease water stress at the
tops of trees, which has been implicated as one of the main factors
limiting absolute heights. Evidence that cold air drainage actually
occurs on the stream comes from the winter storm that occurred at the end
of the last ENTS rendezvous. At middle elevations in NC and TN, the
weather killed many of young leaves on broadleaf trees. Lower Big Creek
was below the elevation generally affected in that fashion, but suffered
the same fate.

Additionally, most of the tall tree areas on lower Big Creek are located
between 1800' and 2400' elevation, fairly low for the southern
Appalachians. The climate associated with those elevations is generally
more favorable than higher elevations in terms of lack of soil freezing,
length of growing season, and other factors.

If any of the basic explanations for how mountains influence climate are
inaccurate, please let me know. I would like to know the correct
information for future reference and do not want to mislead anyone on this
list. Any elaborations or analysis of these speculated interactions would
also be quite welcome. I would like to have as clear of an explanation of
the remarkable growth on lower Big Creek as possible.

Jess Riddle
RE: Topography & Tree Height: Big Creek   Robert Leverett
  May 26, 2005 05:11 PDT 

Jess:

   You've laid it out pretty well for us. Good job as usual. From what I
observe, in latitudes 41-43 degrees, steady supply of water and ample
protection are especially critical. I'm sure deep soil plays a role, but
I find very tall trees growing on tops of rocks in the Berkshire-Taconic
region. It can look bizarre. In some way, the roots are able to absorb
the nutrients that the trees from the scant mineral soils at the base of
the rocks and from organic matter suspended in cracks.

   I'm anxious to hear what Lee has to say.

Bob
Re: Topography & Tree Height: Big Creek   Lee E. Frelich
  May 26, 2005 07:08 PDT 

Jess:

Sounds like a good set of hypotheses for a research project. We don't know
any of these things for sure because no one has collected the appropriate
data to test the hypotheses, even though they are reasonable.

To summarize what you said: there should be an elevation at which growing
conditions for height are optimum, meaning the most even supply of water
and nutrients on two time scales: daily and yearly. At low elevation
drought stress from high temperatures could reduce tree height, and as
elevation increases, so does rainfall, but growing season length gets
shorter and winter extremes are greater. At some point there should be an
optimum balance, and valleys may influence that by modifying the typical
temperature patterns for a given elevation. Within the area with optimum
balance of rainfall and temperature, there should be sites with maximum
protection from strong winds and sites with soils that allow percolation of
water so that roots are always moist but never flooded. That is where the
tallest trees should be.

I would add that more importantly, the maximum height of trees could be
modeled and predicted across the landscape if we had data for the whole
range of conditions.

Lee
RE: Topography & Tree Height: Cold air drainage   Ernie Ostuno
  May 28, 2005 09:16 PDT 

Jess,

I'm not sure that cold air drainage would signifiantly moderate maximum
temperatures in the summer. Cold air drainage primarily occurs at night
when radiational cooling occurs, or when there is snow on the ground at
the higher elevations. After the snow is gone, and when the heating of
the day begins, cold air drainage is probably not much of a factor as
far as cooling the treetops is concerned.

Here's a link that describes some mountain valley circulations:

http://www.infodotinc.com/weather2/3-23.htm

There have been studies of individual mountain/valleys where a dense
network of mini-weather stations were used to measure winds and
temperatures, but they don't specifically address how the summertime
maximum temperatures may be impacted by mountain breezes:

http://www.pnl.gov/VTMX/Costigan2001.pdf

This one is more concerned with cold air drainage into "sinks":

http://www.nr.usu.edu/~timw/final_project.html

Ernie
RE: Topography & Tree Height: Big Creek   Will Blozan
  May 26, 2005 17:41 PDT 
Might the unglaciated Smokies be a refuge for "genetically superior" trees?
RE: Topography & Tree Height: Big Creek   Lee E. Frelich
  May 27, 2005 05:49 PDT 

Will:

The Smokies probably have a lot of diversity, since many tree species had a
refuge there, and probably for many glaciations before the Wisconsin as
well. The trees would have been able to survive glacial and interglacial
periods by moving up and down the slopes. This means that the tree
populations should have a larger accumulation of mutations and genetic
diversity than elsewhere, simply because they have been there for a long
time. One study I saw found no genetic differences in hemlock throughout
its range, including the Smokies. However, our knowledge of tree genetics
is so primitive that I can't conclude anything from those results.

Lee
RE: Topography & Tree Height: Big Creek   Darian Copiz
  May 27, 2005 06:28 PDT 
Will,

I agree with Lee that the Smokies are probably more genetically diverse,
but don't know if this is equivalent to genetically superior. I would
expect that trees at their northern limits slowly become genetically
selected for cold hardiness - this could possibly be at the expense of
other attributes such as maximum height potential. The plants best
adapted to a particular community are usually the ones that survive best
and reproduce best. For many areas growing to be the tallest tree may
not necessarily be what would makes the tree the best adapted individual
for the site. Perhaps in the Smokies, in locations where there are
ideal growing conditions, many other attributes are as important and
tree height has become one of the more important aspects that give trees
in the area a competitive edge.

Darian
RE: Topography & Tree Height: Big Creek   Gary A. Beluzo
  May 27, 2005 07:00 PDT 
Will,

Could it be that because of high precipitation, undisturbed soils
(unglaciated), AND deeply concave terrain (McNab found a strong correlation
between Terrain Shape Index and for exampe tulip tree height) in the coves
that leads ecologically to high maximum heights? In other words, it would
be interesting to see if the really tall trees are "ecotypes" or
"phenotypes" by doing a transplantation study. Would the "Smokies hemlock"
seedlings grow comparatively faster/taller than "Massachusetts hemlock
seedlings"? I know lots of these studies that have been done with
herbaceous plants (we learned this in my Coastal Ecology class in grad
school), but I don't know of studies done with woody plants, specifically
comparing say the Smokies trees to the more northerly ones. Perhaps Lee is
aware of some?

Gary