Rhododendron and Hemlock   Ernie Ostuno
  Oct 16, 2006 08:24 PDT 


Your understory tree measurements are probably every bit
as important as the more attention-getting overstory measurements in
quantifying the health and vitality of an old growth site. Also, this
might be an effective way to measure/document the impact of overgrazing
by deer in recent decades.

I've noticed a great variability in understory characteristics at old
growth sites. I've also noticed that most of the impressive old growth
hemlock sites in PA had large rhododendron. Has anyone done any
height/age measuring of rhododendron? Is there some type of symbiotic
relationship between hemlock and rhodo, besides the hemlock providing
lots of shade? We sometimes have a tendency to ignore some members of
the old growth ecosystem or see them as merely obstacles to be "surfed"
to get to the beloved giants.

Re: Rhododendron and Hemlock   Jess Riddle
  Oct 17, 2006 17:28 PDT 

Hello Ernie,

Good observation about the hemlock and rhododendron. I can't speak
for the northeast, but in the southern Appalachians, large hemlocks
and dense tangles of rosebay rhododendron often go together. Both
species grow relatively well in acidic soils and have a relatively low
fire tolerance. Consequently, positive neighborhood effects for each
species own seedlings wind up also benefiting the other species; the
foliage of both species acidify the soil and their combined dense
shade retain moisture and inhibit fires. Oddly enough, I suspect
rhododendron also benefits hemlock by suppressing tree regeneration;
in thick tangles of rhododendron, trees can only regenerate on top of
logs, which hemlock does well, so hemlock seedlings have fewer species
to compete with and a thinner mid-canopy above the rhododendron.
However, hemlock can reach immense sizes without help from
rhododendron, for instance the third largest hemlock in the Smokies is
16' cbh and 143' with hardly any rhododendron within sight. And the
thickest rhododendron occurs in spruce forests on in balds without any
overstory tree canopy.

Re: Rhododendron and Hemlock   Edward Frank
  Oct 17, 2006 17:36 PDT 

Jess, ENTS,

Last year at the Forest Summit, you and Will showed me a photo of a
massively thick Rhododendron. Where was that specimen from? I would love
to have the photo for the website. Are the stems in a single clump
typically clones of each other? Does anyone have any idea how old individual
stems of rhododendron can be? It might be a case similar to creosote bush
in which the age of a clump that spreads by root sprouts may be
significantly older than individual stems.

Re: Rhododendrons & Hemlock   Jess Riddle
  Oct 18, 2006 16:18 PDT 
Ed, Ents,

If I thinking of the same large rhododendron, it grows outside of
Black Mountain, NC on private property. Will has the photograph.

rhodo_4538.jpg (147986 bytes)

Will Blozan writes: "Here is a shot of the large Rhododendron maximum in Black Mountain, NC. It is 38" girth X 24' height X 24' average spread. It is likely a volume champion as well and Jess and I have thought of measuring it to see how much wood it contains." Oct 22, 2006

I've seen multiple stems growing out of a single burl of wood at the
base, and stems commonly have small sprouts at the base, so all the
stems in a clump must be the same genetic individual. As far as I
know, entire thickets of rhododendron are often a single individual,
similar to aspen forests. Will has been keen on coring rhododendron
in our Tsuga Search plots, so we have recently collected a few cores
from rhododendron over four inches dbh in old-growth hemlock forests.
Half of the cores had over 100 rings.

I would not be at all surprised if rhododendron root systems could
live for thousands of years, but I have absolutely hard no evidence to
back up that opinion. Fungal attacks can occasional kill an acre of
rhododendron here or there, but I haven't seen them eliminate
rhododendron from an entire area. Rhododendron generally scoffs at
storms. Fires, even low intensity ones, will kill rhododendron back
to the ground, but it readily resprouts. Intense fires can kill the
root systems, but rhododendron often grows in very moist sites. Lee
Frelich writes about severe windthrow followed by fire being able to
end sugar-maple hemlock dominance, and such a scenario could be a
major threat to rhododendron even on moist sites. However, I would
thick rhododendron would have at least occasional refuges that would
allow small parts of a colony to survive and reestablish the thicket.
Hence, I think the incredibly dense tangles of rosebay rhododendron in
southern Appalachian spruce forests, with cool climates and 80"+ of
rain annually, would be very difficult to kill between major changes
of climate.


Re: Rhododendrons & Hemlocks   Jess Riddle
  Oct 18, 2006 16:35 PDT 


The areas where rhododendron grows without any overstory trees, known
as balds, heath balds, slicks, or hells, typically occur on exposed
ridges at relatively high elevations on exposed ridges. I don't think
they range as far north as Pennsylvania, but I'm not sure. I think
the latest theory on their formation is that they follow severe fires
in spruce forest. The ridges burn to such an extent that only a layer
of charcoal is left on the bedrock. The dense growth of
rhododendrons, mountain laurel, and occasionally winterberry and other
shrubs and vines excludes tree regeneration. High concentrations of
aluminum in the soil also deter species with weaker constitutions.

Peter S. White has a nice paper on heath balds available as a pdf at