Hemlock
Age 
Charlie
Cogbill 
May
05, 2001 
From: "Charlie
Cogbill"
Date: Sat, 5 May 2001 00:04:26 0400
Fellow Listers,
I have kept an informal database of documented ages of eastern
forest trees and include here an excerpt of eastern hemlock maxima
within various states and provinces. This list is obviously limited and
any additions or comments are actively sought. Several observations are
appropriate to the discussion of hemlock ages and their determination.
The extreme hemlock longevity of 998 years from Pennsylvania reported by
Hough (1960) is a singularity and more than 330 years (or 50%) older
than the penultimate age. I have always wondered if this aberration is
due to a reporting error, an extrapolation gone astray, simply extreme
rarity (maybe never to be repeated), or the small sample size of
reported ages . In any case, the frequency of "old" hemlock ages truly
ends in the 600s and the tail of over 500 years is apparently uncommon
while the 400s are expected maxima in most regions/conditions.
Significantly, the Massachusetts record of 450 years is obviously
exceeded by the 474 minimum count from Dunbar Brook, but still less that
reports from 8 different states and provinces.
This list's discussion has addressed several methods to determine
tree ages lacking a pith core, a subject long pondered by
dendrochronologists and ecologists. The accuracy of these
extrapolations are always questionable and should be of concern when
citing ages, especially extremes (for example, it is unclear if eastern
hemlock can live to 600 or 900 years). Obviously, beyond actual
observation or archival documentation, any cited age not based on a tree
core or cookie is uncertain. Furthermore, dendrochronologists will
argue that any noncrossdated core is to be doubted (at least has some
uncertainty relative to calendar years). Several sources of error in
tree age determinations have been mentioned earlier: missing pith (due
to wood rot; short coreor too big tree; or offcenter core), missed
count (improper preparatione.g. field read?, undercounting narrow
rings, missing rings), coring height above germination height, and
position of pith in relation to tree center (which affects actual
withinbark radius). Interestingly each of these errors generally biases
the age in the same direction (younger) and the actual ring count is a
good starting point for a minimum actual age. Extrapolation beyond this
minimum age is based on several assumptions: primarily radial growth
rates, missing lengthfrom tree center position, and amount of height
growth. For example Bob brackets his Guilder Pond core from 231 to 401
years. I would hesitate to add that a "pure" extrapolation of the
recent growth rate 30.8yr/" to the "remaining 5.7" (is bark thickness
included?) would yield 367 years. Interestingly, a linear regression of
18 record hemlocks from the cited database with both age and diameter
determinations gives an equation of size=3D0.0405dbh(cm) + 39.7cm ; and
the 60.45 cm hemlock above would predict (without a core!) a 513 year
old tree. Note that the regression yintercept is 40 cm dbh, meaning
that the average recent growth rate (approximated slope of the
line=3D0.008"/yr or 125yr/") is much too slow. The relation is also
highly variable explaining only 8% of all the variation, showing that
age is a poor predictor of size and viceversa. In addition, the
average lifetime radial increment of this sample is 26.9yr/", but growth
rates range from 11 to 84 yr/". Given these observations, I would be
hard pressed to speculate on the growth rates of the Guilder Pond
hemlock before1811 (=3D2002191)interestingly what is the error in
this determination? In fact, that was about the end of the Little Ice
Age and the rates were more than likely changing. We might bracket the
extrapolation between the fastest growth rate (11yr/") times the least
missing radius (2"?) giving 22 additional years and the slowest
(125yr/") time the greatest diameter (5.7") giving 712 additional years!
I suspect that Bob's "calculation" of 60 years is a little short, but
an accurate determination is elusive. Doing a proper error analysis is
probably not going to improve this estimate (Bob's range from 200 to
400yr) and my personal solution is to ignore all extrapolations. When
there is no indication of the location of the pith, as seen in narrow
arc to inner rings, one must simply cite the minimum age. Thus by any
standard the 474 ring Dunbar Brook is old (although not the 1020 year
from "pure" extrapolation) and the missing ~ 10 cm (is it really 1 or
20, only pith will tell) is the tease to go back and get more cores. In
fact, ecologically the stand age structure is probably more indicative
of the conditions of the stand than one admittedly indeterminate age.
While I'm lecturing, I would also observe that the growth rates of
hemlocks are highly variable, possibly rapid during release or after
open canopy regeneration, and conversely highly suppressed for many
decades, especially in small advanced regeneration. It is difficult to
deduce this growth from size or recent growth. This is also reflected
by the relatively long time cited by Paul to reach coring height.
Incidentally this average "age to breast height" is a classic case of
the mean being an poor determination of actual values in a highly skewed
or bimodal distribution. Since virtually all the cited tree ages are
from breast height cores why add the same constant to all these ages?
In a parallel argument to the use of an "average" growth rate to
"correct" missing core pieces, why introduce error and increase the
variance into the age determination. It seems better to simply indicate
that the rings were accurately counted and the count is the "age" at
breast height.
Charlie
Charles V. Cogbill
Plainfield, VT 05667
The data:
Hemlock=20
State/Prov, Ring Count, Location, Source,
CN, 350, Colebrook, Nichols 1913
GA, 309, Clayton , Baumgras et al. 1999
IN, 535, Bell Woods, ?
MA, 450, Cold River, Leverett 1991
MA, 365, Alander Mt. , Ed Cook, p.c.
MA, 255, Wachusett Mt., Orwig,
ME, 436, Big Reed Hdwd, Chokkalingam 1998
MI, 509, , Lorimer 1985
MI, 360, Huron Mts., Witty & Coffman
NC, 530, Mt. Guyot/Smokies, Blozan,
NC, 491, Robbinsville, Baumgras et al. 1999
NH, 505, Gibbs Bk. , Cook p.c.,
NH, 400, Cambridge, Lyon 1946
NS, 467, Timber Lake, Lunenburg Co. , Johnson 1986
NY, 470, Shawngunks, Ed Cook, p.c.
PA, 998, , Hough 1960
PA, 607, Tionesta, Morey 1936
PA, 560, East Tionesta, Hough 1936
PA, 492, Hearts Content, Morey 1936 Ecology 17:255
PQ, 661, Abitibi, Frothingham 1915/ Fowles 1975
PQ, 307, Muir's Woods, Nat Areas J. 12 1992
TN, 525, Smokies, Runkle 1982 from Whittaker
TN/NC, 505, Smokies, Blozan/GSMNat. Hist. Assoc.
VT, 486, , Marsh 1864
VT, 419, Gifford Woods, Bormann & Buell 1964
WI, 530, Menominee, Stand 27, Milfred et al 1967
WI, 374, Outer Island, Tyrell & Crow 1994
WV, 375, Cannan Valley, Allard & Leonard 1952

