Re:
OG Definitions and Criteria 
Cogbill 
Sep
18, 2002 08:39 PDT 
Dear
Bob et al.
With all the whirlwind on old growth
criteria, I feel a bit like
Huckleberry Finn at his own funeral.
I am flattered several informal and
anecdotal criteria suggested long
ago are now considered the "standard", but I'm afraid
the original logic and
rigor have been lost, or at least taken out of context. To join
the litany
of questions (actually testable hypotheses) on how the
"5050" age criteria
works, or doesn't work: which trees are included (number,
species, minimum
size, condition, canopy position, spatial sampling pattern)? how
is tree age
determined (field appearance, fieldread cores/stumps, core
preparation,
crossdated cores, pith date, breastheight recruitment,
"corrected",
weighted)?; what are other age determinations (regressions
against size,
reciprocal of the mortality rate, site history; canopy residence
times,
nonparametric or classes)?; how is the age structure expressed
(maximum,
mean, median, distribution parameters such as q or Weibull c
shape; all
ageclasses present)?; what is the maximum longevity
(pathological age,
keyed to local conditions, a confidence interval on the tail of
determined
ages, extreme ever documented)?; and what spatial pattern is
represented
(stand size, spatial averaging, heterogeneity, broadscale
landscape "age"
structure)?. I'm afraid there is no simple answer, but there is
lots wiggle
and research room.
Much of the motivation for the
"5050" rule of thumb was to get an
simple objective indicator of the question of "how old is
old enough"
applicable for a wide range of species and sites. The first
approximation
was to use stand median tree age as a logistically efficient and
statistically interesting quantitative parameter. This was put
in
perspective by comparing it to a local and conceptually logical
constant,
the " maximum longevity". Both empirical data from
sampled stands with
known histories and mathematical theory from different stand age
distributions (truncated evenaged, multicohort additive;
"inverseJ"
thinning; negative exponential or age independent mortality;
probability
density functions from Weibull hazard mortality) indicate that
the median
age in relation to a theoretical "demitile" is an
reasonable (and
dynamically responsive) estimator. Note that the theoretical
maximum is
indeterminate (mathematically, not biologically), so that a
statistical
parameterization of empirical samples must be used to estimate
the "maximum
longevity". Thus two independent concepts have been
combined: median tree
age is a good estimator for stand age structure and the
"old" reference
curve is keyed to the observable maximum. The index was
originally proposed
in its rudimentary form in 1982 (Maine Critical Areas OldGrowth
Report, by
Anonymous) and later elaborated in 1996 with reference to the
principles and
empirical data from red spruce stands (Spruce chapter by myself
in the
Eastern Oldgrowth Book) with a conclusion of "In theory as
well as in
practice, a median age of half the maximum longevity is a good
index of
oldgrowth character." and I must add "in red spruce
stands". I guess you
could quote me here and incidentally, I still like it as a
generic rule of
thumb. In actuality, it is still a working hypothesis, and has
never
received any discussion, critical comment, or indeed actually
been peer
reviewed. Thus despite the tendency to accept it as dogma (or
worse yet
"science"), everyone must be aware of its history,
assumptions, and
limitations and realize it is only a first approximation waiting
for
improvement. I hope the reaction is not summary
dismissal, but instead an
understanding of the underlying issues and complexities. As an
aside, I am
unaware of more than a handful of sites where enough data are
actually
available to determine either the stand age structure or the
stand history
in enough detail to investigate these questions. Thus for now
the
widespread use of this criteria is problematic and probably
moot.
Without revisiting the discussions of
the two "old growth" definition
workshops, or more importantly the vast literature on the topic,
I must
point out that we are still putting the "horse before the
cart". All the
quibbling about criteria still begs the question of a real
definition of
"old growth". Until we can agree (or simply state
one's personal view)
about the concept and get a reasonable definition (be it verbal,
conceptual,
theoretical, or typic with commonly accepted examples), there is
little hope
in agreeing on any criteria which identify such an entity, or is
it
entities. We also are risking tautological arguments, such as
the
traditional "it is 'old growth' because I (or they) say it
is". Even what I
call the Supreme Court definition, Potter Stewart's "I
cannot define it, but
I know it when I see it" needs some examples and
"local community
standards". Inevitably such a definition must address the
history of the
area, extent and intensity of disturbance, the continuity of
composition and
structures, the replacement processes, and the size and
integrity of the
area at various scales. I tend to see "old growth" as
a process (now there
is a conceptual definition) and doubt that specific structures,
predictable
composition, or indicator species will ever be that useful as
parts of a
definition. They might show up in criteria, but we first must
get the
definition straight first.
Enough from the balcony, its time for Huck to float down the
River, no doubt
he will return.
Charlie
Charles V. Cogbill 
Re:
Age / Diameter Relationships 
Cogbill 
Jan
11, 2003 
Bob has indicated that there might be some
general interest in technical comments I had on Tom
Diggins's question on size/age data. Here is my reply to
Tom.
Tom,
Although
most of the work on agediameter relationships seems to have
been done on managed forests or evenaged conifer systems, there
is a scattered literature on ages of eastern hardwoods in older
unmanaged forests. These contain big, hard and commonly hollow
trees and present a problem in dendroecology, but there are
indeed some data on their age structure. Here are some relevant references and a brief summary of the
methodology and efficacy of using size as a predictor of age.
Lorimer
C.G., S.E. Dahir, & EV. Nordheim. 2001.
Tree mortality rates and longevity in mature and
oldgrowth hemlockhardwood forests.
J. Ecol. 89:
960971
Hough,
A.F. 1943. Soil
factors and stand history in a virgin forest valley on the
northern Allegheny Plateau.
Soil. Sci. 43: 1928.
Hough,
A.F. 1932. Some
diameter distributions in forest stands of northwestern
Pennsylvania. J.
For. 30: 933943.
Hough,
A.F. & R.D. Forbes. 1943.
The ecology and silvics of forests in the high plateaus
of Pennsylvania. Ecol.
Monogr. 13:301320.
Chokkalingam,
U. 1998. Spatial
and temporal patterns and dynamics in oldgrowth northern
hardwood and mixed forests of northern Maine.
Ph.D. thesis, University of Maine, Orono, ME.
Gates,
F.C. & G.E. Nichols. 1930.
Relation between age and diameter in trees of the
primeval northern hardwood forest.
J. For. 28:
395398.
Lorimer
C.G., 1980. Age
structure and disturbance history of a southern Appalachian
virgin forest. Ecology
61: 11691184.
Tubbs,
C.H. 1977. Age and structure of a northern hardwood selection
forest, 19291976. J.
For. 75: 2225.
Leak,
W.B. 1975. Age
distribution in virgin red spruce and northern hardwoods.
Ecology 56: 14511454.
Ross,
M.S., T.L. Sharik, & D.M. Smith 1982.
Agestructure relationships of tree species in an
Appalachian oak forest in southwest Virginia.
Bull. Torrey Bot. Club 109: 287298.
Sano,
J. 1977. Age and size distribution in a longterm forest dynamics.
For. Ecol. & Mgt. 92: 3944.
Blum,
B.M. 1961. Agesize relationships in allaged northern hardwoods.
For. Res. Note 125, USFS, NE For. Expt. Sta. Upper Darby,
PA.
Kenefic,
L.S. and R.D. Nyland. 1999.
Sugar Maple heightdiameter and agediameter
relationships in an unevenaged northern hardwood stand.
Northern J. Appl. For. 16: 4347.
Comments
All
studies find a statistically significant relationship between
age and diameter (no surprise as trees do grow larger with age),
but the correlation coefficients between these variables within
a species in the same stand for dominant hardwoods (i.e. sugar
maple, beech, oak, tulip, white birch) is highly variable
(growth is not constant and is relatively independent of
size). In fact, the
correlation coefficient (r) has an average of 0.73 in 6 of the
studies cited above (range from 0.26 to 0.94). This implies
that only 53% (maximum 88%) of the variance in the ages
is explained by knowing the diameter of a tree in a specific old
stand. Many
studies go on to use an ageondiameter regression to derive a
predictive relationship for age.
In some cases (e.g. Lorimer 1980, Leak 1975) this
regression equation is used to expand a sampled diameter
frequency (size structure) into an age frequency (age
structure). Curiously
using a monotonic deterministic equation produces an age without
any variability. In
essence it does not derive the age structure but only RESCALES
(by converting units) the diameter distribution.
As seen above there is a
rather large error even in age determination based on
diameters in the same stand.
Predicting
age from trees outside the sampled stand introduces even greater
variability. The
accuracy of a generic calibration of age from size can be
illustrated by a quick analysis from my eastern age database for
sugar maple ages derived from actual ring counts (no corrected,
estimated, or
extrapolated ages) using 94 sugar maples of measured diameter up
to 91 cm dbh growing in numerous old stands in the East.
Technically this analysis is a
model 1 regression, while the more appropriate analysis
could be a model 2 regression of diameteronage.
However the predictive equation from a linear regression
of the scatter of dbhage determinations gives y (age in years)=
2.46 x (dbh diameter in cm) + 40.95 with a correlation (r) of
0.53; that is 72%
of the variance in the ages is unexplained by the regression.
Specifically the standard error of any specific determination of
age from this
equation (s sub Yhat) is 67.0 years.
Importantly the confidence limits around this prediction
line expand with distance from the mean and, in fact, many large
trees exceed the domain of the regression.
Ignored are the facts that the variability also increases
if the assumptions of determining ages from ring counts at dbh
and the uncertainty in treating diameter as a truly independent
variable without error are accomodated by the model. Thus
this analysis indicates that for a 90 cm diameter sugar maple
the liberal 95% confidence interval for a single age
determination is +/ 134 years around the predicted mean of 262
years. Significantly the range of ages can be confidently placed
only between 128 and 396 years for a 90cm tree!
Obviously using better fitting curvilinear regressions
and more sitespecific samples might narrow this range, but
given the large variability in the determinations it is
problematic how accurately age can be determined by any
regression calibration.
Of
course much of this is argument is about statistics and they can
easily mislead, but I am convinced that age cannot generally be
derived by extrapolations or predictive relationships.
This is the basis of my “second rule of forestry”
that “one cannot tell the age of a tree by its size”.
Since there are direct means of deriving tree ages and
more importantly the age structures of stands, I encourage not
compounding the errors by secondary methods.
Thus I am interested in any actual ages you might have
found in the Zoar Valley. However scanty, these
are the appropriate data that are needed for the description of
the age structure of this site.
Charlie
Cogbill

