OG Definitions and Criteria
18, 2002 08:39 PDT
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
of questions (actually testable hypotheses) on how the
"50-50" age criteria
works, or doesn't work: which trees are included (number,
size, condition, canopy position, spatial sampling pattern)? how
is tree age
determined (field appearance, field-read cores/stumps, core
crossdated cores, pith date, breast-height recruitment,
weighted)?; what are other age determinations (regressions
reciprocal of the mortality rate, site history; canopy residence
non-parametric or classes)?; how is the age structure expressed
mean, median, distribution parameters such as q or Weibull c
age-classes present)?; what is the maximum longevity
keyed to local conditions, a confidence interval on the tail of
ages, extreme ever documented)?; and what spatial pattern is
(stand size, spatial averaging, heterogeneity, broad-scale
structure)?. I'm afraid there is no simple answer, but there is
and research room.
Much of the motivation for the
"50-50" rule of thumb was to get an
simple objective indicator of the question of "how old is
applicable for a wide range of species and sites. The first
was to use stand median tree age as a logistically efficient and
statistically interesting quantitative parameter. This was put
perspective by comparing it to a local and conceptually logical
the " maximum longevity". Both empirical data from
sampled stands with
known histories and mathematical theory from different stand age
distributions (truncated even-aged, multi-cohort additive;
thinning; negative exponential or age independent mortality;
density functions from Weibull hazard mortality) indicate that
age in relation to a theoretical "demi-tile" is an
dynamically responsive) estimator. Note that the theoretical
indeterminate (mathematically, not biologically), so that a
parameterization of empirical samples must be used to estimate
longevity". Thus two independent concepts have been
combined: median tree
age is a good estimator for stand age structure and the
curve is keyed to the observable maximum. The index was
in its rudimentary form in 1982 (Maine Critical Areas Old-Growth
Anonymous) and later elaborated in 1996 with reference to the
empirical data from red spruce stands (Spruce chapter by myself
Eastern Old-growth Book) with a conclusion of "In theory as
well as in
practice, a median age of half the maximum longevity is a good
old-growth 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
received any discussion, critical comment, or indeed actually
reviewed. Thus despite the tendency to accept it as dogma (or
"science"), everyone must be aware of its history,
limitations and realize it is only a first approximation waiting
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
available to determine either the stand age structure or the
in enough detail to investigate these questions. Thus for now
widespread use of this criteria is problematic and probably
Without revisiting the discussions of
the two "old growth" definition
workshops, or more importantly the vast literature on the topic,
point out that we are still putting the "horse before the
cart". All the
quibbling about criteria still begs the question of a real
"old growth". Until we can agree (or simply state
one's personal view)
about the concept and get a reasonable definition (be it verbal,
theoretical, or typic with commonly accepted examples), there is
in agreeing on any criteria which identify such an entity, or is
entities. We also are risking tautological arguments, such as
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
standards". Inevitably such a definition must address the
history of the
area, extent and intensity of disturbance, the continuity of
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,
composition, or indicator species will ever be that useful as
parts of a
definition. They might show up in criteria, but we first must
definition straight first.
Enough from the balcony, its time for Huck to float down the
River, no doubt
he will return.
Charles V. Cogbill
Age / Diameter Relationships
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
most of the work on age-diameter relationships seems to have
been done on managed forests or even-aged 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.
C.G., S.E. Dahir, & EV. Nordheim. 2001.
Tree mortality rates and longevity in mature and
old-growth hemlock-hardwood forests.
J. Ecol. 89:
A.F. 1943. Soil
factors and stand history in a virgin forest valley on the
northern Allegheny Plateau.
Soil. Sci. 43: 19-28.
A.F. 1932. Some
diameter distributions in forest stands of northwestern
For. 30: 933-943.
A.F. & R.D. Forbes. 1943.
The ecology and silvics of forests in the high plateaus
of Pennsylvania. Ecol.
U. 1998. Spatial
and temporal patterns and dynamics in old-growth northern
hardwood and mixed forests of northern Maine.
Ph.D. thesis, University of Maine, Orono, ME.
F.C. & G.E. Nichols. 1930.
Relation between age and diameter in trees of the
primeval northern hardwood forest.
J. For. 28:
C.G., 1980. Age
structure and disturbance history of a southern Appalachian
virgin forest. Ecology
C.H. 1977. Age and structure of a northern hardwood selection
forest, 1929-1976. J.
For. 75: 22-25.
W.B. 1975. Age
distribution in virgin red spruce and northern hardwoods.
Ecology 56: 1451-1454.
M.S., T.L. Sharik, & D.M. Smith 1982.
Age-structure relationships of tree species in an
Appalachian oak forest in southwest Virginia.
Bull. Torrey Bot. Club 109: 287-298.
J. 1977. Age and size distribution in a long-term forest dynamics.
For. Ecol. & Mgt. 92: 39-44.
B.M. 1961. Age-size relationships in all-aged northern hardwoods.
For. Res. Note 125, USFS, NE For. Expt. Sta. Upper Darby,
L.S. and R.D. Nyland. 1999.
Sugar Maple height-diameter and age-diameter
relationships in an uneven-aged northern hardwood stand.
Northern J. Appl. For. 16: 43-47.
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
studies go on to use an age-on-diameter 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
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.
age from trees outside the sampled stand introduces even greater
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,
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 diameter-on-age.
However the predictive equation from a linear regression
of the scatter of dbh-age 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 site-specific samples might narrow this range, but
given the large variability in the determinations it is
problematic how accurately age can be determined by any
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.