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Comments on "Some Effects of Thinning on Increment
in Douglas.fir in Western Washington"!.
The last conclusion presented in
the paper "Some Effects of Thin­
ning on Increment in Doug'las-fir
in Western Washington" published,
in the February issue of the JOUR­
NAL is subject to serious question.
The acceptance, without qualifica­
tion, of the statement "residual/
volume was the only factor tha
influenced increment. significantly",
could have widespread effect on'
silvicultural practices. I believe the
authors may have overlooked two
important growth concepts in
reaching this conclusion, and, if
this be so, then the results of the
investiga'tion must be re-evaluated in light 'of these concepts before any conclusion can be reached. First, basal area in well-stocked,
untouched stands of a given species
and age is a measure of site qual­
ity. The higher the basal area the
better the site. Thus, when the
sample plots used in this investiga­
tion' were re-arranged to study the
relationship of residual growing
stock to subsequent growth, i is
quite likely that the re-arrange­
ment was on the basis of inherent
site quality with basal area as the
index. The' range in average origi­
, nal basal area from 146 to 177
square feet per acre might evi­
dence' considerable variation in
site quality. The scheme of remov­
ing a fixed percentage of the vol­
ume in the first thinnings retained
much of the basal area ranking of
plots 'that existed before treatment.
Therefore, the analysis of subse­
quent growth on residual basal
area could very well have been a
determination of the effect of site
on growth. .In this case the conclu.·
sion should have been ((site quality
was the only factor that influenced
'Worthington, NOl'man P., Donald L.
Reukema, and George Staebler.
Some
effects of thinning on increment in Doug­
las-fir in Western Washington. Jour. For­
estry 60: 115-119. 1962.
Reply
We would like to express our
appreciation to Professor Barrett
for his careful tudy of our article
"Some Effects of Thinning on lri­
crement in Douglas-fir in Western
Washington," which appeared in ..
the
February JOURNAL. Further .'
Secondly, site index as measured
by height over age may bear little analysis has been made to check
raised in his.
relation to basal area and volume on some of the points
.
comments.
increment. To illustrate, assume
Analysis of the data from· the
two' sites with equally favorable
Voight
Creek plots shows that
soil moisture regimes in the spring
basal
area
prior to treatment was.
(during the time of height growth
and diameter growth) but one has not related to site index as meas­
much less available soil moisture ­ ured by height and age-the cor­
in the summer (height growth relation coefficient being only .27
(38 d.f.). Therefore, one must be
stopped but diameter growth con­
a
better measure of site quality
tinues). Then two.stands of the
same species and age on these sites than the other. Because basal area
may
have
comparable height is so strongly affected by stocking,
growth but unlike basal area and we have assumed that the conven-·
volume increments. The writers tional height-age relationship is the
of prod­
note this situation in the paper more acceptable measure
)
but stiil use the height over age uctivity.
Hence, the conclusion that in­
index as a measure of production
crement
in basal area after thin­
potential homogeneity among plots.
ning
is
not
related to productivity
The original plot basal areas, that
as
measured
'by site index does
indicate a difference in site quality
seem
valid.
However,
cubic volume
does exist, may have been a much
better index of potential incre­ increment is related to site index,
presumably because the added di­
ment than tree heights.
mension of height increment is in­
The two conclusions, first that
volved.
growth is little influenced by the
The conclusions that increment
intensity of the thinning operation
is little influenced by thinning :in­
and second that g rowth is related
tensity, and that it is related' to
to residual volumes, both within
residual volumes, although perhaps
the limits described in this paper,
superficially
contradictory, are
are contradictory. It should be
borne out by statistical analysis.
noted that the study was initially
The explanation lies in differences
designed to provide information
in pretreatment basal area between
about the effects of thinning on
plots within any given treatment.
growth but not about the relation­
Increment was significantly related
ship of growth to residual volumes.
to these basal areas. As 'pointed
I believe the second conclusion
out by Professor Barrett, the treat­
might rest on evidence that IS con­ ments tended to result in retention
founded by site differences among
of this pretreatment ranking of
the plots and is not a valId deduc..
plots within 'treatment, areas, but
tion from present data.
not between treatments.
JOHN W. BARRETT
GEORGE R. STAEBLER
.A8sooiate Professor of Silvioulture,
DONALD L. REUKEMA
Stme Un1iJj ersity Oollege of Forestry
NORMAN P. WORTHINGTON
at. Syr a cuse Univers-ity; N. Y.
increment significantly." This con­
clusion is certainly a far cry from
the one proposed in the paper, but
it may be logically reached with
the information at hand.
Seotion oont1nues on page 416
413