Effect of Leader Clipping on Height Growth of
Young Coastal Douglas-Fir
(Pseudotsuga menziesii var. menziesii)
Frank C. Sorensen, USDA Forest Service, Forestry Sciences Laboratory, 3200
Jefferson Way, Corvallis, OR 97331.
ABSTRACT: Leaders of 3- through 7-yr-old Douglas-fir seedlings were left unclipped or were clipped for
1 to 4 consecutive years. Terminal removal reduced height increment in a significantly linear fashion and was
about 19 cm for each year clipped. All treatments had comparable height increment in the 4 yr after clipping,
and the initial depressing effect on height was still present at age 23. Diameter increment was reduced but not
significantly. West. J. Appl. For. 17(2):75–77
Key Words: Height and diameter growth, animal damage.
Douglas-fir (Pseudotsuga menziesii) has relatively high
susceptibility to animal damage (Moore 1940). The damage
may remove primarily foliage, lateral branches, or terminal
shoots, and it may occur at various phenological stages (e.g.,
in winter when the shoots are dormant, in spring when new
growth is elongating, and in summer when new growth is
mostly completed but not hardened off).
Past studies have evaluated the effect of animal damage in
terms of immediate growth loss but less often have addressed
the persistence of growth loss after the damage has ceased.
Fisch and Dimock (1978) reported that winter shoot clipping
by Douglas squirrels (Tamiasciurus douglasii) resulted in a
mean height loss of 15 cm in the year of clipping, with
complete recovery of height loss the following year. One to
five annual incidents of hare (Lepis americanus) clipping of
summer leaders from recently planted seedlings reduced
height by about 25 cm per clipping event. The heightreduction response was nearly linear with years of clipping,
and reduction was still present 3 yr after the last clipping.
The effect of repeated leader clipping of Douglas-fir by
blacktailed deer (Odocoileus hemoinus columbianus) is re­
ported from several studies. Dimock (1970) observed growth
loss in a western Washington plantation of 9 cm/yr for 1 to 7
yr of clipping. Season of browsing was not reported. Roy
(1960) studied the consequence of summer browsing in
northwestern California and found no growth loss with 1 yr
NOTE: The author can be reached at (541) 750-7300; Fax: (541) 758-7760;
E-mail: fsorensen@fs.fed.us. The author thanks Richard Miles for
many years of plantation care, Roger Petersen for statistical advice,
and Floyd Freeman, Randy Johnson, Steven Sharrow, Tally Patten,
and an anonymous referee for helpful comments on an earlier draft.
This article was written by U.S. Government employees and is
therefore in the public domain.
of clipping but 20 cm/yr for 2 to 5 yr of clipping. Mitchell
(1964), on Vancouver Island, BC, in a retrospective evalua­
tion of 4 yr of winter clipping at two plantations and 5 yr of
damage at a third, reported 9, 19, and 20 cm of loss per
incident of leader removal. Except for the first year of
browsing reported by Roy (1960), the annual response of
growth loss was very close to linear in all these tests. Mitchell
(1964) observed that leader removal did not affect rate of
height growth if no subsequent damage was incurred.
Sharrow et al. (1992) reported on the effect of brief
grazing by domestic sheep in May, August, and the following
May. Clipping was heaviest on lateral shoots but included
zero to three removals of terminals. Plants were measured 3
and 8 yr after grazing. Impact of terminal clipping on height
was approximately linear and averaged 44 cm per clipping 3
yr after grazing and 61 cm 8 yr after grazing.
In some early long-term genetic field tests established on
lower, western slopes of the Cascade Range in central Or­
egon, a slightly different pattern of damage was observed,
namely, a brief period of deer browse beginning 2 to 3 wk
after bud flush. Feeding removed various amounts of newly
flushed leaders and a few laterals. Not all plants were browsed.
In terms of foliage removed, the damage was negligible.
Because persistent growth loss can affect variances in genetic
tests (Shaw et al. 1988) and because damage was a little
different than previously reported, a study was installed to
evaluate immediate and longer term effects of spring brows­
ing on height increment.
In this study, two levels of clipping of newly flushed
shoots were applied to plantation seedlings for 1 to 4 consecu­
tive years, and height and diameter were measured during and
for several years after clipping. Immediate and longer term
effects of leader removal on growth are reported and related
to results from previous tests.
WJAF 17(2) 2002
75
Materials and Methods
Results
An open-pollinated seed mix from the west side of the
Cascades Mountains in central Oregon (44˚35’N, 122˚42’W,
260–300 m) was sown and seedlings raised in an experimen­
tal nursery in Corvallis, OR (44˚35’N, 123˚20’W, 75 m).
Two-year-old seedlings were outplanted in January 1979
southwest of Monmouth, OR (44˚48’N, 123˚18’W, 100 m)
on abandoned agricultural land that formerly was a Douglasfir, white fir (Abies concolor), Oregon white oak (Quercus
garryana) site. Soil type was well-drained silty clay loam
(Hartung 1970), slope was about 5%, and mean annual
precipitation about 1,250 mm. Nearby plantings were expe­
riencing little browse damage.
Seedlings were planted at a 3 × 3 m spacing. Competing
vegetation was controlled with mowing. The test area was
surrounded by one border row. There was no thinning
during the test; crown closure occurred about age 11 to 12
yr. Experimental design was a randomized complete block
with three blocks. Clipping treatments were applied to sixseedling row plots within blocks. Treatments were (1) two
levels of clipping—(a) half of or (b) all of the newly
flushed leader removed when the shoot was 6 to 10 cm
long; and (2) 0 (unclipped control), and 1, 2, 3, and 4
consecutive years of clipping. There were two unclipped
control plots in each block to balance the two levels of
clipping. Treatments and controls were assigned randomly
to row plots within blocks.
Seedlings were allowed to establish for 1 yr, and treat­
ments started in the spring of the fourth growing season. All
treatment plots were clipped the first treatment year, those
plots that were to have 2 or more years of treatment were
clipped again the second year, and so on through 4 yr.
Height of each seedling was measured to the base of the
terminal bud. Measurements were made in the winter before
the first clipping and annually in winter after each of the
next 8 growing seasons. Also, each seedling was painted
with a dot 15 cm above ground level, and diameter was
measured at that point when height was measured. Because
clipping effect was still present at age 11, a final measure­
ment of height and of diameter at 140 cm above ground level
was made at age 23 yr.
Analysis was based on plot means using the following
preliminary statistical model:
Annual height increment of the control (no clipping)
treatment averaged 60 cm during the 4 yr over which clipping
was applied. By age 7 the plants were on the linear phase of
the height growth curve, and annual increment averaged a
little over 1 m during the 4 yr after clipping.
Plots did not differ in mean height or diameter prior to
clipping (Table 1, line 1). During the 4 yr of clipping, the
linear effect was very highly significant (P = 0.0004) and
amounted to a decrease in increment of about 19 cm/yr of
clipping (Figure 1A, Table 1). In the 4 yr after clipping, there
was a slight but nonsignificant recovery of the clipped plants
(Figure 1B). The linear effect of clipping was still significant
(P = 0.0148) 4 yr later (age 11) and amounted to a loss of
about 15 cm/yr of clipping (Table 1). At age 23, the linear
effect was no longer significant (Table 1, P = 0.0710), but
growth reduction persisted at 17 cm/yr of clipping.
Clipping also reduced diameter increment, but the re­
sponse was not statistically significant (Table 1).
Deviation from linearity was nonsignificant (P > 0.05) for
both height and diameter at all measurements.
Xijk = µ + Bi + A j + Yk + ( AY ) jk + ∈ijk
where Xijk = plot mean, µ = grand mean, Bi = effect of ith
block, Aj = effect of jth amount clipped, Yk = effect of kth
fixed years of clipping, (AY)jk = interaction of jth amount
clipped and kth years, and ∈ijk = experimental error; i = 1...3
blocks, j = 1,2 amounts clipped, and k = 1...4 yr of clipping.
In the analysis, control (no clipping) was contrasted with
clipping, and clipping was applied at four equally spaced
intervals. In preliminary analyses, Aj and (AY)jk were always
insignificant. Consequently, a second analysis deleted Aj and
(AY)jk, based the plot mean for year on both clipping amounts,
and treated control and years of clipping as five levels of the
year treatment, 0...4.
76
WJAF 17(2) 2002
Discussion
All observations reported above were made in the western
part of the range of coastal Douglas-fir. With two exceptions
(see below), results from different studies were quite consis­
tent. When damage was predominantly leader removal, re­
sponse to years of removal was approximately linear and
ranged from 9 to 25 cm (mean 16 cm ± 6 cm) reduction in
height per year of leader removal (Roy 1960, Mitchell 1964,
Dimock 1970, 1971). Exceptions occurred if seedlings were
small (Dimock 1970) or laterals were heavily browsed (Osman
and Sharrow 1993). In these cases, height reduction was
amplified, probably because of increasing competition from
other vegetation. In two cases, 1 yr of leader clipping did not
reduce later height (Roy 1960, Fisch and Dimock 1978).
Amount of leader removed did not influence growth re­
sponse.
There was considerable recovery in the year of leader
removal. Unclipped controls averaged 60 cm annual height
Table 1. Change in height and diameter per year of clipping, and
significance of the linear effect associated with that change.
Age (yr)
3†
7††
9
11
23
3–7§
7–11||
11–23#
Height
Change* (cm)
+1.0
–13.0
–16.0
–14.7
–17.2
–19.1
+3.6
0
P
0.0938
0.0001
0.0048
0.0148
0.0710
0.0004
0.2098
0.5969
Diameter
Change* (mm)
P
+0.2
0.2075
–0.7
0.1139
–2.0
0.1081
–2.7
0.1118
–4.2
0.0248
–1.1
0.0523
–1.8
0.1206
–1.6
0.2142
* Change in total height or diameter at 15 cm (at 140 cm for year 23) per year of
clipping; e.g., at age 7 mean height was reduced by 13 cm if leaders were removed
1 yr, 26 cm if leaders were removed 2 consecutive years, etc.
† Prior to first clipping.
†† After 4 yr of leader clipping.
§ Increment during the 4 yr of clipping.
|| Increment during the 4 yr following the last clipping.
# Increment during years 11 to 23.
250
A. Years 3-7
expense of slightly reduced diameter growth. The results of
Sharrow et al. (1992) indicate that terminal removal had an
effect on diameter increment independent of lateral brows­
ing, but their results partially confounded terminal removal
and degree of lateral browsing. Expanding new shoots and
needles are strong carbohydrate sinks (Krueger and Trappe
1967, Waring 1987, Kozlowski 1992), and replacement of
clipped shoots could have slightly reduced diameter growth
in the year of clipping. Another possibility is related to
hormonal effects. The terminal meristem is a source of
auxin, which facilitates cell expansion (Sharrow et al.
1992). Removing the leader would temporarily remove this
auxin source. If subsequent expansion was geometric (i.e.,
of uniform rate) from cambia of different circumference,
the diameter effect would increase over time.
In conclusion, Douglas-fir suffers considerably from
animal damage. Leader removal reduces plant height and
the reduction persists, particularly if damage is repeated.
Growth loss at the level observed here is probably not
sufficient to warrant expensive protection measures in pro­
duction plantations. However, because the damage does
alter genetic variances (Shaw et al. 1988), protection prob­
ably is warranted in genetic tests such as the ones that led to
the installation of this study.
B. Years 7-11
Literature Cited
225
Height Increment (cm)
200
175
150
475
450
425
400
375
0
1
2
3
4
Years of Clipping
Figure 1. Effect of 0–4 yr of leader clipping on (A) cumulative
height increment during the 4 yr of clipping and on (B) cumulative
height increment during the 4 yr after clipping.
increment during ages 3 to 7. Leader removal reduced this by
about 19 cm/yr or about 30%. In the years after removal, all
treatments grew equally, as observed by Mitchell (1964) in a
retrospective test. Statistical significance of the clipping
treatment decreased with age because the error mean square
increased with plant size, whereas the treatment mean square
did not change. Under other conditions with more microsite
variation and various damages occurring in the stand, the
clipping effect probably soon would become statistically
nonsignificant (Fu et al. 1999), even though recovery had not
occurred.
The effect of clipping on diameter also was weakly
negative and nearly significant (P = 0.0523) during the
years of clipping. The trend could have been an artifact of
minor site variation because it was still present at age 23.
Alternatively, because the effect was not present at age 3
(Table 1, line 1), its development during clipping might
have been due to compensatory height recovery at the
DIMOCK, E.J., II. 1970. Ten-year height growth of Douglas-fir damaged by
hare and deer. J. For. 68:285–288.
DIMOCK, E.J., II. 1971. Influence of Douglas-fir seedling height on browsing
by black-tailed deer. Northwest Sci. 45:80–86.
FISCH, G.G., and E.J. DIMOCK II. 1978. Shoot clipping by Douglas squirrels in
regenerating Douglas-fir. J. Wildl. Manage. 42:415-419.
FU, Y.B., A.D. YANCHUK, AND G. NAMKOONG. 1999. Spatial patterns of tree
height variations in a series of Douglas-fir progeny trials: implications for
genetic testing. Can. J. For. Res. 29:714–723.
HARTUNG, R.E. (COMP.). 1970. Soil-woodland interpretations for soils in the
Willamette Basin, Oregon, progress report. USDA Soil Conserv. Serv.,
Portland, OR. 22 pages + 8 unnumbered leaves.
KOZLOWSKI, T.T. 1992. Carbohydrate sources and sinks in woody plants. Bot.
Rev. 58:107-222.
KRUEGER, K.W., and J.M. TRAPPE. 1967. Food reserves and seasonal growth
of Douglas-fir seedlings. For. Sci. 13:192-202.
MITCHELL, K.J. 1964. Height growth losses due to animal feeding in Douglas
fir plantations, Vancouver Island, BC, For. Chron. 40:298-307.
MOORE, A.W. 1940. Wild animal damage to seed and seedlings on cut-over
Douglas fir lands of Oregon and Washington. USDA Tech. Bull. 706,
Washington, DC. 28 pages.
OSMAN, K.A., and S.H. SHARROW. 1993. Growth responses of Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco) to defoliation. For. Ecol. Man­
age. 60:105–117.
ROY, D.F. 1960. Deer browsing and Douglas-fir seedling growth in north­
western California. J. For. 58:518–522.
SHARROW, S.H., W.C. LEININGER, AND K.A. OSMAN. 1992. Sheep grazing
effects on coastal Douglas-fir forest growth: a ten-year perspective. For.
Ecol. Manage. 50:75–84.
SHAW, D.V., A. HELLBERG, G.S. FOSTER, AND B. BENTZER. 1988. The effect of
damage on components of variance for fifth-year height in Norway
spruce. Silv. Genet. 37:19–22.
WARING, R.H. 1987. Characteristics of trees predisposed to die. Bio­
science 37:569–574.
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