Effect of Animal Damage on a Young
Coniferous Plantation in Southwest
Washington
ANIMAL DAMAGE is one hazard be­
setting young coniferous planta­
tions.
Plantations are usually
established in cut- and burned­
over areas where ecological condi­
tions have been so radically altered
that animal populations are often
high. The sequence in the Pacific
Northwest is a familiar one: re­
moval of the forest, often followed
by slash or accidental fire, results
in a great increase in the vegeta­
tion preferred by many animals,
and populations are quick to build
up. Frequently, deer, rabbits, and
rodents, particularly mountain bea­
vers, become so plentiful that the
animals deplete their new food
source and are forced to eat the
less palatable plant species, includ­
ing forest seedlings that may have
been planted. This threat to plan­
tation success is often a serious
one.
The Puget Sound Research Cen­
ter, a branch of the Pacific North­
west Forest and Range Experi­
ment Station; Weyerhaeuser Tim­
ber Company; U. S. Fish and
Wildlife Service ; and Western
Washington Experiment Station of
the State College of Washington
have cooperated in a study of this
problem. The experiment, intended
to explore the nature of the prob­
lem and the type of study required,
was begun in 1947. It is designed
to trace the effects of deer, rabbits,
mountain beavers, and cattle on a
young plantation of Douglas-fir,
Port-Orford-cedar, and western
hemlock established in an area of
high animal populations on the
Clemons Tree Farm in southwest­
ern Washington.
Animals Responsible for Damage
The deer inhabiting the area is
the black-tailed deer ( Odocoileus
hemionus columbianus) , common
to western Washington. This deer
is overnumerous in the area (popu­
lation estimated at 30 or 40 per
square mile1) and deer have con­
tinually utilized many of the
browse plants so that a browse line
is readily apparent.
The mountain beaver (Aplodon­
tia rufa rufa) is a burrowing ro­
dent of robust build commonly
found west of the Cascade range.
Its food consists of almost any
vegetation that grows in its forag­
ing area, including bracken fern.
The animal frequently girdles tree
roots below ground, killing the tree
or causing it to be blown over. It
also clips twigs or branches up to
% inch in diameter; sometimes it
climbs small trees, prunes off all
branches and cuts the leader.
Mountain beavers were numerous
on the area chosen for the experi­
ment and were presumed to have
been at least partly responsible for
earlier plantation failures in the
vicinity.
The "rabbit" of western Wash­
ington is, strictly speaking, a hare,
Lepus
americanus
washingtonii.
When abundant, the animals are
destructive to newly planted seed­
lings, clipping them off often ·just
above the ground. Rabbits follow
a population cycle and although at
times practically none are present
they may, as this experiment has
shown, increase enormously in a
short time.
Domestic cattle frequented the
area from ranches at lower eleva­
tions surrounding the experimen­
tal area. Undoubtedly, cattle do
damage seedlings and plantations
in certain circumstances, but dur­
ing the course of this experiment
so few of them were on the area
that they were not important.
Other animals, notably grouse
(which eat tree buds and needles)
and mice, do at times damage plan­
tations, and some injury by these
animals was noted during the ex­
periment. Ants, also, are at times
1 Populatio1,1 estimates were made by
the Washington State Game Commission,
who also have cooperated in other phases
of this study.
730
George R. Staebler,
Paul Lauterbach, and
A. W. Moore
S. Forest Service, Weyerhaeuser Tim­
ber Company, and U. S. Fish and Wild­
life Service, respectively.
U.
destructive; they girdle stems just
above the root collar and the
girdled portion becomes infested
with aphids, which the ants "cul­
ture. '' Damage by these animals
was not followed in this experi­
ment.
Description of tlte Experiment
The original objective of the ex­
periment was to evaluate damage
to a young coniferous plantation
by deer, cattle, and mountain bea­
ver. Almost no rabbits were on the
area when the experiment was
planned. Hence, no account was
taken of them in designing the
experiment, but rabbits immedi­
ately staged an upsurge in numb rs
and soon became the most damag­
ing animal, preventing to some ex­
tent a rigorous statistical analysis
of experimental results.
Three cover types were recog­
nized on the area, which had been
logged in 1927 and 1928 and slash­
burned in 193 0. These were: (1)
grass type, characterized by stands
of vel vet grass ( Holcus lanatus),
with lesser amounts of bracken
(Pteridium aquilinum), brush, and
sword fern (Polystichum muni­
tum) ; (2) low brush, mostly Ore­
gon grape ( Odostemon aquilfol­
ium) less than 3 feet high; ( 3 )
bracken, characterized by heavy
growth of bracken.
Scattered trees from a 1941
planting were present on the area
and almost all bore some sign of
animal damage in 1947. The ear­
lier plantat1ons had been judged
a near failure, the most apparent
reason being animal interference.
The experiment consists of
twelve 1;4 -acre plots planted in
March 1947, to mixtures of 1-0
Douglas-fir and Port-Orford-cedar,
and wildling western hemlock.
Three additional plots 25 feet
square were planted to Douglas-fir
in April 1948. All planted seed­
.
731
OCTOBER 1954
lings were staked with cedar stakes.
Six of the large plots are in an
area from which the mountain bea­
ver have been elim nated by trap­
ping and poisoning. The other six
are in an area where this animal
has been undisturbed. Of the six
plots in each series, two are in each
of the three cover types : grass, low
brush, and bracken. Of each pair
of plots, one is fenced against cat­
tle and the other is not. The three
species of trees are planted on each
plot. The three small plots are
fenced against rabbits, deer, and
cattle, and are located on· the
mountain beaver-free areas, hence
provide total exclosures.
It will be noted that each plot is
different in the treatment received,
without replication. A rough anal­
ysis of variance of results is pos­
sible, however, by using treatment
interactions as experimental error.
The plots were inspected ten
times after planting, between
spring 1947 and spring 1951. At
each inspection, survival was de­
termined, each seedling was ex-·
amined for damage, and the dam­
age classified as ruminant (cattle
or deer-damage not separable
from examination of seedling),
mountain beaver, or rabbit. Each
year the average height of the seed­
lings was determined from sample
measurements. ·Thus, s u r v i v a 1,
height, and damage on the different
plots may be compared. The effect
of cover type, tree species, moun­
tain beaver, and cattle fences on
these variables may be evaluated.
In addition, changes in the small
plots, from which all animals have
been excluded, may be compared to
results found on the larger plots.
Results of the Experiment
The four-year-old plantation is
at least moderately successful, in
spite of exposure to animal dam­
age. Survival of all species except
hemlock has been satisfactory, and
the poor recorQ. for hemlock is not
due to animals. Height growth,
though reduced on damaged trees,
is satisfactory. A comparatively
high percentage of the surviving·
seedlings are deformed, most like­
ly as a direct result of animal dam­
age.
Damage to Seedlings
The percentage of trees deformed
(Table 1) was used as one measure
of the extent of damage. Abnorm­
ally bushy trees or trees with more
than one leader were classed as
deformed. Undoubtedly, some de­
formity is a natural occurrence, but
in this area it is probable that the
majority is a result of animal dam­
age. In the small plots, from which
all animals were excluded, 23 per­
cent of the Douglas-fir_ seedlings
were deformed after three years,
indicating that some trees will be
deformed even where no animal
damage occurs. On these plots it
is believed that the deformity is
at least partly a result of frost
damage in the nursery before the
stock was lifted. The percentage
of trees deformed cannot be com­
pared directly to the damage after
four years on the larger plots.
Three important results are
brought out by the four years of
record and observation:
1. Rabbits did the most damage
to planted trees. Deer were next in
importance, then cattle. Although
deer and cattle damage could not
always be positively segregated, in
all likelihood the cattle damage was
of little significance. Mountain
beaver damage was negligible dur­
ing the course of the experiment.
2. Rabbit damage was heaviest
in the bracken and was largely re­
sponsible for the high percentage
of deformity in this type (80 per­
cent of. the Douglas-fir, 67 percent
of the cedar, and 34 percent of the
hemlock). Most. of the damage oc­
curred in the summer months.
3. Douglas-fir throughout the
experiment was the most damaged
species, hemlock the least, Port­
Orford-cedar suffered little dam­
age until it was 3 years old, when
deer began to browse it. Deer; .ap­
parently, have to acquire a taste
for this exotic species. After 4
years, 71 percent of the surviving
Douglas-fir was deformed, 46 per­
cent of the Port-Orford-cedar, and
20 percent of the hemlock.
Unfortunately, the experiment
was not designed to e aluate rab­
bit damage, and the 'presence· of
rabbits on all plots has more or less
upset the statistical evaluation of
damage caused by the other ani
mals. Even if not pl- anned for,
however, the sudden increase in
rabbits provided useful informa­
tion. The trend in damage is amp e
evidence of the rapid increase:
Season
Percent of live
trees cropped
Summer 1947
Winter 47 -48
Summer 1948
Winter 48- 49
Summer 1949
Winter 49- 50
Winter 50 -51
5
23
15
15
21
35
38
Survival apparently was not ·ap­
preciably reduced in this planting,
but rabbits have been known to
completely wipe out a plantation.
It is likely that this one survived
the damage because there were few
rabbits when the trees were plant­
ed. By the time the rabbits became
numerous, the seedlings had one or
two seasons of growth and were
better able to withstand the high
incidence of damage.
The difference in deformity be­
tween "mountain beaver present"
and ''mountain beaver absent,''
TABLE I.-PERCENTAGE OF LIVE T&EES DEFOEMEI>, BY
Treatment
Port-Orford­
cedar
Western
hemlock
Percent
Percent
Percent
------"---------------------
---- ---------
VARIATION' TES 'l!lD
Douglas-fir
plots
.... -----------------------------------Mountain beaver present
Mountain beaver absent
l<'enced
Unfenced
Grass type
Low brush type
Bracken type
All
SOtTRC!EIS OF
--------------------- ----
----- -------------------------- ---- ----
------- -------------- -- ------------------ ----- ------------------ -------'------------------ ------- ---------- ------- ------ ___________
---------- ----------- ---------------- ----------------- ------------ ---- -- ------- --------------------------- -----
"---"-·---------·---------------------------------------·-
71
67
74
66
75
64
69
80
46
42
51
41
51
50
22
67
20
12
28.
25
14
12
13
34
732
JOURNAL OF FORESTRY
TABLE
2.-8URVIVAL
OJ!' V.Al!.IATION TES'TFJD
Western
Port-Orfordcedar
hemlock
Douglas-fir
PEROEN'JJAGES,
BY
Treatment
All plots
Mountain beaver present - Mountain beaver absent
--
------------ ----------------------
-
SouRc'ES
Percent
71
75
68
72
71
76
79
60
77
81
- 72
--------------------- --------
-
- ------------------------------ ------
-------
-----
-
------ -------------------
: : ed··::::::=:::::::::::::::=::::::::::::::::::::::::::=:::::::::::::
i
��:��q� :::::::::::::::::::::::::::::::::::::=::::::::::::=::: ii
though significant, is not a real re­
sult of the treatment, but is due to
variations in rabbit damage, a fac­
tor not under experimental con­
trol. At the start of the experi­
ment, c a s a l observation h d
seemed to indicate that mountam
beavers had been an important
factor in the failure of earlier
plantations. Certainly there was a
high population of the animals, and
their burrows and tunnels were
everywhere on the area. Under­
mined trees and mountain beaver­
pruned trees were easily found.
Yet careful observations made on
the plots planted in 1947 showed
the damage done by mountain bea­
vers was so small as to be insig­
nificant. Never more than one or
two percent, usually none, of the
trees were found to have been dam­
aged by this animal. lt is probable
that the earlier damage was over­
estimated because of its conspicu­
ousness in the older, understocked
stands.
Between fenced and unfenced
plots, the differences are too small
to be safely ascribed to the fences,
because of high experimental error.
Survival
After four growing seasons, sur­
vival of the planted trees was:
Douglas-fir, 77 percent; Port-Or­
ford-cedar, 71 percent; Western
hemlock, 27 percent (Table 2).
Survival is usually quoted to indi­
cate the degree of success of a plan­
tation. The plantation represented
in the experimental plots must_ be
considered successful,_ except for
hemlock, in spite of exposure to
animal damage. The poor showing
of hemlock is less a result of dam­
age than of other causes, notably
the use of wildling stock, much of
27
30
23
27
27
25
28
27
it too large for successful plant­
ing. Also, it is a known fact that
hemlock is difficult to establish in
plantations.
Survival was substantially less
on plots with no mountain beaver
and in the bracken type. Statistical
analysis shows that these differ­
ences are not a chance result. As
in the analysis of deformity, sur­
vival differences between plots with
and without m,ountain beaver are
probably a result of rabbit damage
and not ascribable to anything
under experimental control.
The poor survival in the bracken
type (69, 60, and 27 percent for
the three species) is undoubtedly a
real result and may be partly due
to the heavy damage that rabbits
inflicted on the trees in that type.
Also, the bracken type, because of
intense competitien and deep_
shade, is an inhospitable site for
seedlings. It is interesting to note
that although Douglas-fir and Port­
Orford-cedar survival is much low­
er in the bracken type, hemlock
survival is just as good as in the
other types (27 percent in the
bracken, 25 and 28 percent in the
grass and brush types). During
the :first years of the experiment
hemlock, probably as a result of
its tolerance, actually survived
better in the bracken.
The three plots having the high­
est percentage of deformed Doug­
las-fir-from which we may infer
that they suffered most from ani­
mal damage-also had the lowest
survival of Douglas-fir. Hence,
animal damage probably caused
some mortality in addition to de­
forming surviving trees.
·
Tree Heights
·
After four growing seasons in
the :field the effect of damage on
height growth was quite pro­
nounced. Average heights of the
well-formed and deformed trees
were:
Well­
formed Deformed
Species
Douglas -fir
Port-Orford- cedar
Western hemlock
Inches
_______ ________________
•
__________
______________
30.5
14.1
16.1 Inches
20.9
11.9
9.2
Separation of the a v e r a g e
heights by sources of variation
tested showed that differences in
heights among species and among
cover types are large and unmis­
takable (Table 3 ). It is also ap­
parent that bracken sharply re­
stricts the height growth of Doug­
las-fir but has almost no effect on
hemlock. Port-Orford-cedar is in­
termediate. Differences between
plots with and without mountain
beaver and between fenced and un­
fenced plots are hardly explain­
able in terms of animal damage
actually controlled in the experi­
ment. More likely they are due to
vagaries of rabbit distribution and
the partial exclusion of deer on
fenced plots.
The difference between species is
most likely an inherent quality and
not connected with animal damage.
Height difference between cover
types is probably due partly to
damage and partly to the fact that
Douglas-fir and Port-Orford-cedar
grow more slowly in the bracken
type than in the other two types.
Hemlock, being more tolerant,
would be less affected. This obser­
vation is born out for Douglas-fir
by the trees in the total exclosures,
where the rank in height '(with no
damage) for the three cover types
is the same as on the larger plots.
However, there is no doubt that
the differences are accentuated by
heavier rabbit damage occurring
m the bracken.
Summary and Conclusions
This experiment was intended
to be exploratory in nature. The
:findings are based on the results
of only one planting on only one
OCTOBER 1954
733
area. Some of the findings are un­
doubtedly chance occurrences that
would not always be repeated un­
der similar circumstances. Never­
theless, the study has produced
valuable information on what hap­
pens to a plantation in the first
four years of its life in an area of
high animal populations. The im­
portant conclusions are:
1. The plantation was successful
in an area where mountain beaver
and deer populations were high.
After one or two growing seasons
it also withstood heavy rabbit dam­
age. Extensive damage, while caus­
ing a high percentage of deformed
trees and severe reduction of
height growth, did not lessen sur­
vival appreciably.
2. Rabbit populations built up
extremely fast, resulting in exten­
sive dam age. It should be assumed
that the experimental planting suc­
ceeded because of an opportune
timing of establishment.
TABLE
3.-HEIGHTS OF 8EED'LINGS,
SOURCES OF VARIA.TION
Douglas-fir
TrAatment
All plots
Mountain beaver present --------------------------------Mountain beaver absent -----------------------------------Fenced ------------------------------------------------------------Unfenced ------------------------------------------------Grass type ------------------------------------------------------Low brush type --------------------------------------------------Bracken type -----------------------------------------------------
---------------------------------------------------------------
-
3. Damage from mountain bea­
ver was negligible during the four
years of observation.
4. In this mixed planting, Doug­
las-fir was the most damaged spe­
cies, although deer did considerable
damage to Port-Orford-cedar after
the third year. Hemlock seedlings
were apparently avoided.
5. Douglas-fir, even though most
damaged, was superior to the other
species in survival and height
growth.
H. W. Shawhan
31
BY
Port-Orford·
cedar
Inches
22.2
24.0
20.4
22.3
22.1
28.4
26.7
11.6
(1890-1954)
28, 1890 in Cynthiana, Ky., he was a graduate of the old
1913; F.E. 1914).
Biltmore Forest School (B.F.
Following employment with lumber companies he was appointed state for­
1933, and director of the state conservation commis­
1934, which position he held until 1940.
ester of West Virginia in
sion in
He served as an Army officer in both World Wars, rising from lieutenant
to colonel.
12.0
13.1
10.8
14.1
9.8
13.3
13.8
8.8
Western
hemlock
Inches
14.5
15.2
13.9
16.2
12.9
15.5
14.2
13.9
Of the cover types observed,
heavy bracken seemed to be the
poorest risk for a plantation. This
was partly due to competition from
the bracken and partly to concen­
tration of rabbits in the type, par­
ticularly during the s u m m e r
months.
7. Not all deformity was due to
animal damage. Some trees were
deformed even where damage was
completely prevented.
6.
H. W. Shawhan, former West Virginia conservation director, died August
at Goshen, W. Va. He was 63 years of age.
Born November
Inches
TESTED