Genetic Resistance in Douglas ..fir to Damage by
Snowshoe Hare and Black..tailed Deer
EDWARD J. DIMOCK II
ROY R. SILEN
VIRGIL E. ALLEN
Abstract. Genotype of Douglas-fir significantly affected feeding selection for foliage by
both snowshoe hare and black-tailed deer in pen tests with captive animals. Nine clones
were rated independently for each animal species. Genotypes preferred by deer and hare
ranged up to 64 and 178 percent more attractive, respectively, than those least preferred.
Order and magnitude of damage resistance in pen tests, as predicted for full-sib F, prog­
enies based on preference shown among clones, closely conformed to resistance traits
indicated for parents. In one 4-family test, captive deer selected between resistant and
susceptible families with feeding incidence levels of 41 and 78 percent, respectively, at
the point of maximum difference between extremes. In another 4-family test, captive
hare also showed comparable selection of 35 and 82 percent between extremes. Resis­
tance to wild hare, in a 4-family field test with seedlings, also conformed closely to that
predicted from preferences established in clonal pen tests. Damage incidence levels
ranged from 56 to 86 percent for most resistant and susceptible families, respectively,
after one winter's exposure to severe hare clipping. A later 4-family trial with seedlings
exposed to wild hare in the field established close agreement among related materials in
clonal pen tests, family pen tests, and family field tests. Differences were highly signifi­
cant with the most resistant family damaged 37.5 percent and the most susceptible 62.5
percent after one winter. In this test, moreover, significant damage resistance was shown
by 2 families during a second winter of exposure. Genetic analysis suggests that resis­
tance to animals based on nonpreference is strongly inherited and chiefly additive. forest
Sci. 22:106-121.
Additional lcey words. Animal damage control, heritability, Lepus americanus, Odocoi­
leus hemionus columbianus, Pseudotsuga menziesii, seedling survival.
NATURAL RESISTANCE in trees to damage
by animals remains essentially unrecognized
and unexploited. Within-species variations
in such damage are not extensively docu­
mented and have usually been noted as in­
cidental traits in tree provenance studies.
Among important western conifers, only
ponderosa pine (Pinus ponderosa) has been
clearly shown to elicit variable foraging re­
sponses by herbivores. Documented obser­
vations concerning this species include:
mule deer (Odocoileus hemionus hemionus)
in South Dakota (Bates 1927; Leopold
1933, p. 273); mule deer, snowshoe hare
(Lepus americanus), and porcupine (Erethi­
zon dorsatum) in Oregon and Washington
106
I Forest Science
(Squillace and Silen 1962); and black-tailed
jackrabbits (Lepus californicus melanotis)
in Nebraska (Read 1971).
Douglas-fir (Pseudotsuga menziesii) is
an obvious candidate for scrutiny. A spe­
cies of relatively high susceptibility to ani­
mal damage (Moore 1940), Douglas-fir in
The authors are, respectively, principal silvicul­
turist, Pacific Northwest Forest and Range Ex­
periment Station, USDA Forest Service, Cor­
vallis, Oreg.; principal plant geneticist, Pacific
Northwest Forest and Range Experiment Station,
USDA Forest Service, Corvallis, Oreg.; and
forestry technician, Olympic National Forest,
USDA Forest Service, Shelton, Wash. Manu­
script received March 10, 1975.
..
.
_
young plantations frequently needs protec­
tion. Genetic resistance in the form of non­
preference might prove a useful adjunct to
traditional methods of damage control. De­
termination of possible benefits, however,
must await demonstration that resistance
traits not only exist but also affect damage
incidence by practical amounts.
Preliminary studies by Dimock (1971)
suggested that black-tailed deer ( Odocoi­
leus hemionus columbianus) would discrim­
inate as much as 2 : 1 between local races
of Douglas-fir, but almost entirely because
of differences in seedling size. With this
lead, we began further trials aimed at two
animals considered serious obstacles to re­
forestation in the Pacific Northwest-snow­
shoe hare and black-tailed deer.
The experimental approach was sequen­
tial. In a succession of interrelated studies,
we sought answers to the following ques­
tions regarding Douglas-fir: (1) Do geno­
typic differences in foliage affect feeding
selection by deer and hare; and, if so, how
consistently and to what degree? (2) Are
resistance traits due to nonpreference for
foliage transmittable through tree breeding;
and, if so, how predictably and to what
extent? (3) Can nonpreference be exploited
to give practical and predictable amounts
of animal resistance to seedlings in the
field? Answers to the first question were
obtained in 1967 and 1968 with foliage
materials from grafted clones tested on cap­
tive deer and hare. Insights from this ex­
perimental series led to similar preference
trials on captive animals in 1970 with fo­
liage from selected progeny of the clonal
parents previously tested. Finally, we prop­
agated seedlings from the same parents for
testing on free-ranging hare in the field.
Two separate trials were initiated-the first
in 1970 and the second in 1972-and each
was monitored for a 2-year period.
Parent Pen Tests
Procedures. Preference tests with cuttings
from 9 Douglas-fir clones were conducted
on captive snowshoe hare and black-tailed
deer in large outdoor pens maintained by
the U.S. Fish and Wildlife Service at Olym­
pia, Washington. The enclosure containing
snowshoe hare was 0.4 hectare ( 1 acre)
in area and, at various times during the
course of our studies, held 7 to 12 ani­
mals of both sexes. A 1-hectare (2.5-acre)
portion of a 4-hectare ( 10-acre) enclosure
holding about 25 animals was used for test­
ing cuttings on deer. Mixed-sex groups of
5 to 8 animals were randomly chosen for
individual tests, and numbers were held
constant during each trial. Hare and deer
moved freely within their respective enclo­
sures and at all times had access to cover,
a maintenance diet, and some natural for­
age. Preference testing with captive ani­
mals followed general procedures described
by Cardinell and Hayne (194 7) and Hil­
dreth and Brown (1955), with specific
methods of design and analysis outlined by
Dodge and others (1967) and modified by
Dimock ( 1971 ) .
Our selection of clones for parent pen
tests was both limited and arbitrary, and
necessarily reflects only part of the varia­
tion likely in a natural gene pool. The
Olympic National Forest's Dennie Ahl
Seed Orchard near Shelton, Washington,
provided a source of clones that had been
grafted from superior phenotypes growing
at middle elevations wtihin one township
on the northwestern part of Washington's
Olympic Peninsula. Since clones were not
equally represented in the seed orchard, the
sole criterion for selection was availabil­
ity. We therefore concentrated on clones
with best representation and confined selec­
tion to larger ramets about 5 to 8 meters
(16-26 feet) tall. Cuttings were chosen for
morphological similarity both within and
between clones, and about ten 30-centi­
meter cuttings were taken from the lower
crown of each selected ramet. To mini­
mize possible confounding due to genotype­
microsite interactions, the same part of the
orchard was used to sample all clones used
in any particular test. Also, insofar as pos­
sible, all cuttings were collected and subse­
quently tested during cool, rainy winter
months to minimize reductions in palat­
ability through desiccation. Normally, time
between collection and test installation dtd
not exceed 24 hours.
Heavy concentration of deer and hare in
volume 22, number 2, 1976 I 107
DAYS UNTIL BROWSED
DAYS UNTIL CLIPPED
8
~DEER
7
A
3
~HARE
6
2
.3
2
o--­
1. Mean time required for incidence of feeding by captive animals on Douglas-fir cuttings from
5 clones tested separately on deer and hare. (Browsing represents combined selection by 5 deer; clip­
ping represents combined selection by 8 hare. Each bar is the mean of 100 observations. Within
each test, means not superposed by a common letter differ significantly at p < 0.05.)
FIGURE
restricted areas provided extreme feeding
pressure and rapid testing. Cuttings were
offered to test animals simply by tamping
them into prepared spots so that each cut­
ting simulated a live seedling. As small
dtfferences in seedling size have been pre­
viously shown to influence feeding selection
by deer (Dimock 1971 ), all cuttings were
presented to both deer and hare at a con­
stant 25-centimeter (10-inch) height. Cut­
tings were checked daily for evidence of
feeding, and the day of earliest hare clip­
ping or deer browsing was noted for each.
Examinations in each test continued until
all cuttings had been fed upon--or for pe­
riods varying from 1 to 4 weeks.
A randomized block design was used in
all pen tests. Each trial contained 5 clones
with 10 cuttings per clone replicated in 10
108
I Forest Science
blocks. Cuttings were individually random­
ized at a 0.9- by 0.9-meter (3- by 3-foot)
spacing to assure that item-by-item discrim­
ination on the part of test animals would
be responsible for any observed differences
among clones. Two measures were used
to evaluate relative preference: ( 1 ) mean
exposure in days required for all cuttings
within a clone. to be clipped or browsed
and (2) where applicable, mean residual
height of cuttings within each clone after
1 week's exposure. Analysis of variance,
supplemented by the "Q" method, was
used to compare individual clonal means,
and linear correlation analysis to compare
clonal means in duplicate tests (Snedecor
and Cochran 1967). Results were consid­
ered significant at p < 0.05; highly signifi­
cant at p < 0.01.
DAYS UNTIL
BROWSED
DAYS UNTIL CLIPPED
8
w
7
ll
5
DEER_
~HARE
6
4
-
5
4
'L
2
2. Mean time required for incidence of feeding by captive animals on Douglas-fir cuttings from
5 clones tested separately on deer and hare. (Browsing represents combined selection by 6 deer; clip­
ping represents combined selection by 7 hare. Each bar is the mean of 100 observations. Within
each test, means not superposed by a common letter differ significantly at p < 0.05.)
FIGURE
A series of 6 winter-season tests were
used to evaluate the 9 clones upon which
subsequent work was based. Initially, we
conducted 2 simultaneous tests of the same
5 clones (8, 10, 13, 15, and 17) on deer
and hare in early 1967. Again including
clone 13 as a common standard, we added
4 different clones ( 1, 19, 22, and 23) in a
similar pair of simultaneous tests conducted
1 month later. Finally, to confirm differ­
ences observed in the latter 2 tests, we re­
peated them both with the same 5 clones
(1, 13, 19, 22, and 23), but with different
ramets and different groups of deer and
hare, in the winter of 1968.
Results and Discussion. For comparative
purposes, results for deer and hare are
shown together for the first pair of 5-clone
tests (Fig. 1). Significant clonal preferences
were demonstrated by each species of test
animal (F = 5.59 and 3.33 for deer and
hare, respectively, with 4 and 36 df).
Clone 13 was consistently least preferred
by each animal--differing significantly from
all other clones in the case of deer, and
from clones 10 and 15 in the case of hare.
A reasonably consistent order of prefer­
ence among all five clones was also shown
by both deer and hare, with only clone 15
ranking noticeably out of line. In view of
the close experimental control maintained
in testing, genotypic variation seemed the
most likely explanation for the differences
observed.
In the second pair of 5-clone tests in
1967 (Fig. 2), clonal preferences of obvi­
ously high significance were again shown
volume 22, number 2, 1976
I 109
196 7
DAYS UNTIL
BROWSED
DAYS UNTIL
~
A
6
1968
BROWSED
1967
-­
3
4 -
2
2­
--1
13
22
23
CLONE
19
3. Mean time required for incidence of browsing by captive deer on Douglas-fir cuttings from
5 clones tested separately in each of two successive years. (The 1967 test represents combined selec­
tion by 6 deer; the 1968 test represents combined selection by 8 deer. Each bar is the mean of 100
observations. Within each test, means not superposed by a common letter differ significantly at
p < 0.05.)
FIGURE
by both test animals (F = 32.90 and 32.22
for deer and hare, respectively, with 4 and
36 df). Of the 10 possible paired compari­
sons between clonal means in each case, 6
were significant for deer and 7 for hare.
However, though clone 13 was again least
preferred by deer, three others (1, 22, and
23) were even less preferred by hare. Only
one clone in each trial differed significantly
from all others-clone 13 with deer and
clone 22 with hare. Barring the inconsis­
tency of clone 13's preference ranking with
each animal species, the remaining 4 clones
ranked identically for deer and hare.
Though the 1968 test with deer pro­
ceeded more rapidly than its counterpart
in 1967, due to the vagaries of animal feed­
ing habits or to other variables affecting
clone palatability, deer discriminated among
clones in an order identical to that of a
11 0 I Forest Science
year earlier (Fig. 3). The correlation be­
tween both years' preference rankings for
all five clones was highly significant ( r =
0.98 with 3 df). Though some sensitivity
of the 1968 comparison was lost because
of heavy feeding pressure (F = 7.23 with
4 and 36 df), clone 13 significantly dif­
fered from all other clones as it had in
1967.
Results of the 5-clone test repeated on
hare in 1968 were also completely consis­
tent with 1967 data. Because of difficulties
in anticipating and subsequently regulating
animal feeding pressure, much sensitivity of
comparison by the measure previously used
(mean days of exposure) was lost due to
an excessively rapid test. In fact, none of
the clonal means compared in that way
differed significantly. An alternative mea­
sure, residual height of each cutting after
HEIGHT
AFTER
ONE WEEK (em)
2or---------------------------------------------------.
A
16
-
12
-
8
-
4
-
~
1967
~
1968
0
19
4. Mean residual heights of 25-centimeter Douglas-fir cuttings from 5 clones exposed for 1
week in separate tests to captive hare in each of two successive years. (The 1967 test represents com­
bined selection by 7 hare; the 1968 test represents combined selection by 12 hare. Each bar is the
mean of 100 observations. Within each test, means not superposed by a common letter differ sig­
nificantly at p < 0.05.)
FIGURE
1 week's exposure to hare clipping, gave
more readily interpretable results (Fig. 4).
It not only reflected a tendency for hare
to clip preferred clones repeatedly but also
provided a highly significant comparison
(F = 51.62 and 45.06 for 1967 and 1968
tests, respectively, with 4 and 36 df). Of
the 10 possible paired comparisons between
clonal means in each year, 7 differed sig­
nificantly in 1967 and 8 in 1968. Of
perhaps greater importance are the close
agreements between both years in order
and relative magnitude of preference (r =
0.92; p < 0.05 with 3 df). The high pref­
erence expressed for clone 19 in both trials
is also clearly evident.
Progeny Pen Tests
Data from the preceding tests both estab­
lish and confirm that variations in genotype
of Douglas-fir consistently influenced am­
mal feeding preference. Up to this point,
however, we had no evidence regarding ca­
pacity of the underlying factors involved to
combine through breeding and be passed
on to succeeding generations. We there­
fore proceeded to seek evidence of inher­
ited resistance stemming from nonprefer­
ence among F1 progeny.
Procedures. Trees in the Dennie Ahl Seed
Orchard within families averaged about 5
years old and 2 meters (7 ft) tall in 1970,
and were thus able to provide limited
amounts of foliage as a source of cuttings
for use in pen tests for comparing selected
families. Six of the clones previously tested
(1, 8, 10, 13, 19, and 22) were represented
as both parents (female x male) in each of
seven families (8 X 1, 10 X 1, 10 X 8, 13
volume 22, number 2, 1976
I 111
X 22, 19 X 1, 19 X 8, and 22 X 1). Ac­
cordingly, we conducted pen tests in early 1970 with 4 families (8 X 1, 10 x 1, 13 X 22, and 19 X 1 ) on deer and with 4 fam­
ilies (10 x 8, 13 x 22, 19 X 8, and 22 x 1) on hare. Assuming that previously established
preferences for different clones might be
ranked to predict approximate damage re­
sistance in their progeny, we combined all
9 clones used in preceding trials into a
composite preference array based on 1967
data (Figs. 1 and 2). Clone 13 served as
the common standard for combining data
by direct proportion for each test animal
separately. Then, again by direct propor­
tion, these data were converted to mean
exposure preference index (MEPI) values
by further adjusting so that the clones most
resistant to each test animal ( 13 with deer;
22 with hare) equaled 100. Relative mag­
nitude of differences between clones, as
demonstrated previously, thus remains un­
changed. MEPI values were then divided
into three subjective levels of estimated re­
sistance based on relative position in the
array:
Deer
Hare
MEPI
Clone value
Clone
13
22
15
100
127
130
22 23
17
8
23
135
135
147
1
10
19
149
152
164
MEPI
value
l
Estimated
resistance
level
100
127
141 Resistant 13 17
8
179!
204 .
238 Intermediate 19 15
10
250
256
278 Susceptible l
As ranked above for the 6 parents (under­
lined) represented in the 4-family tests de­
scribed previously, common levels of resis­
tance against both target animals appear associated with four clones (8, 10, 19, and 22) ; differing levels for deer and hare with two (1 and 13 ) . Predictions for full-sib families were
based on the premise that male and female
parents were equally capable of transmit­
ting traits leading to damage resistance or
112 I Forest Science susceptibility. Hence, in the 4-family trial
conducted on penned deer in 1970, the
following resistance levels were forecast for
the progeny under test:
Family
13 X 22
8X 1
10 X 1
19 X 1
Parental traits
Resistant X Resistant
Intermediate X Susceptible
Susceptible X Susceptible
Susceptible X Susceptible
Predicted
resistance
Resistant Intermediate Susceptible Susceptible Similarly, in the 4-family trial conducted
on penned hare during the same period, the
following resistance levels were forecast:
Family
22 X 1
13 X 22
19 X 8
10 X 8
Parental traits
Resistant X Resistant
Intermediate X Resistant
Susceptible X Intermediate
Susceptible X Intermediate
Predicted resistance Resistant Intermediate Susceptible Susceptible Testing procedures were similar to those
used in parent pen tests and we employed
the same facilities. Study design with mixed­
sex groups of deer ( 6 animals) and hare
(10 animals) was the same in each test: 4
families with 10 cuttings per family repli­
cated in 10 blocks. From 5 to 10 lower­
crown cuttings were taken per tree within
each cross until 100 cuttings had been ac­
cumulated per family. Randomized individ­
ually at a 0.9- by 0.9-meter (3- by 3-foot)
spacing, cuttings were tamped into place as
before at a constant 25-cm height and ex­
amined daily for first incidence of feeding
on main stems. Observations were contin­
ued until all cuttings had been browsed or
clipped. Percent differences in feeding se­
lection among families were evaluated daily
by analysis of variance to bracket that
portion of the test period in which signifi­
cant differences occurred. Percentages were
analyzed in raw form and as transformed
to angles by arc sine. Since transformed
BROWSING IN
PERCENT
100
IIII
80
i
i
........
-·-·-· 10 X 1
-19X1
60
i ~·
i
--- 8 X 1
- ..- 13 X 22
/i /
/
40
.,.""·'
, ....· I
20
·::.--'I /
;:r
..--·
...
/
/
,
..-
·:
.......J
~·
/
//
I,.._/
.
./ 1 - - ../
1:
1/
I
I
I: ,.,.f
__ ,.
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.J
I: /'
__
_.......·
/
-/
! ,-- , ..
_;,I I '
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/
...................
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//
/
/
//
:
/
I
!
. . . -· -· -· :;;__-:,_-:....-._-.::::::_-=-;;:::::-..:.;;~
/__ . . ,.r··-··-.. --./
~ ...../·
,.,.,. ;·
~·
I III I I I
I I
I
~./'
0._.__._._.._._.__._.__._.__._._.__._.__._._.._._.__._.__._._.
0
5
10
15
20
25
DAYS OF EXPOSURE
5. Cumulative incidence of browsing by 6 captive deer on main stems of Douglas-fir cuttings
from 4 full-sib families. (Each family comprises 100 cuttings. Differences exceeding bar lengths are
significant at p < 0.05.)
FIGURE
and untransformed percentages gave SIIDl­
lar results, raw percentages are presented
for clarity.
Results and Discussion. The 4-family trial
on penned deer required 26 days for all
cuttings to be browsed (Fig. 5). Discrimi­
nation among families began immediately
and differences were significant from the
5th through the 18th day, or for about one
half of the test period. Moreover, results
agreed closely with our predictions. Family
13 X 22 was most resistant of the four;
10 X 1 and 19 X 1 were most susceptible;
and 8 X 1 fell into an intermediate position.
From test beginning, incidence of browsing
on the most susceptible family (lOx 1) was
approximately twice that on the most resis­
tant ( 13 X 22), and it remained so on a
cumulative basis throughout the lOth day.
Penned hare clipped all cuttings from
the 4 families exposed to them in 15 days
(Fig. 6). In this case, discrimination among
families was not apparent until the 5th day
when preference for two families suddenly
became significant and remained so through
the 11th day. Highly significant differences
were clearly evident during this portion of
the trial and reached a maximum separa­
tion of 35 and 82 percent between family
extremes at 1 week. Although agreement
between test results and prediction was less
than perfect, it was nevertheless quite close.
At midtest, those families predicted to be
most susceptible (19 X 8 and 10 X 8) were
highly preferred over those rated interme­
diate (13 x 22) and resistant ( 22 x 1 ) .
The above results appear notable for
several reasons. Tests with progeny re­
vealed differences in animal feeding prefer­
ence that were sharply defined and similar
in magnitude to those previously shown
with parents. The differences seemed un­
volume 22, number 2, 1976
I 113
CLIPPING
IN
PERCENT
-· -·-_.-A--.._----­
""""'·-· . -· -·.....
""'·
100
~·
I I
--19 X 8
60
-·-·10X8
---13 X 22
· · .. 22 X 1
i
i
i
i
i
i
.....
~·
/""
/
~·
/
80
~·
.
i
.;.""""
/
/
.... /
,"' •I
:1
·I
:I
'I
:I
i
;I
•I
:I
'I
40
.i
-·
•I
Y'
,('
/·
.
:
.
..
/.
/
20
/
/
:,...,-;.·
/
III I I
5
10
DAYS
15
OF EXPOSURE
6. Cumulative incidence of clipping by 10 captive hare on main stems of Douglas-fir cuttings
from 4 full-sib families. (Each family comprises 100 cuttings. Differences exceeding bar lengths are
significant at p < 0.05.)
FIGURE
likely to be chance in view of both statisti­
cal significance and consistent demonstra­
tion with two species of test animal. Most
importantly, the relative animal resistance
of first-generation progeny appears to be
duectly predictable from parental charac­
teristics with a fair degree of accuracy.
Progeny Field Tests
Both magnitude and consistency of animal
preferences shown in pen tests suggested
that resistance in the form of nonpreference
might give effective protection in the field.
However, we had only been able to specu­
late that animals would discriminate under
field conditions in ways consistent with
their behavior as captives. Moreover, ef­
fective protection would require that con­
siderable numbers of seedlings be damaged
lightly or not at all over at least the peak
damage period in a typical field situation.
114 I Forest Science
Accordingly, we describe two different trials aimed at snowshoe hare as the target animal. Both studies included Douglas-fir seedlings rated for resistance to hare clip­
ping on the basis of their parentage. The first of these, installed in November 1970, was an attempt to discover if field perfor­
mance of full-sib families could be pre­
dicted from parental attributes as rated by clones in pen tests. In the second trial,
installed in November 1972, we attempted
to assess the reliability of clonal prediction
plus the comparability of both pen and field
tests with identical family groups.
Procedures. Seedlings for the 1970 study
were grown in cold frames at the Dennie
Ahl Seed Orchard for 1 year, then were
transplanted to cold frames at Olympia
in early 1970 for an additional season's
growth. The four families selected (1 X 22,
22 X 1, 1 x 8, and 8 X 10) had been arti­
:
·-
ficially bred from clones tested for hare
preference in 1967. To minimize possible
confounding due to variations in nursery
bed environment, we systematically spaced
portions of each family throughout each
cold frame; all families were subjected to a
common regime of irrigation and nitrogen
fertilization.
We also prepared in early 1970 to propa­
gate seedlings for the 1972 study-a 4­
family field test designed to duplicate the
1970 pen test on hare with cuttings. Using
standard controlled breeding methods, we
crossed selected ramets to reproduce the
families (22 X 1, 13 X 22, 19 X 8, and
10 X 8) ultimately needed for field testing.
Cones were gathered in late 1970 and
transported to Corvallis, Oregon, where
seed was processed and seedlings started
under greenhouse conditions in styrofoam
containers. Transferred to Olympia in mid­
1971, seedlings continued growth under a
uniform greenhouse regime of irrigation
and fertilization with nutrient solution. All
families were transplanted to cold frames
near the end of the growing season, and
cultivated by procedures similar to those
described for the 1970 study through the
1972 growing season.
Relative resistance for families used in
the 1970 field study was predicted, as be­
fore, from the array of preference charac­
teristics derived from 1967 clonal tests.
Only one of these families ( 22 X 1 ) had
been previously tested on captive hare. We
therefore predicted that this family and its
reciprocal cross (1 X 22) would show about
the same level of resistance, and that both
would rank more resistant than either of
the two families compared against them:
Family
22 X 1
1 X 22
1X8
8 X 10
Parental traits
Resistant X
Resistant
Resistant X
Resistant
Resistant X
Intermediate
Intermediate X
Susceptible
Predicted
resistance
Resistant
Resistant
Intermediate
Susceptible
Similarly, since the families compared in
the 1972 field study were identical to those
already pen tested as cuttings in 1970, we
would also predict unchanged resistance
levels for them: 22 x 1 (Resistant), 13
x 22 (Intermediate), 19 x 8 (Susceptible),
and 10 X 8 (Susceptible). In addition, we
had opportunity in this case to compare di­
rectly the results from three distinct ex­
perimental phases: pen tests with clones,
pen tests with families, and field tests with
families.
Field testing procedures varied but little
in the two studies, and chiefly due to dif­
ferences in numbers of available seedlings.
Both studies were installed as randomized
blocks. The 1970 trial contained 400 seed­
lings-four families with 20 seedlings per
family replicated in five blocks; the 1972
trial contained 576 seedlings-four families
with 36 seedlings per family replicated in
four blocks. As in pen tests, all seedlings
were individually randomized, but at a
spacing of 2.4 by 2.4 meters (8 by 8 feet)
to approximate commonly accepted stan­
dards of plantation density. Uniformity of
seedling height was sought in each trial­
32 centimeters (12.6 inches) in the 1970
study; 40 centimeters (15.7 inches) in the
1972 study-and attained by deep planting
of larger stock to a measured height ap­
proaching that of smallest seedlings. Vari­
ations among families in mean height at
planting were thus held to a minimum­
maximums of 3.5 centimeters (1.4 inches)
in the 1970 study; 2.2 centimeters (0.9
inch) in the 1972 study-and in no case
were they significant (p < 0.05). Both
studies were located within 25 miles of
Olympia. Test areas were selected to pro­
vide maximum exposure to snowshoe hare
on clearcuts logged 5 to 30 years previously
and were sufficiently separated to insure
that each replicate catered to a different
hare population.
In both 1970 and 1972 studies, seed­
lings were checked weekly during the first
winter after planting, and damage by hare
to terminal shoots was recorded as it oc­
curred throughout the season when clipping
of Douglas-fir seedlings is normally most
prevalent. We continued observations in
each study until about a month past May
bud burst to assure that all damage by hare
volume 22, number 2, 1976 I 115
CLIPPING
IN
PERCENT
100
IIIIII
80
/,/·~·-·-•''
60
,.~·
,.
......
-·-·-·-·-·
i
i
i
III I I I
p/
Jvl
/·
5
NOV
_
?/
.--:-.:.:.=,;:=:.:......f;;;;o
/~20
___
,------------------------­
. .. .. .._.. .. _.. _,,_
... /
//
i
--------------------------­
~··-"_"_
-·/'
i
40
,·-·-·''
,.,.,·''
DEC
-­
-·-·-·
--- ..-
10
JAN
WEEKS
FEB
8X10
1X8
22X1
1 X22
15
20
OF EXPOSURE
I
MAR
I
APR
25
MAY
7. Cumulative incidence of clipping (terminals only) by wild hare on Douglas-fir seedlings from
4 full-sib families, 1970-71 field test. (Each family comprises 100 seedlings. Differences exceeding
bar lengths are significant at p < 0.05.)
FIGURE
would be documented. Seedlings in both
studies were again evaluated following their
second winter's exposure to hare clipping,
but not on a weekly basis. (Confounding
of results due to deer browsing was not
encountered, as damage by this animal oc­
curred to less than 1 percent of the seed­
lings in each study.) Cumulative levels of
terminal shoot clipping in percent were
periodically analyzed during the first winter
by the same procedures used in family pen
tests. Analysis of variance was also used
to compare seedling status in terms of sur­
vival, total height, and damage to new ter­
minal shoots after two winters of exposure
to hare clipping.
Results and Discussion.
Snowshoe hare (1970).-Seedlings were
damaged severely from time of planting in
November 1970, to the end of February
116
I Forest Science
1971, when virtually all hare clipping
ceased. Only three seedlings were clipped
thereafter prior to resumption of damage
during the following winter. Discrimination
by hare began almost immediately (Fig. 7).
Differences between families were signifi­
cant by the 3rd week, highly significant by
the 4th week, and significant from the 13th
week throughout the remainder of the first
winter. Damage to terminal shoots at 25
weeks ranged from 56 percent for the most
resistant family ( 1 X 22) to 86 percent for
the most susceptible (8 x 10). The 30­
percent difference suggests that resistant
families could give practical amounts of
protection over at least one winter season.
Terminal clipping of the susceptible family
(8 X 10) accumulated to over twice that
on each of two resistant families (1 x 22
and 22 x 1) for about 2 months during the
period of most severe damage just after
planting.
CLIPPING
IN
PERCENT 80
60
II
- - 19X8
-·-· 10X8
---13 X 22 • • • • 22 X 1 .... ...... .......... 40
20
.. ..
..
5
NOV
DEC
••
I III
10
WEEKS OF EXPOSURE
JAN
I
FEB
I
I
15
MAR
8. Cumulative incidence of clipping (terminals only) by wild hare on Douglas-fir seedlings from
4 full-sib families, 1972-73 field test. (Each family comprises 144 seedlings. Differences exceeding
bar lengths are significant at p < 0.05.)
FIGURE
:
Of equal or possibly greater interest,
wild hare followed the predicted preference
order without exception. Nearly identical
resistance was shown by families 1 X 22
and 22 x 1 throughout the test, and there
was no evidence that either male or female
characteristics predominated in contributing
to resistance. Furthermore, the susceptible
(8 X 10) and intermediate (1 X 8) families
differentiated early and maintained their
relative rankings as predicted.
The capacity of resistance characteristics
to protect Douglas-fir seedlings against
snowshoe hare damage in the field for peri­
ods exceeding one season was not evident
in the 1970 study. Heavy animal pressure
resumed during the second winter after
planting, and there were some indications
that first-year effects confounded any com­
parisons that could be made thereafter.
Most notably, numbers of heavily damaged
seedlings differed significantly among famt­
lies after one winter's clipping. Though not
closely monitored, seedlings from suscepti­
ble families grew fewer and smaller shoots
than those from resistant families as a con­
sequence of having sustained heavier dam­
age the previous winter. Such effects, we
believe, influenced animal feeding selection
and thus biased subsequent comparison.
Reexamined the second winter, 49 weeks
after planting, the 1970 field study did not
reveal any significant differences among
families in mean seedling survival, seedling
volume 22, number 2, 1976
I 117
height, or incidence of damage to new ter­
minal shoots :
Family
Survival
(percent)
Height
(em)
Terminals
clipped
(percent)
1 X 22
22 X 1
1X8
8 X 10
99
97
93
95
27
25
27
27
55
55
48
49
Though resistance characteristics could well
have continued to operate, their effects may
have been nullified, as previously specu­
lated, by unequal availability of new foliage
among families. In any event, the param­
eters measured in the above study failed to
mdicate second-year protection.
Snowshoe hare (1972).-The 1972 field
study was checked first at 6 weeks after
November installation, weekly thereafter
until March 1973, then sporadically until
June 1973. Severe hare damage to seed­
lings occurred early and continued for 12
weeks until late February. Terminal clip­
ping during the first winter ceased at 16
weeks.
Field results (Fig. 8) corresponded well
with those from pen trials among both
clones and families. Family differences
were highly significant at 6 weeks and re­
mained so throughout duration of the test.
Maximum separation between resistant (22
x 1) and susceptible (19 x 8) families oc­
curred at 8 weeks with 27- and 58-percent
terminal clipping, respectively. The 31-per­
cent difference decreased only slightly to
25 percent at the end of the winter. Dif­
ferences between the two susceptible fami­
lies ( 19 X 8 and 10 X 8) were not signifi­
cant, a result agreeing fully with predicted
resistance as determined by clonal traits
and with demonstrated family resistance as
rated by pen testing (Fig. 6). The inter­
mediate position of family 13 x 22 in the
field agrees closely with prediction but is
somewhat inconsistent with the nearly iden­
tical ranking of 13 X 22 and 22 X 1 in the
pen. All elements considered, however,
concurrence among 3 independent evalua­
tions-prediction, the 1970 pen test, and
the 1972 field study-is generally high.
Seedlings were reexamined in early spring
118
I Forest Science
of 1974 at 68 weeks after planting. From
their practical ramifications, results were
distinctly more encouraging. Differences
in mean seedling survival and seedling
height, as in the 1970 field study, were not
significant. However, differences in hare
damage to new terminal shoots, though not
overwhelming, were clearly consistent with
results from previous pen tests and the
preceding year's damage patterns among
the same seedlings. (Family means not
followed by a common letter differ signifi­
cantly at p < 0.05.):
Family
22
13
19
10
X1
X 22
X8
X8
Survival
(percent)
Height
(em)
95
96
93
91
38
36
34
34
Terminals
clipped
(percent)
54
56
65
70
(a)
(a)
(ab)
(b)
The two most resistant families (22 x 1 and
13 X 22) were damaged 16 and 14 percent
less, respectively, than the one most sus­
ceptible ( 10 x 8). Though family differ­
ences in terminal shoot clipping were only
about half those of the previous year, they
were nevertheless significant (F = 6.46 with
3 and 9 df). Resistance characteristics were
evidently strong enough in this case to over­
ride any family biases due to previously
sustained damage.
Genetic Analysis
Procedures. Genetic analysis was performed
by use of parent-offspring correlations ac­
cording to the method of Falconer (1960).
Since different measures were used to eval­
uate results from pen and field trials, a
common statistic that could be applied to
all tests was determined for pen tests with
cuttings and for field tests with seedlings.
We termed this measure mid-test selection
index (MTSI) and defined it as the pro­
portion of each clone or family clipped or
browsed when half the materials in a par­
ticular trial had been fed upon.
By methods of direct proportion previ­
ously described, we combined results from
1967 clonal tests by using clone 13 as a
common standard to generate MTSI values
for all nine clones originally compared. As
cant for deer and also for hare (r = 0.90
and 0.97, respectively, with 7 df). There­
fore, we concluded that both MTSI and
MEPI should be similarly effective as pre­
dictors of progeny performance.
We then correlated actual performance
of each family, as determined by its MTSI
value, with its predicted performance as
estimated by mid-parent value (MPV).
This latter measure was calculated as the
mean index value--determined separately
by both MTSI and MEPI values-for any
two parents in a given cross and a statistic
expected to relate to the combined additive
genetic component of resistance expressed
in first-generation progeny.
Finally, we tested the actual correlation
between deer and hare preferences on the
basis of demonstrated clonal traits as mea­
sured by both MEPI and MTSI.
TABLE 1.
Mid-test selection index
(MTSI) and mean exposure preference in­
dex (MEPI) values for deer and hare among
nine Douglas-fir clones. 1
Deer
Hare
Clone
MTSI
MEPI
MTSI
MEPI
1
8
10
13
15
17
19
22
23
53
44
53
37
48
55
60
47
53
149
135
152
100
130
135
164
127
147
42
67
69
47
67
47
59
23
29
141
238
278
179
256
204
250
100
127
1
Each statistic represents a mean of 100 ob­
servations.
before, deer and hare were rated sepa­
rately. We then compared MTSI values to
the corresponding mean exposure prefer­
ence indices (MEPI's)-the values actually
used to rank predicted progeny resistance­
for the same clones (Table 1). Correlation
between the two indices was highly signifi­
Results and Discussion. Although limited
by the small number of families (10) tested
in pen and field on deer and hare, the cor­
respondence between parent and offspring
gave ample evidence of strong additive ge­
netic variation (Table 2). Correlations be-
TABLE 2. Relationship between mid-parent value (MPV) and mid-test selection index
(MTSI) in jour trials with full-sib Douglas-fir families. 1
Trial
Test
animal
MPV
Family
Predicted
resistance
MEPI basis
MTSI basis
MTSI
Resistant
Intermediate
Susceptible
Susceptible
34
46
67
53
1970
pen test
Deer
13 X
8X
10 X
19 X
22
1
1
1
113.5
142.0
150.5
156.5
42.0
48.5
53.0
56.5
1970
pen test
Hare
22
13
19
10
X
X
X
X
1
22
8
8
120.5
139.5
244.0
258.0
32.5
35.0
63.0
68.0
Resistant
Intermediate
Susceptible
Susceptible
31
31
65
73
1970--71
field test
Hare
22
1
1
8
X
X
X
X
1
22
8
10
120.5
120.5
189.5
258.0
32.5
32.5
54.5
68.0
Resistant
Resistant
Intermediate
Susceptible
33
34
62
71
1972-73
field test
Hare
22 X
13 X
19 X
10 X
1
22
8
8
120.5
139.5
244.0
258.0
32.5
35.0
63.0
68.0
Resistant
Intermediate
Susceptible
Susceptible
35
45
62
58
1
In all three 1970 trials, each MPV and each MTSI represent means of 200 and 100 observations,
respectively; in the 1972 trial, means of 288 and 144 observations, respectively.
volume 22, number 2, 1976
I 119
tween MPV and MTSI were constrained
by only two degrees of freedom in each of
the four 4-family trials. Therefore, signifi­
cance could be demonstrated only by very
high correlation coefficients (r values ex­
ceeding 0.95 at p < 0.05 = *, and 0.99 at
p<0.01=**):
MPV-MTSI
correlation coefficients
Trial
1970 pen test
1970 pen test
1970-71 field
1972-73 field
on deer
on hare
test on hare
test on hare
MEPJ
MTSI
basis
basis
0.818
0.992**
0.969*
0.951 *
0.790
0.998**
0.991 **
0.931
Although MPV -MTSI correlations in the
1970 pen test on deer were not significant,
they were nonetheless similar and encour­
agingly high. In the case of hare, all three
trials indicated that MPV's for clones could
effectively predict relative levels of resis­
tance for progeny. Moreover, MPV's based
on either MEPI or MTSI appeared equally
effective as predictors.
Heritability values can be estimated
directly from the regression coefficients
of parent-offspring relationships (Falconer
1960). If MTSI-based MPV's are pro­
portionally adjusted to a scale equivalent to
that of MTSI values for offspring, resulting
regression coefficients for each of the pre­
ceding family trials are 1.74, 1.19, 1.03,
and 0.62, respectively. Heritability esti­
mates exceeding 1.00 may arise by chance
with so few observations, but more prob­
ably stem from nonrandom choice of ex­
perimental materials. Thus, the above
estimates are unusable for prediction of
potential gain, but obviously express a high
component of additive genetic variation.
Similarities between deer and hare in
feeding preferences for identical Douglas­
fir genotypes are not as close as indicated
by subjective levels of resistance estimated
from data in Figures 1 and 2. These levels,
which depend upon ranking in an array
rather than the magnitude of individual in­
dex values, lend an exaggerated impression
of preference agreement. In fact, agree­
ment in feeding preference between the
120 I Forest Science
two animals is anything but close when ac­
tual index values are compared (Table 1 ).
Correlations between deer and hare prefer­
ences among all nine clones were not sig­
nificant for either index (r = 0.26 and 0.03
for MEPI and MTSI, respectively, with 7
df). Therefore, although preferences shown
by both animals agree in certain gross re­
spects, underlying factors that govern palat­
ability of Douglas-fir foliage to each ani­
mal species probably differ in more ways
than they agree.
Conclusions
The preceding series of interrelated experi­
ments provides abundant evidence that ge~
netic factors in Douglas-fir can measurably
influence the palatability of its foliage to at
least two animal species that damage forest
plantations by their feeding activities. Pref­
erences for morphologically similar but ge­
notypically different foliage approached a
ratio of 5 : 3 for black-tailed deer and ex­
ceeded 5 : 2 for snowshoe hare. Deer and
hare showed both parallel and differing
preferences for genetically alike material.
However, preference agreement was almost
certainly more apparent that real. Both
animal species probably react to a complex
of underlying factors that variously affect
the palatability of Douglas-fir to each.
That factors affecting preference could
be predictably transmitted to full-sib Doug­
las-fir progeny by crossing clonal parents
was clearly demonstrated. Hence, we sug­
gest that animal resistance through non­
preference for seedlings could become a
practical aim for tree improvement pro­
grams. Moreover, such animal resistance
appears to be strongly inherited, and the
genetic component of variation appears to
be chiefly additive. Differences between
susceptible and resistant progenies were on
the order of 2: 1 in pen tests with captive
deer and hare and also in field tests. with
wild snowshoe hare over a full winter sea­
son. These results imply that not only is
practical animal protection in the field at­
tainable, but also that field performance
of progeny is predictable. Correlations be­
tween performance of parent and off­
spring were sufficiently high to suggest that
full-sib progeny resistance could be accu­
rately estimated from parental characteris­
tics alone; that is, without the necessity of
progeny tests.
As forest practices become more in­
tensive, most commercial forests in the
Douglas-fir region are now beginning tree
improvement programs. The potential for
animal resistance as a forest protection tool
is high. Obviously, it must be compatible
with more highly sought after traits affect­
ing quantity and quality of tree growth.
Our demonstration of damage resistance as
a function of varying animal preference
among superior phenotypes within a local
race of Douglas-fir is especially encourag­
ing. Exploitation of local variation would
seem a more promising approach toward
attaining practical resistance than one aimed
at utilizing racial variation among widely
differing provenances.
Our work merely shows that practical
levels of animal resistance in Douglas-fir
exist and that their exploitation is possible.
Success will hinge upon development of less
costly and more rapid methods for evalu­
ating parents and progeny. Considerably
~ore research will be needed to expand
upon our findings and translate them into
useful future applications.
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25:610.
CARDINELL, H. A., and D. W. HAYNE. 1947.
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Stn Q Bull 29:303-315.
DIMOCK, E. J., II. 1971. Influence of Douglas­
fir seedling height on browsing by black-tailed
deer. Northwest Sci 45:80-86.
DoooE, W. E., C. M. LoVELESS, and N. B.
KVERNO. 1967. Design and analysis of forest­
mammal repellent tests. For Sci 13:333-336.
FALCONER, D. S. 1960. Introduction to quanti­
tative genetics. Ronald Press Co, New York,
363 p.
HILDRETH, A. C., and G. B. BROWN. 1955. Re­
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age by rabbits. US Dep Agric Tech Bull 1134,
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LEOPOLD, A. 1933. Game management. Charles
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volume 22, number 2, 1976 I 121