BASKIN, C. C. 1981. Accelerated aging test. In Handbook of vigour test methods (D. A. Perry, ed),
p 43-48. Int Seed Test Assoc, Zurich. 72 p.
BoNNER, F. T., and T. R. DELL. 1976. The Weibull function: a new method ofcomparing seed vigor.
J Seed Techno11:96-103.
BoNNER, F. T., and J. A. Voz:z.o. !983. Measuring southern pine seed quality with a conductivity
meter-does it work? In Proceedings 1982 southern nursery conferences (J. P. Brissette and C.
W. Lantz, comps), p 97-105. USDA Forest Serv Tech Pub! R8-TP4, 312 p. South Reg, Atlan­
ta, GA.
CAMPBELL, R. K., and F. C. SoRENSON. 1979. A new basis for characterizing germination. J Seed
Techno! 4:24-34.
CzABATOR, F. J. 1962. Germination value: an index combining speed and completeness of pine seed
germination. Forest Sci 8:386-396.
GooDCHILD, N. A., and M.G. WALKER. 1971. A method of measuring seed germination in phys­
iological studies. Ann Bot 35:615-621.
GRABE, D. F. 1964. Glutamic acid decarbosylase activity as a measurement of seedling vigor. Proc
Assoc Off Seed Anal 54:100--109.
JANSSEN, J. G. M. 1973. A method of recording germination curves. Ann Bot 37:705-708.
MATTHEWS, S., and A. A. PowELL. 1981. Electrical conductivity test. In Handbook of vigour test
methods (D. A. Perry, ed), p 37-42. lnt Seed Test Assoc, Zurich. 72 p.
MooRE, R. P. 1971. Tetrazolium evaluation of tree and shrub seeds. 16th ISTA Congress. Preprint
No 69, 7 p.
MooRE, R. P. 1973. Tetrazolium staining for assessing seed quality. In Seed ecology (W. Heydecker,
ed), p 347-366. Penn State Univ Press, London. 578 p.
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individual sugars and total soluble carbohydrate materials in plant extracts. Agron J 55:523-525.
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Int Seed Test Assoc 33:531-540.
PERRY, D. A. 1981. Topographical tetrazolium test. In Handbook of vigour test methods (D. A.
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WooDSTOCK, L. W., and D. F. GRABE. 1967. Relationship between seed respiration during imbibition
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Forest Sci., Vol. 32, No. 1, 1986, pp. 178-184
Copyright 1986, by the Society of American Foresters
Estimates of Genetic Parameters for Deer Browsing of Douglas-fir
Roy R. Silen, William K. Randall, and Nancy L. Mandel
ABsTRACT. This report evaluates genetic variation in wintertime deer browsing among full-sib
families ofDouglas-fir. Cuttings of 10-year-old progeny of a mating ofsix trees as females crossed
with II trees as males provided estimates of heritability, combining ability, and genetic gain in
browsing resistance. Open-pollinated progeny and cuttings from parent trees were also included
in the study. An average of 46 percent of the cuttings were browsed after 38 days exposure.
Browsing among seedling families ranged from 39 to 69 percent. Sixty-five percent of the total
genetic variation was additive. Reciprocal, maternal, and specific combining ability components
were nonsignificant. Family heritability was estimated at 0. 73. Gain per generation, based on
The authors are, respectively, Project Leader, USDA Forest Service, Pacific Northwest Forest and
Range Experiment Station, Forestry Sciences Laboratory, Corvallis, Oregon; Geneticist, USDA Forest
Service, Pacific Northwest Region, Siuslaw National Forest, Corvallis, Oregon; and Statistical Math­
ematician, USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Forestry
Sciences Laboratory, Corvallis, Oregon. Manuscript received 23 July 1984.
178 I
FOREST SCIENCE
selecting 10 percent of the most resistant families, was estimated at II percent.
178-184.
ADDITIONAL KEY WORDS.
FoREST
Sa. 32:
Pseudotsuga menziesii, selection, animal damage, resistance.
IN CONTRAST with the low carrying capacity for black-tailed deer (Odocoileus hemionus
columbianus) in shaded virgin timber stands, high numbers of deer now inhabit the brushy
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) clearcuts ofwestern Oregon and Wash­
mgton. In this region terminal shoots on an estimated 20 percent of seedlings are browsed
annually during their first S years (Black and others 1979), resulting in about a 20-percent
loss ofvolume growth after 16 years (Batdorffand Fauss 1981). Most feeding occurs early
m the spring period of new growth, although heavy browsing sometimes occurs in winter.
Browsing levels are highly variable and seedling mortality is rare. Where browsing is light
or moderate, control is probably not needed. Where repeated browsing encourages brush
species to overtop seedlings, however, poorly stocked stands may result. Such areas are
predictable by biologists. Artificial barriers, special hunting permits, provision of more
preferable food, and use ofseedlings resistant to browsing provide possible control methods.
No method is totally satisfactory, but use of resistant seedlings may be the least costly if
there is no adverse genetic correlation with growth and survival. This report evaluates
variation in browsing by black-tailed deer among full-sib families of Douglas-fir. Estimates
are provided ofheritability, general and specific combining ability, maternal and reciprocal
effects, and genetic gain for resistance to browsing. In addition, we explored the phenotypic
correlation of resistance to deer browsing versus tree growth.
LITERATURE REVIEW
This study follows directly from earlier research on browsing of Douglas-fir by deer and
hare. Longhurst and others ( 1968) present evidence for resistance of trees and grasses to
browsing by deer and show that bottle-fed deer have virtually the same feed preferences
as do wild deer. Dimock and others ( 197 6) tested seed orchard clones and cross-pollinated
Douglas-fir families in deer and hare enclosures. They confirm that tests with snowshoe
hare in pens and in the field correlate well and also found that the same order of preference
among Douglas-fir genotypes occurs with captive deer. The difference among genotypes
was highly heritable and primarily additive, although the studies provide no estimates of
heritabilities. Among seedlings of varying height, however, deer were found to prefer taller
plants (Dimock 1971); hence, test plants have to be presented at equal heights. Foliage of
mature plants, or their clones, was much more rapidly consumed than was foliage from
their progeny (Dimock 1974). The order of preference among progeny, however, was still
predictable based on parental characteristics. Using similar plant material, Radwan and
Crouch (1978) show no relationship between preference and essential oils ofleaves, but
levels of chlorogenic acid do appear to be related to preference by the deer.
Silen and Dimock ( 197 8) demonstrate such a high rate of consistency in the preference
by penned deer for Douglas-fir clones that once feeding rates are indexed for several clones,
actual browsing patterns can be predicted with considerable accuracy.
Such results had practical application in 1981 with 225 Douglas-fir clones from the
Beaver Creek Seed Orchard near Corvallis, Oregon. Cuttings were given to a captive herd
of young orphaned deer that were maintained by the Oregon Department of Fish and
Wildlife. A selected group of 20 clones with relatively low preference for winter browsing
now provides a special seed mix from the orchard for clearcuts where browsing was a
serious problem. The prospect that this practical genetic application might become wide­
spread provided incentive to quantify genetic parameters needed for a breeding effort.
Thus, a followup study using the same herd and testing methods was planned for the 1982
spring browsing period.
METHODS
Unusual genetic material was available for this study in the form of 10-year-old Douglas­
fir progeny from a mating of eleven Willamette Valley parents near Corvallis. One of us
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TABLE 1.
Diagram of mating. X denotes families used in this study.
Female parents
Male parents
I
(•)
2
3
4
5
6
X
7
8
9
10
II
X
X
X
X
X
X
X
X
2
3
4
5
6
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
• Selfed families were not included.
(Silen) successfully crossed all possible parent-tree combinations on six of the parents in
1971. This resulted in a 6 x 11 crossing matrix consisting of a 6 x 6 diallel plus a 6 x 5
factorial with progeny in every cell. One cell had too few seedlings to include in this test
and selfed families were also excluded because oftheir low vigor. Thus, a complete analysis
of genetic components was possible, and the convenience of the penned deer made an
experiment using about 5,000 cuttings practical.
Parameters ofinterest were the standard general and specific combining ability, maternal
and reciprocal effects, and heritability estimates of preference that such a mating permits
In addition, we looked at the phenotypic correlation between resistance to deer browsing
and tree growth.
A diagram of the 6 x 11 mating (Table 1) illustrates the test design and analysis. In
addition to cuttings from 59 outcrossed families, cuttings from six open-pollinated families
of the female parents and from nine of the parents themselves were also included in the
test. Cuttings from the outcrosses, from open-pollinated families, and from parents were
tested together but were considered as three experiments and analyzed separately.
Lateral branch tips of the outcrossed families were collected at the 2-m level from the
5-m-tall trees in mid-January 1982, stored in a refrigerator in plastic bags, and exposed to
deer the following day.
The test area was a 6.3-ha fenced field. Six deer were allowed to freely browse the cuttings
and natural food in the field. A maintenance diet of alfalfa hay and commercial calf feed
was also provided.
Experimental design was a randomized complete block with eight replications. Eight
cuttings were represented in the study from each of 10 members of the 65 families (59
outcrosses plus six open-pollinated families), plus 24 cuttings from each of 9 mature, 18­
to 33-mm-tall parent trees. One 30-cm-long cutting of a family member per plot was stuck
with 20 em exposed above ground so that each replication had 677 cuttings at 1- by 1-meter
spacing. Distribution of the 5,416 cuttings in the 8-replication experiment was as follows:
59
6
10
1
8
9
3
8
5,416
full-sib families
open-pollinated families
members per family
cutting per member per replication
replications
plus
parent trees
cuttings per parent per replication
replications
total cuttings
Mean value of outcross and wind-pollinated family of the 6 x 11 mating was based on
180 /
FOREST SCIENCE
TABLE 2. Percent ofremaining cuttings at 38 days among families in the 6 x 11 mating,
and parental cuttings at day 38.
Female parent
Parent
2
3
4
5
6
58
64
69
57
Male Original
parent parent
mean
mean
Male parent
1
2
3
4
5
6
7
8
9
10
11
Female parent mean
Open-pollinated mean
X
57
55
X
54
60
X
46
50
57
53
41
52
54
56
58
54
55
62
55
63
60
53
45
48
53
44
39
59
53
48
57
51
61
67
49
52
48
46
46
44
45
X
48
48
63
58
56
60
49
59
60
54
54
50
53
59
60
60
53
56
58
51
54
57
51
52
55
55
55
52
44
56
57
55
42
47
40
58
49
55
54
54
48
X
X
4
17
17
14
12
17
8
4
4
11
80 observations (8 replications of 1 cutting times 10 family members); thus, each female
parent was tested by 800 observations and each male parent by either 400 or 480 obser­
vations. Each mature parent tree was evaluated by a mean of24 observations. We recorded
the day when a cutting was first browsed beginning on the morning of day 2 and continued
at approximately 3-day intervals for a total of 38 days. Unlike the 1981 tests with less
material and a larger deer herd that quickly browsed every cutting, at day 38 deer browsing
had virtually stopped and many cuttings had become unpalatable. This unexpected outcome
limited us to an analysis on percent of family that was unbrowsed and not to the time of
browsing of each cutting. Heritability calculations were thus limited to family means and
not to an individual cutting basis. For the analysis of variance we used the plot mean based
on 10 family members within a plot. Using zero for browsed and 1 for unbrowsed the plot
mean is proportion unbrowsed. For example, ifthere were six browsed and four unbrowsed,
the plot mean was 0.4. For ease ofunderstanding we refer to these plot means as percentages
hereafter.
A separate analysis of variance was made for each type of material: full-sib crosses, open­
pollinated families, and parent-generation trees. Full-sib crosses were analyzed genetically
using Griffing's (1956) random Model II Method 3 for a full diallel with one set of F 1's
with reciprocals. The North Carolina State University diallel analyses computer program
ofSchaffer and Usanis (1969) was used for the 6 x 11 matrix. Significant differences among
the female means were detected by ANOVA. Differences among specific females were
detected by least significant difference analysis. We assumed that parent trees represented
a random sample of Douglas-fir in western Oregon.
Ten-year family mean heights, available for the same families of the 6 x 11 matings in
1971, were analyzed for possible correlation with average percent unbrowsed cuttings at
38 days.
REsULTS
Browsing was slow compared with earlier studies. In this test, an average of only 46 percent
of cuttings were browsed. This level was found to be nearly optimum in previous testing
(Dimock and others 1976). Because there were so few deer for the number of cuttings in
the test, the test was not as sensitive as were earlier studies. There was large random
variation in percent of unbrowsed cuttings among full-sib families within each male and
female parent. Family means ranged from 39 to 69 percent unbrowsed, a difference sig­
nificant at the 5-percent level. Among parents, only females 4 and 5 showed significant
VOLUME
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1, 1986/ 181
differences with 47 to 58 percent unbrowsed cuttings, respectively (Table 2). Even so, the
susceptibility of parent 4 as either a female or male parent is striking. In each of the ten
matings involving female parent number 4, progenies were more susceptible to browsing
than the average family of each male parent (Table 2) and parent 4 as a female was the
most susceptible of the six comparable families in 9 of the 11 cases. Even as the male,
parent 4 was more susceptible than average in four of the five crosses.
Means of open-pollinated and full-sib values in Table 2 were similar (48 versus 54
percent unbrowsed, respectively). The test was not sensitive enough to reveal significant
differences among parent trees or among open-pollinated families. As in past studies, parent
tree cuttings were much more heavily browsed than were cuttings from 10-year-progeny
(11 versus 54 percent unbrowsed, respectively).
Despite the large variability in the experiment, the analysis of genetic components gave
quite clear results. General combining ability, which is a measure of parent effect when
crossed with many genotypes, was highly significant (Table 3). Genetic effects of reciprocal,
maternal, and specific combining ability were not significant. Sixty-five percent of total
genetic variance is estimated to be additive using the formula:
2
Percent additive variance= 100
Uga
uga
2
+
(Jd
2
where
ga = additive variance component, and d = nonadditive variance component. Our calculation of heritability (h 2 ) is applied to selection of half-sib families using the
variance components of our analysis and specifically for repeated seed collection from the
selected parents. To illustrate an intended commercial application, we used half-sib selec­
tion with two replications and ten seedlings per family per replication for each of 260
families (260 x 20 = 5,200) as was tested in 1981. On that basis, h2 = 0.73.
where
additive genetic variance
plot variance
within plot variance
r = number of replications
n = number of observations per family per rep.
ug/
u/
uw2
If selection is for the top 10 percent of the families (26 families from 260), gain in percent
unbrowsed seedlings over the mean of unselected families is calculated to be 12 percent.
G
=
Sh 2
where
G
amount of gain, and
S = selection differential.
DISCUSSION
Despite the slow rate of browsing with only six deer exposed to over 5,000 cuttlngs and
the high variability that resulted, the main goal was achieved. Rates of gain in browsing
resistance per generation can now be estimated.
About 65 percent of the total genetic variation was shown to be additive, with specific,
maternal, and reciprocal effects not significant. From results ofprevious studies, we expected
most families would be completely browsed, in which case variability would be considerably
less. Differences in preference for the array ofparents in this study were within the expected
range. Error was too large, however, to show statistically significant differences among more
than two of eleven parents. The consistency of browsing by deer for particular genotypes,
182 /
FOREST SCIENCE
TABLE 3. Mean squares, estimated variance components, and their standard deviations
for Douglas-fir browsing resistance to black-tailed deer.
Source of
variation
Replication
General
Specific
Maternal
Reciprocal
Plot error
Error within plot
Degrees of
freedom
Mean square
7
10
34
5
9
406
4,120
3.142156•
.081839•
.023931
.033434
.016010
.021034
.021238
Estimated
component
Standard
deviation
0.05290
.00070
.00037
.00019
0.02511
.00046
.00059
.00021
.00044
.00147
(")
.00005
.02054
• Denotes significant differences at the 0.05level. Plot error mean square was used as the denominator
to test reciprocal, maternal, and specific effects; the specific mean square is the denominator to test
for general combining ability.
• Negative values are considered zero.
though striking for families of parent number 4, was not as good generally as in past tests.
Order of preference was similar between full-sib and open-pollinated families, even though
this order was not displayed with parent tree material. For these reasons our conclusions
are probably conservative.
No correlation was found with 10-year family growth and percent unbrowsed plants.
This suggests no adverse growth effect might result from use of a browse-resistant seed
mix; however, the test is not definitive. This consideration is so important that stronger
tests must be conducted in the future. The preference by deer for taller seedlings (Dimock
1971) cautions that a possible selective advantage may exist for slower growing families,
another example ofthe general inverse relationship between growth and various expressions
of hardiness. The low sensitivity of our test also cautions against early inferences about
growth versus browsing.
Past research (Dimock 1974, Radwan 1975, and Tucker and others 1976) shows that
cuttings from older parent trees are more heavily browsed than are those from younger
material. In confirmation, our parent cuttings were also browsed nearly twice as much as
were cuttings from their progeny (89 versus 46 percent, respectively). Parent material was
browsed to near the ground line, but most progeny cuttings lost only 2 to 5 em of height.
This phenomenon is also regularly observed where green limbs from old trees have fallen
to the ground and their new foliage has been completely browsed while nearby seedlings
are only slightly browsed. The consistency between observations and studies suggests that
Douglas-fir may have developed resistance to browsing for the seedling stage only. If such
early resistance is maintained with chemical compounds, which might require considerable
expenditure of energy or photosynthate, then mature trees with less resistance would have
a selective advantage.
Tucker and others (1976) and Radwan (1975) suggest that browsing preference may be
related to increases in chlorogenic acid. The U.S. Fish and Wildlife Service Laboratory in
Denver evaluated chlorogenic acid content for 10 of the most preferred and 10 least
preferred clones from the previous year's test of 225 clones. The correlation (r = 0.106)
was much too weak to permit effective selection for palatability. Chemical evaluation for
browsing preference may become important after the technique is refined. For the present,
analysis of chlorogenic acid does not appear to be as effective nor as economical for
determining the rate of genotypic browsing as is possible with penned deer. The recent
reports ofhare showing similar preference tendencies related to levels ofpinosylvin methyl
ether in buds and stems provides a new direction for research (Bryant 1981, Bryant and
others 1983).
Deer browse dormant seedlings during the winter and again during the early growing
season in May and June. Browsing in winter is severe in certain areas, but damage early
in the growing season is probably more widespread and serious (Campbell and Evans 197 8).
A determination ofwhether browsing preference, for specific genotypes, is constant between
the major browsing periods is necessary.
VOLUME
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In perspective, the study clearly accomplished the main objective of estimating the
heritability and proportion ofadditive genetic variation for winter deer browsing resistance.
This was missing from earlier studies. The rate of potential improvement from a practical
breeding program can now be estimated at about 11 percent per generation under our test
conditions. Many past observations about the trait were also supported but not as well in
some cases as had been documented in earlier studies.
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of four Alaskan trees. Science 213:889-890.
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FOREST SCIENCE