Forest Sci., Vol. 28, No.2, 1982, pp. 283-292
Copyright 1982, by the Society of American Foresters
Inbreeding Depression in Height, Height
Growth, and Survival of Douglas-fir,
Ponderosa Pine, and Noble Fir
to 10 Years of Age
FRANK c. SORENSEN
RICHARDS. MILES
ABSTRACT. Self-, outcross-, and wind-pollination progenies of Douglas-fir, ponderosa pine, and
noble fir were measured at outplanting and annually to age 10 years. Inbreeding depression in
survival the first 2 years in the plantation ranged from 3 to 16 percent, and thence to age 10 years
ranged from 0.4 to 3 percent. Inbreeding depression in plant height ranged from 24 to 30 percent
at outplanting and ranged from 29 to 36 percent at age 10.
Gompertz growth curves were fitted to annual height measurements and elongation rates, based
on the curves, determined for ages 5 and 9 years and for a common height. Average inbreeding
depressions in elongation rates (growth in centimeters/year) were 35 percent (age 5), 28 percent
(age 9), but only 10 percent when adjusted to a common height. The interpretation was that
relative inbreeding depression at any age consists of a genetic inbreeding depression of growth
rate and an additional effect resulting from self and outcross plants being at different points on
a more or less common growth curve.
Losses in productivity of wind-pollinated Douglas-fir and ponderosa pine plantations at age 10
were estimated based on observed inbreeding depressions in survival and height and on expected
frequencies of self seedlings in the wind-pollination populations. Losses were estimated to be
about 5 percent in Douglas-fir and about 8 percent in ponderosa pine. FOREST Sci. 28:283-292.
ADDITIONAL KEY WORDS.
Pseudotsuga menziesii, Pinus ponderosa, Abies procera.
WE REPORT SURVIVAL, height, and annual height growth through age 10 of self­
(S), outcross- (C), and wind- (W) pollination families. This follows earlier publi­
cation of seed yield and nursery heights for the same species (Sorensen and Miles
1974, Sorensen and others 1976).
The purpose of the article is (I) to compare inbreeding depression during the
field growth phase to inbreeding depression at the time of outplanting, and (2) to
estimate the decrease in productivity of a 10-year-old wind-pollination plantation
caused by natural inbreeding. With regard to the first point we will try to show
that the inbreeding effect, as measured in a young field test, consists oftwo parts,
an early genetic effect and a growth-curve effect (Burdon and Sweet 1976, Over­
ton and Ching 1978). The latter effect apparently occurs because self (S) and
cross (C) plants occupy quite different "age" positions on exponential growth
curves at the time of outplanting.
The authors are with the U.S. Department of Agriculture Forest Service, Pacific Northwest Forest
and Range Experiment, at its Forestry Sciences Laboratory, Corvallis, OR 97331. They thank Nancy
Mandel for assistance with the mathematics and Timothy Max for helpful comments on the statistics.
Manuscript received 23 February 1981.
VOLUME 28, NUMBER 2, 1982 / 283
MATERIALS AND METHODS
Results are based on S-, C-, and W-pollination progenies from individual seed
trees. Numbers of seed trees, numbers of seedlings per family, and ages at out­
planting are given in Table 1.
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees were sampled at five
locations in western Oregon ranging in elevation from 45 to 1,340 m. All areas
had been naturally regenerated. Four stands were one-storied and pure Douglas­
fir; one stand had scattered old-growth Douglas-fir over a mixed coniferous under­
story also containing Douglas-fir. Ponderosa pine (Pinus ponderosa Dougl. ex.
Laws) trees were sampled at two locations, 1,200 and 1,650 m, in central Oregon.
Both stands were nearly pure and nearly even-aged ponderosa pine with a minor
percentage of lodgepole pine. Both stands had been naturally regenerated and
had more or less regular spacing after some artificial thinning. Very scattered old­
growth ponderosa pines were present in both stands. The noble fir (Abies procera
Rehd.) trees were growing at 1,550 m in the central Oregon Cascades. This stand
had scattered small openings and a large clearing upslope from the study area.
All sampling sites were parts of extensive natural forests. Further details on the
stands are given in Sorensen 1973, Sorensen and Miles 1974, and Sorensen and
others 1976.
Seedlings were raised at our genetics nursery (elevation 75 m) in western Or­
egon. Douglas-fir and noble fir were transplanted to a genetics test site (elevation,
100--125 m) in western Oregon. Ponderosa pine progenies were planted at two
sites near the seed-tree locations. Spacings in the plantations were 3.7 x 3.7 m
(Douglas-fir), 3.0 x 3.0 m (ponderosa pine), and 2.4 X 2.4 m (noble fir). Seedling
heights were measured at the time of outplanting and each autumn thereafter,
except Douglas-fir which was not measured at the end of the ninth growing sea­
son. Each mating type (S, C, and W) was represented by nearly equal numbers
of seedlings.
Male parentage for C pollination was an equal-volume mix of fresh pollen from
4 to 10 trees growing in the same locality as, but not neighboring, the seed trees.
Open-pollination cone collections were made in the upper one-third to one-half
of the crowns.
In the field tests the seedlings were planted in single family rows grouped by
seed trees, that is the C-, S-, and W-pollinated family rows from a single seed
tree were always planted neighboring one another with the S family between C
and W families. (This arrangement was used so that later thinning would leave
S rows widely spaced.) The seed tree, or maternal parent, served as replication.
The Douglas-fir and noble fir plantations were established on agricultural land
which had originally been a Douglas-fir site. Planting spots were scalped before
planting, and the seedlings planted with a shovel. Competing vegetation was
TABLE 1.
Seed tree and seedling numbers and outplanting age for each species.
Species
Douglas-fir
Ponderosa pine
Noble fir
284 /
FOREST SCIENCE
Seed trees
Seedlings
per family
Age at outplanting
(seedbed­
transplanted)
Number
Mean number
Years
19
17
7
23
39
22
2-0
2-1
3-0
controlled through age 8 with spring herbicide applications. Ponderosa pine was
auger planted at two pumice soil sites from which competing vegetation (predom­
inantly sclerophyllous shrubs or grass) had been scraped by bulldozer after log­
ging. Competing vegetation had not reoccupied the site during the period covered
by this report, and crowns had not closed by year 10 in any of the plantations.
Some root competition between trees was possible, particularly in the Douglas­
fir test. It is not likely that mutual shading or competition from other vegetation
for moisture affected height growth.
The following traits were analyzed:
1. survival during the first 2 years in the plantation,
2. survival after the first 2 years in the plantation (growing seasons 5 through
10 for Douglas-fir and 6 through 10 for ponderosa pine and noble fir),
3. seedling total height after outplanting (age 2 for Douglas-fir and 3 years for
ponderosa pine and noble fir),
4. plant height at age 10.
Because the observations on seedling height and plant height at age 10 years
were on the same plants, the two analyses were not independent and a split-plot
in time was used. Heights were transformed to log 10 which equalized the variances
for the two ages.
Survival percentages were transformed to square roots if family survival per­
centages were all above 80 percent. Arc sine transformation was used if survival
percentages varied above and below 80 percent (Steel and Torrie 1960, p. 158).
Pollen types (S, C, W) and ages were treated as fixed effects, seed trees and
planting locations (ponderosa pine only) as random. Although pollen-type rows
were not randomized within seed-tree groups, they were considered as such for
purposes of analysis. Because subordinate vegetation had been controlled and
tree crowns were still fully open at age 10, lack of randomization within seed­
tree groups should have had little effect on inbreeding depression in height.
The split-plot analysis of variance for Douglas-fir and noble fir seedlings and
plant heights had the form,
Expected
d.f.
mean squares
Source ofvariation
2
2
a - 1
Ages (A) U"a + ptKA
U"a 2 + apa-r2
t - 1
Seed trees, or replications
U"a 2
(a - l)(t - 1)
Error (a) a-b2 + atKp2
p-1
Pollen types (P) (a - l)(p - 1)
PxA a-b 2 + tKAP 2
a-b2
a(p - l)(t - 1)
Error (b) <n
Analysis of ponderosa pine data included, within the subunits, additional terms
for planting site and its interactions with the other factors.
RESULTS
Height.-Heights at time of outplanting and at plant age of 10 years are given in
Table 2. Inbreeding depression was 24-30 percent (average for the three species,
27 percent) at the time of outplanting and 29-36 percent (average, 32 percent) at
age 10 years. When plant heights were expressed in logs 10 , the interaction term,
pollen type X age, was not significant for any species. That is, there were no
significant differences in relative inbreeding depression between outplanting and
age 10. In other words, the relative elongation rates for S, C, and W progenies
were similar. Nevertheless, inbreeding depression in actual height increased
greatly-for example, from 5 em to 1.5 min Douglas-fir.
VoLUME
28,
NuMBER
2, 1982 I 285
TABLE 2. Average plant heights ofoutcross-, wind-, and selfpollinationprog­
enies at time offield planting (seedling age 2 years in Douglas-fir and 3 years in
ponderosa pine and noble fir) and at 10 years.
Plant height
Time of measure­
ment and species
At field planting
Douglas-fir
Ponderosa pine
Noble fir
At age 10
Douglas-fir
Ponderosa pine
Noble fir
Outcross
Wind
Self' ··---····-·· ·-·····-·-·-····-········· ..• em··-·················-··········-·-·-·-··
20.7
22.2
24.1
21.7
22.6
22.5
16.5 (24.0)"
15.8 (30.1)2
16.4 (27 .1)2
···-·-·····-······················--·-·­ m ···············-·········---·-··········
5.31
5.09
3.77 (29.0)"
0.97
0.99
0.62 (36.1)2
1.93
1.84
1.35 (30.1) 2
Values in parentheses are inbreeding depression in percent.
Null hypothesis: no difference in height between cross- and self-pollination families, rejected
(P < 0.01).
1
2
Survival.-Survival percentages are given in Table 3. Survival was good in all
cases except for ponderosa pine the first 2 years after outplanting. Relative sur­
vival of S-pollinated plants was not influenced noticeably by general plantation
survival. On the sites with the lowest overall survival (ponderosa pine planting
sites), S plants survived nearly as well as C plants. On the other hand, in the
Douglas-fir test, S plants had relatively low survival even though the survival of
C plants approached 100 percent.
DISCUSSION
Inbreeding depression relative to C plants in 10-year height, which we found to
be 29-36 percent, is in line with that reported in the literature for other coniferous
TABLE 3. Survival of outcross-, wind-, and selfpollination progenies of Doug­
las-fir, ponderosa pine and noble fir for first 2 years after field planting, and
thereafter to age 10.
Survival time
and species
Survival
Outcross
Wind
SelP-
First 2 years
Douglas-fir
Ponderosa pine
Noble fir
····-·-··-··-···········-··-·-·­
98.7
72.4
97.0
Percent• ······-----····················­
83.3 (15.6)3
94.0
75.4
68.8 (5.0)
94.4 (2.7)
96.2
To age 10
Douglas-fir
Ponderosa pine
Noble fir
······-··-···················-·­
99.9
99.3
100.0
Percent• ····-···-·-·········-···········
99.8
98.8 (1.1)
99.6
96.1 (3.2)5
99.9
99.6 (0.4)
Values in parentheses are inbreeding depression in percent.
Retransformed from arc sines.
3
Null hypothesis: no difference in height between cross- and self-pollination families, rejected
(P < 0.01).
4 Retransformed from square roots.
5 No difference in survival (P < 0.05).
1
2
286 I
FoREST SciENCE
TABLE 4. Percentage inbreeding depression in height and survival in other
coniferous species after several years in field tests.
Inbreeding depression
Species
Larix
Picea
Picea
Pinus
Pinus
Pinus
Pinus
Pinus
europaea
abies
glauca
elliottii
elliottii
monticola
strobus
sylvestris
Mean
Height
Survival
Age
Percent
19.9
47.9
25.5
25.8
34.0
29.7
17.0
32.0
Percent
Years
29.0
19.4
13
II
11.4
7-18
8-9
5
9-12
10
6-7
21.7
22.9
21.4
Control
Reference
cross
wind
wind
cross
wind
cross
cross
cross
Langner (1951)
Langlet ( 1940)
Ying (1978)
Gansel (1971)
Snyder (1972)
Barnes (1964)
Kriebel (1974)
Dengler (1939)
species at comparable ages (Table 4). Percentage inbreeding depression in sur­
vival (3-15 percent from outplanting to age 10) was generally less than that for
other species (Table 4).
In our study, Douglas-fir had the least inbreeding depression in height but the
greatest inbreeding depression in survival. Mortality is not random with regard
to plant size; it is heaviest among the smallest seedlings. Because height was
measured on surviving trees, greater mortality of the S plants resulted in smaller
inbreeding depression in average height of the surviving S trees.
In general, in tests of this type, differential mortality would bias size compar­
isons among mating types. It would also affect comparisons with other tests
having more or less inbreeding depression in survival.
Inbreeding Depression in Rate of Stem Elongation.-A surprising result was the
lack of significance for the interaction term contrasting inbreeding depression in
log height at outplanting and at age 10 years. In another test of young plantation­
grown Douglas-fir, Overton and Ching (1978) found no difference between the
height growth trajectories of average and superior nursery selections through
plantation age 9. Because the elongation paths did not differ, the authors inferred
that the nursery differences were cultural in origin. In the present test, however,
nursery differences were not cultural but were genetic in origin (Sorensen and
Miles 1974, Sorensen and others 1976). Nevertheless, the lack of significant pollen
type x age interaction in height suggests that S, C, and W progenies within a
species must all be on quite similar growth curves in the field.
To investigate this in more detail, Gompertz growth curves were fitted to annual
height measurements (Bliss 1970) and annual rates of elongation compared at
ages 5 and 9 and at a common height for each species-200 em for Douglas-fir,
50 em for ponderosa pine, and 100 em for noble fir. These heights represented
sizes attained by the plants 3-4 years after outplanting, that is, after the plants
were well established and presumably no longer affected by transplanting shock.
Growth curves were fitted to species mean heights based on all families within
a pollen type. Only seedlings still alive at age 10 were included.
The Gompertz growth curve expresses the height-to-age relationship as,
y
= abcz (1)
where y is height in em at age x in years,
a is upper asymptote,
VoLUME 28, NuMBER 2, 1982 I 287
1096.6
e7
403.4
es
-5148.4 .!:..-
e5
E
-
1­
J:
1-
(!)
J:
(!)
~ 54.6
jjj
J: e4
c::::
...J
20.1
e3
7.4
e2
.2
.3 .4
.5
.6
.7
.8
.9
AGE (Cx, where x=years)
I. Plot of height and In height against age term, ex, where x is age in years. Parameter c is
defined in text. Gompertz growth curves were fitted to annual height measurements, ages 2 through
10, of cross-, wind-, and self-pollination Douglas-fir families. Lines represent the fitted curves.
Crosses, circles, and dots are the observed values.
FIGURE
b is a measure of the location of the curve, and
c is a term relating the loge (In) responses of mean heights in the
first, second, and third thirds of the time variate x (Bliss 1970,
p. 195).
Plots of In y against ex gave good fits to the actual heights. There was a slight
deviation during the 2 years immediately after planting, but it was probably due
to planting shock, because there was no systematic deviation thereafter. Figure
1 gives an example of the fit of the curve to the Douglas-fir data. Fits to ponderosa
pine and noble fir measurements were comparable.
Methods for determining a, b, and c are given in Bliss (1970). The age, x, for
any height, y, can be found by
X
=
1og (log ylog- blog
a)
------~~------
log c
(2)
The stem elongation rate in centimeters per year at any age, x, is given by the
first deviate of equation (1),
dy = abcz(ln b)cx(ln c).
(3)
dx
Inbreeding depression in elongation rate was present in all species comparisons
(Table 5). For Douglas-fir, the only species in which elongation had reached the
288 I
FoREST SciENCE
TABLE 5. Elongation rates of outcross-, wind-, and selfpollinated families of
Douglas-fir, ponderosa pine, and noble fir. Elongation rates are given at ages 5
and 9 years and at a common height representative for the species 3-4 years
after outplanting. Rates are calculated from Gompertz growth curves (Bliss 1970)
fitted to the data. Further explanation in text.
Elongation rate
Age or height and species
5 years
Douglas-fir
Ponderosa pine
Noble fir
Outcross
Wind
Self'
__ ________ ___ __________ ___________ c mlyr _________________________________ _
47.7
8.0
14.5
43.7
7.9
13.5
28.3 (40.6)
5.2 (35.0)
10.4 (28.3)
9 years
Douglas-fir
Ponderosa pine
Noble fir
108.8
14.9
45.0
107.1
16.6
42.5
92.3 (15.2)
9.4 (36.9)
30.8 (31.6)
Common height
Douglas-fir
Ponderosa pine
Noble fir
72.6
10.7
33.8
71.8
11.1
32.3
69.0 (5.0)
9.1 (15.0)
30.0 (11.2)
1
Values in parentheses are inbreeding depression in percent.
linear phase, inbreeding depression in elongation rate was less at age 9 than at
age 5. For all species, inbreeding depression in rate of stem elongation was much
less when taken at a common height than when taken at a common age. This
suggests that the size difference observed after outplanting and attributed to in­
breeding has two sources, (I) a true genetic inbreeding effect in elongation rate,
and (2) an effect due to being at different points on the elongation curve-an
inbreeding aftereffect, as it were.
At common heights, inbreeding depression in elongation rate was 5-15 percent
(average about 10 percent). We propose that these smaller values might approx­
imate the true genetic inbreeding depression in elongation rate during the period
of linear height growth, at least before plant competition and interplant shading
occur. If inbreeding depression in total height is 25 percent at time of outplanting
and 10 percent in elongation rate during the grand period of elongation, percentage
inbreeding depression in total height will, after a certain time, decrease with age.
The only long-term data on inbreeding come from a Swedish plantation of Wand
S progenies of Picea abies. Inbreeding depression in total height decreased from
61 percent (age 19) to 54 percent (age 27) to 22 percent (age 61) (estimated from
Figure 4, Eriksson and others 1973).
These observations, along with those of Overton and Ching (1978), also appear
to be pertinent to other genetic comparisons, particularly in young field trials.
They suggest that some reassortment of within- and among-group variances may
occur because accumulated seedling history, genetic and cultural, has put differ­
ent individuals and groups at different points on the growth curve at a common
age. If the growth curve is linear, variances should not change with time. If
growth is in an exponential phase, however, variances and variance ratios should
change with time even if a common growth curve is involved. Further investi­
gation is required to determine if variance changes due to an exponential growth
curve are important. It is remarkable in the present test that S and C elongation
rates, if adjusted to a common position on the curve, differ by only about 10
VoLUME
28,
NuMBER
2, 1982 I 289
percent, when inbreeding depression in total height is about 35 percent at age
5 years. The difference in these values suggests that variance changes might be
sizeable, particularly if the genetic differences were small relative to differences
in position on the curve.
Interaction of inbreeding depression with age is also reported in maize. Ex­
periments comparing the relative growth rates of hybrids and their parental inbred
lines showed that differences in relative growth rates diminished with age, es­
sentially disappearing by age 7 weeks (Donaldson and Blackman 1973). Some of
the inbreeding effect was, in fact, associated with germination. Extension rate of
the embryo was greater in the hybrid seeds (Donaldson and Blackman 1974),
putting inbred and hybrid plants at different points on the growth curve very
early in development. This is similar to the pattern we suggest for trees, but on
an annual rather than a perennial time scale.
Inbreeding Depression in Productivity in Wind-Pollination Plantations.-An es­
timate of the productivity loss at 10 years due to the natural inbreeding present
in a plantation of W-pollination plants can be calculated for Douglas-fir and pon­
derosa pine. We assume no culling in the nursery and cultural treatment com­
parable to that of this experiment.
Proportions of S seedlings in the W-pollination progenies have been estimated
at 7 percent and 11 percent for Douglas-fir (Sorensen 1973) and ponderosa pine
(Sorensen and Miles 1974), respectively. (These proportions may include milder
forms of inbreeding as well as selfing; however, the amount of inbreeding has
been estimated as equivalent to 7- and 11-percent self seedlings.) No estimate of
natural inbreeding is available for noble fir. Because diameters were not mea­
sured, we assume a percentage inbreeding depression in diameter equal to that
in height and that tree volume can be represented by the volume of a cone. For
C plants the average tree volume would be 7Td2 h/12, which equals 0.26d2 h, where
d and hare the average diameter and height of the C plants. At 10 years, Douglas­
fir had an average inbreeding depression in height of 0.29, giving an average tree
volume for the surviving S trees of 7T(0.71d)2 (0.71h)/12, which equals 0.09d2 h.
The comparable value for ponderosa pine would be 7T(0.643d)2 (0.643h)/12, which
equals 0.07d2 h.
If productivity of a plantation without S plants is set at 1.00, the productivity
of a plantation established from W seedlings would be the sum of­
1.00 for the proportion of C plants, plus
0.00 for the proportion of S plants which do not survive, that is the additional mortality due to inbreeding, plus (0.09/0.26) or (0.07/0.26) for the proportion of S plants of Douglas-fir and pon­
derosa pine, respectively, that do survive. For the two species, we obtained the following values for productivity­
Douglas-fir: (1.00)(0.93) + (0.07)(0.17)(0) + (0.07)(0.83)(0.09/0.26) = 0.95 and
ponderosa pine: (1.00)(0.89) + (0.11)(0.08)(0) + (0.11)(0.92)(0.07/0.26) = 0.92.
At 10 years, the productivity of a Douglas-fir plantation established from W­
pollination plants is estimated to be reduced by 5 percent (4 percent due to growth
loss, 1 percent due to increased mortality) due to natural selfing. The reduction
for ponderosa pine is estimated to be 8 percent (7 percent growth loss, 1 percent
increased mortality). Increasing the number of planted seedlings to allow for some
culling could ameliorate this loss.
The calculated growth loss in Douglas-fir compares well with the 4 percent
reduction in height observed at age 10 in the wind-pollination progenies in Table
2. However, for ponderosa pine the wind-pollination progenies in our plantations
290 / FOREST SCIENCE
showed no reduction in height (Table 2, age 10). We have no explanation for this
other than the possibility that one or more of the pollens making up the outcross
polymix had unusually low breeding values.
Nursery culling or seedling culture under very favorable conditions (under
plastic or in glasshouses) might change these figures somewhat. Nursery rogueing
to remove inbreds is inefficient because of the overlap in size of S and C plants
(Snyder 1968). Nevertheless, a light culling would probably eliminate some weak
inbreds (Sorensen and Miles 1974). Presumably this would reduce the mortality
due to inbreeding and keep some of the slowest growing inbreds out of the plan­
tation.
Seedlings cultured under very favorable growth conditions show less inbreeding
depression than seedlings grown under normal nursery conditions (Eriksson and
Lindgren 1975, Lindgren1 ). Culling of S seedlings is not possible under the fa­
vorable conditions, but the S and C plants show less separation on their growth
curves because of less average inbreeding depression. Since genetic differences
in growth rate and positions on the growth curve both contribute to inbreeding
depression in the plantation, it is possible that larger size of the S plants at the
time of outplanting will counteract the inability to cull. At least it should moderate
the inability to cull.
LITERATURE CITED
BARNES, B. V. 1964. Self- and cross-pollination of western white pine; a comparison of height growth
of progeny. USDA, Forest Serv, Res Note INT-22, 3 p. Intermt Forest and Range Exp Stn.
Buss, C. I. 1970. Statistics in Biology. Vol Two. McGraw-Hill, New York. 639 p.
BURDON, R. D., and G. B. SwEET. 1976. The problems of interpreting inherent differences in tree
growth shortly after planting. In Tree physiology and yield improvement (M.G. R. Cannell and
F. T. Last, eds), Chap 29, p 283-502. Academic Press, London, New York. 567 p.
DENGLER, A. 1939. Uber die Entwicklung kiinstlicher Kiefernkreuzungen. Zeitschrift f. Forst- u.
Jagdwesen 71:457-485.
DoNALDSON, C., and G. E. BLACKMAN. 1973. A further analysis of hybrid vigour in Zea mays during
the vegetative phase. Ann Bot 37:905-917.
DONALDSON, C., and G. E. BLACKMAN. 1974. The initiation of hybrid vigour in Zea mays during the
germination phase. Ann Bot 38:515-527.
ERIKSSON, G., and D. LINDGREN. 1975. Nagra genetiska reflexioner kring plantsortering. (Some
genetic aspects on the grading of plants.) Sveriges Skogsvardsfiirbunds tidskrift 73:615-621.
ERIKSSON, G., B. ScHELANDER, and V. AKEBRAND. 1973. Inbreeding depression in an old experi­
mental plantation of Picea abies. Hereditas 73:185-194.
GANSEL, C. R. 1971. Effects of several levels of inbreeding on growth and oleoresin yield of slash
pine. Proc South Forest Tree lmprov Conf 11:173-177. (Sponsored Pub!. No. 33 of the Southern
Forest Tree Improvement Committee, Eastern Tree Seed Laboratory in cooperation with South­
eastern Area, State and Private Forestry, and Region 8, U.S. Forest Service.)
KRIEBEL, H. B. 1974. Inbreeding depression in eastern white pine. In Proc Central States Forest
Tree lmprov Conf, Ames, Iowa, 9:48-55. North Cent Forest Exp Stn and Northeast Area, State
and Priv For, Upper Darby, Pa. 172 p.
LANGLET, 0. 1940. Om utvecklingen av granar ur frii efter sjalvbefruktning och efter fri vindpolli­
nering. (Uber die Entwicklung von teils nach kiinstlicher Selbstbestaubung, teils nach freier Wind­
bestaubung entstandenen Fichten.) Meddelanden fran Statens Skogsfiirsiiksanstalt 32(1): 1-22.
LANGNER, W. 1951. Kreuzungsversuche mit Larix europaea D. C. und Larix leptolepis Gord. Zeit­
schrift f. Forstgenetik u. Forstpftanzenziichtung. I :40-56.
OVERTON, W. S., and K. K. CHING. 1978. Analysis of differences in height growth among populations
in a nursery selection study of Douglas-fir. Forest Sci 24:497-509.
1
Lindgren, Dag. 1975. Inbreeding-disadvantage or tool in tree breeding. Royal College of For­
estry, Stockholm. 28 p. (Mimeographed.)
VoLuME
28,
NuMBER
2, 1982 I 291
SNYDER, E. B. 1968. Seed yield and nursery performance of self-pollinated slash pines. Forest Sci
14:68-74.
SNYDER, E. B. 1972. Five-year performance of self-pollinated slash pines. Forest Sci 18:246.
SORENSEN, F. C. 1973. Frequency of seedlings from natural self-fertilization in coastal Douglas-fir.
Silvae Genet 22:20-24.
SORENSEN, F. C., and R. S. MILES. 1974. Self-pollination effects on Douglas-fir and ponderosa pine
seeds and seedlings. Silvae Genet 23:135-138.
SORENSEN, F. C., J. F. FRANKLIN, and R. WOOLLARD. 1976. Self-pollination effects on seed and
seedling traits in noble fir. Forest Sci 22:155-159.
STEEL, R. G. D., and J. H. TORRIE. 1960. Principles and procedures of statistics. McGraw-Hill, New
York. 481 p.
YING, C. C. 1978. Performance of white spruce (Picea g/auca (Moench) Voss) progenies after selfing.
Silvae Genet 27:214-215.
Forest Sci., Vol. 28, No. 2, 1982, pp. 292-296
Copyright 1982, by the Society of American Foresters
N2 Fixation in Brown-Rotted Soil Wood in an
Intermountain Cedar-Hemlock Ecosystem
Michael J. Larsen, Martin F. Jurgensen, and Alan E. Harvey
ABSTRACT. Nitrogen-fixation rates in brown-rotted wood (soil wood) were shown to be linearly
related to moisture contents for a western conifer site. Rates of fixation for this substrate are
reported as 273 g N/halyr. The potential importance of soil wood in this capacity is noted for dry
sites and for dry periods on wet sites. Furthermore, this substrate does not appear to accumulate
N beyond ca. 0.6 percent, which appears to be a "steady state" value. It is tentatively concluded
that the importance of the contribution of N from N2 fixatio.n in soil wood is similar to that of
humus on this site, but is probably outweighed by N inputs from other sources. Functionally,
soil wood may be of greater importance if it provides a medium in which a N source is readily
available for mycorrhizal feeder roots. FoREsT Sci. 28:292-296.
ADDITIONAL KEY WORDS.
Thuja p/icata, Tsuga heterophyl/a.
RECENT STUDIES (Cornaby and Waide 1973; Roskowski 1977, 1980; Larsen and others 1978)
have demonstrated that woody residues in various stages of decay are suitable substrates
for dinitrogen fixation. Furthermore, differences in rates of fixation were detected (Larsen
and others 1978) between kinds of decay (brown vs. white) and kinds of wood (gymno­
spermous vs. angiospermous). Jurgensen and others (unpublished data) have noted dif­
ferences in rates of N fixation in residues between various sites and also seasonal differ­
ences. Larsen and others (1979) have also provided data on a variety of characteristics of
brown cubical decayed wood that support the view that intact brown-rotted wood in soil
(soil wood) functions in a manner similar to soil humus, an observation also made by
Rypacek and Rypackova (1975).
Place (1950) measured moisture contents (as a function of electrical resistance) in ''rotten
wood" and "adjacent humus" and provided a striking graphical representation of the
The authors are, respectively, Mycologist, USDA Forest Service, Center for Forest Mycology
Research, Forest Products Laboratory, Madison, WI 53705; Professor, Department of Forestry,
Michigan Technological University, Houghton, MI 49931; and Plant Pathologist, USDA Forest Ser­
vice, Intermountain Forest and Range Experiment Station, Ogden, UT 84401, stationed at the For­
estry Sciences Laboratory, Moscow, ID 83702. They thank K. White and P. Cattelino for their
significant contributions through their technical assistance. Manuscript received 3 November 1980.
292 /
FOREST SCIENCE