Effects of defoliation on growth of certain conifers : a summary of research literature

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1

Defoliation by spruce b unworn ....

Defoliation by other insects .... 3

Defoliation by fire ......... 6

Defoliation by fungi and hail .... 7

Artificial defoliation experiment . . 8

Conclusions ............. 10

Literature cited

11

EFFECTS OF DEFOLIATION ON GROWTH
OF CERTAIN CONIFERS

A SUMMARY OF RESEARCH LITERATURE

by

Thomas W. Church, Jr.2
Silviculturist

Northeast cm Forest Experiment . Station

Defoliation by insect pests has often resulted in heavy mortality
in the forests of North America. Among the more notable offenders and
their coniferous hosts are the spruce budworm on spruce and balsam fir,
the pine butterfly on ponderosa pine, the gypsy moth on white pine, the
hemlock looper on hemlock, the tussock moth on douglas-fir, and the
larch sawfly on tamarack. Under favorable conditions these insects can
build up to epidemic proportions and sweep disastrously through millions
of acres of valuable timber. Defoliation by forest fires and needle-
cast fungi also cause mortality in conifers.

Trees that undergo partial defoliation insufficient to cause
death suffer a consequent reduction in growth, because of the smaller
quantities of food materials produced. This phenomenon, although less
spectacular than mortality, is nevertheless important.

Defoliation By Spruce Budworm

r\

Swaine and Craighead (18_)^ point out that during the spruce bud-
worm outbreak of 1909-13 in Canada the trees that survived suffered a
loss of 3 to 5 years' increment. From observations made during a. bud-
worm infestation, Craighead (6) observed that the growth of annual rings

Stationed at the Northeastern Forest Experiment Station's
Adirondack Branch, Paul Smiths, N. Y. , which is maintained in coopera-
tion with Paul Smith's College.

? . •
'Numbers in parentheses refer to Literature Cited, page .11,

1

Showed different degrees of retardation in different portions of the
crown. The first year of a budworm attack on both spruce and balsam
fir is characterized by a severe reduction of growth in the top of the
tree, no noticeable change in the middle, and a rather decided increase
near the base. In red spruce this effect is not so pronounced as in
balsam fir, and in large blocks of nearly pure black spruce the first
affected ring is 1 year later than in balsam.

There may be a second depression or reduction in growth because
of the prolific production of cones immediately after the budworms stop
feeding. Continued defoliation by the budworms for several years is
marked by a rapid decline in the growth of all parts of the tree. "In
both spruce and balsam the greatest reduction in the terminal and basal
sections takes place the same year, in spruce 4 years after the first
feeding, in balsam 5*n

Following a spruce budworm attack with its resultant defoliation,
an almost total suppression of rings wag commonly observed in balsam,
and occasionally as much as 3 years' growth was lacking on certain
portions of the trunk. The spruce that recovered showed no indication
of missing growth rings, but recovery of both spruce and balsam to their
normal growth rates preceding the attack took 12 to 15 years.

Another interesting aspect of the last budworm outbreak in Canada
was that many of the trees that survived and lived through the worst
years of defoliation died 6 to 10 years later. These trees had all
regained their normal amounts of foliage, but never attained their
normal growth rate, as shown by the narrow annual rings 0 Craighead (6)
reports that an extensive study of these trees was made by W. E. Hi ley
(School ox Forestry, Oxford, England), who concluded that they were
dying from a lack of rings large enough to transport required amounts of
wrater to the crown.

I

Craighead (6) also noted that shortly after the defoliation of
balsam fir by the budworm, the absorbing rootlets began to die; the
number of dead rootlets bore a marked correlation to the severity of the
defoliation. On red spruce, however, the death of these rootlets was a
much more gradual process.

Closely integrated with the death of these small roots is the
correlation that exists between the recovery of trees following defol-=
iation and the amount of water available. Surveys show a higher
percentage of living spruce and balsam along streams, in depressions,
and on soils having a high water-retaining capacity; whereas on thin,
shallow, sandy soils the death rate following defoliation is always
higher.

Winter-killing is common among severely defoliated trees. The
ability to withstand freezing temperatures is associated largely with
the concentration of the cell sap, and trees that have been severely

2

defoliated cannot manufacture sufficient food to maintain high concen-
trations of cell sap.

<& All these observations on defoliation by the spruce budworm tend

to point out that the vigor or vitality of a stand at the time of
severe budworm attack determines to a large extent ho?j that stand vd.ll
survive. Young, vigorous growing stock stands a much better chance of
survival than old, overmature timber; and it is on the basis of this
that much of our present experimental budworm control work is being
carried on.

Defoliation By Other In sect s

Following the 1922 outbreak of the pine butterfly on portderosa
pine in Idaho, Evenden (10; reported that defoliation by this insect
resulted in a marked reduction in the basal growth of all 100 sample
trees, with 91 percent failing to add any basal increment. This per-
iod of no basal growth occurred in 96 percent of the trees that
succumbed and 89 percent of the trees that recovered, and it varied in
length from 1 to 11 years with an average of 2.6 years..

Evenden (10) and Craighead (6) agree that vigor, as expressed by
basal increment, has a definite influence on the recovery of injured

Table 1 . —Relation of defoliation by gypsy moth to
diameter increment of ~wh.it e pine, 1912-21

Degree of
defoliation ’
(mean percent)

-

Trees

—

Decline (from
average) in
radial increment"5

Number

Percent

0-20 •

268

21-40

17

21

41-60

32

41

61-80

20

52

81-100

12

38 •

^ Me an d.b.h. of white pine in 1912, 10.7 in.

, Dominant trees only.

■^Includes decline from all causes. Decline
from causes other than defoliation is practically
constant since the same trees were used in all
defoliation classes.

trees, the degree of defoliation, however, being the most important
factor governing their subsequent death or recovery.

The gypsy moth is primarily a defoliator of hardwoods, but has
been known to cause as much as 30 percent mortality in stands of heavily
damaged young white pine. From his observations on the effects of
gypsy moth defoliation on the growth of white pine. Baker (l) reported
the existence of a direct correlation between the degree of defoliation
and the decline in radial growth. As shown in table 1, the decline in
radial increment was 52 percent greater on white -pine that wore -1 -100
percent defoliated than in those that were defoliated 20 percent or less.

Since the greatest decline in radial increment took place in the
61--80 percent defoliated group, it is probable that the more lightly
defoliated trees represented average conditions. The surviving trees
in the most heavily defoliated class represented the most vigorous and
resistant individuals, because the majority of white pines of this size
are killed by a single complete defoliation.

The hemlock looper infestation in Oregon, which became epidemic
in 1915, ran as high as 4' million loopers to the acre and resulted in
the death of 40 million feet of timber before effective control measures
were employed (14 ) . Since the big consideration was to stop the rav-
ages of this pest and prevent further mortality, 'partial defoliation and
loss of increment were small factors.

In 1923, however, an outbreak of the hemlock looper occurred in
the spruce-fir forests of Quebec, north of the Gulf of St. Lawrence.
Watson (19) reported that a number of heavily defoliated balsam firs
exhibited no suppression of ring growth at the end of the first year of
feeding. The full effect of the 1923 feeding was not apparent until
the following year, when the 1929 ring showed a marked reduction, even
in those trees that did not undergo any further defoliation that year.

During the looper infestation, the growth of spruce remained
normal, since only the small trees suffered any defoliation, . Further
studies on the reduced increment of balsam fir were made in 1930, the
results of which are illustrated in figure 1.

The relatively slow growth of balsam fir in the Trinity River
section is due to the fact that the trees from, which the measurements
were taken were growing in a muskeg, associated with black spruce.

These trees were killed in 1923 without any apoarent reduction in
increment .

The Douglas -fir tussock moth is another defoliator that builds
up to epidemic proportions and causes widespread damage. The Canadian
Forest Insect Survey (5) reports that from observations made during
former outbreaks, it appears that trees completely defoliated will die,
and in many cases parts of trees that have been stripped fail to
recover.

4

Figure 1. — Hemlock looper damage to balsam fir,
as reported by Watson (19) .

The first indication of reduced growth due to defoliation by the
larch sawfly is reported by Harper (12) to be the absence of thickened
tracheids in the autumn wood. Later there is a reduction in the width
of annual rings and, in some instances, increment in. the basal portion
of the tree may be nonexistent. Although the growth may be insignifi-
cant at the base of the tree, there is an annual ring formed in the crown
every year until death.

The fact that defoliation of the crown by insect attack immed-
iately diminishes the increment has also been established by Btisgen and
Mlinch (4). They point out that the scanty amount of assimilated mat-
erial in partially defoliated trees is extracted from the descending sap
stream in the upper part of the stem, and in the lower portion growth is
practically at a standstill.

These investigators report that, in conifers, the needles of for-
mer years must assist in producing the spring shoots. Spruces complete-
ly defoliated by caterpillars in the summer invariably die because their
reserve materials can produce only weak shoots inadequate to nourish the
tree. Pines, if defoliated late in the season, can retain their
vitality better by the production of substitute shoots, but only at the
expense of reserve materials. Gradual recovery of these trees may
manifest itself in the production of short "brush shoots" the summer

5

following defoliation, and by long shoots with few needles during the
sec ond sunnier »

Defoliation By Fire

Most fires in coniferous stands result in a reduced leaf area
either from heat defoliation, which causes a gradual shedding of the
needles, or from direct scorching and destruction by flames.

Certain types of forest fires were noted by Craighead (jO to
produce characteristic growth- ring patterns that are, in many ways,
similar to those produced by several leaf-feeding insects. Cross
sections from a number of ponderosa pines that withstood a midsummer
f i re indicated that reduction in basal increment was proportional to the
amount of defoliation suffered by the tree. Certain trees failed to
add any wood in the lower stem for two seasons, while in the crown these
effects were less pronounced. These growth characteristics are similar
to those observed by Craighead (6) in his observations on the spx’uce
budworm. •

Observations of fire damage in the California pine forests by
Show and Kotok (15) also substantiate the fact that reduction in dj_am~
eter growth is proportional to the percentage of the crown killed by
fire. Table 2 illustrates this point, data being taken from increment
borings of several species 5 years after the burn.

Table 2 . — Effect of crown in, jury on rate of growth

Amount of
crown killed
(percent)

Reduction of
diameter growth

Basis

Trees

studied

Average

height

Percent

Number

F eet

17

11.0

9

68

25

28.5

12

71

33

32 c 0

19

68

50

39.0

10

58

67

56. 5

4

57

In a study of the effect of fire on the taper of yourfg longleaf
pine, Stone (16) shews that after a moderate to severe fire with
resultant defoliation of 50 percent or more, there is a marked reduction

7 TCTVT’' rv, 7 -j

6

in the next season's growth. As shown in table 3> the maximum reduction
in growth takes place at breast height and below, while the ring widths
at higher levels are affected only slightly. The net result is a de-
crease in stem taper.

Table 3 • — Comparative ring widths at various heights
before and after fire^

Height of section
above ground
(feet )

Tree Mo. 1

Tree

No. 2

Before

After

Before

“I

! After
I

Inches

Inches

Inches

;

Inches

4

0.20

0.02

0.22

0.11

8

.19

.04

.20

.09

12

.25

.06

.23

.12

16

.26

.08

.22

*15

20

.24

.11

.29

.18

24

.25

.12

.29

.22

28

*31

.19

—

.28

^"Radial growth in 1936-37 in stand of young long-
leaf pine near Saucier, Miss., burned January 8, 1937*

The fact that reduction in growth due to defoliation by fire does
not take place uniformly over the stem is in agreement with the observ-
ations on defoliation by insects, in which the greatest reduction in
increment takes place in the lower portion of the tree. Similar
results were noticed by Cummings (9) following pruning in a young short-
leaf pine plantation. For two growing seasons following pruning, there
was a marked, reduction in diameter growth at the 1.5“ and 4-5-foot
levels, but no significant difference at 7*5 feet. This decrease in
diameter growth in the basal portion of the stem was due to the reduct-
ion of the living crown, as was similarly the case in defoliation by
insects and fire.

Defoliation By Fungi And Hail

In addition to insects and fire, there are several other agencies
of defoliation that are resppnsible for less extensive damage. One of
these is hail, which, according to Stone and Smith (17.) has been known

to defoliate trees partially or wholly and to break through the bark oi
small branches. Their observations of hail damages in a you ng stand oi
longleaf pine indicate that severe defoliation retards diameter growth.
One particular storm, on April 30, 1937, occurred after most of the
springwood had formed, but the effects of defoliation took effect
immediately and only a narrow band of summerwood was laid down during
the remainder of that year. In 1933 the springwood zone was much less
than normal, the' summerwood exceeding it in width. The follov/ing year

ring widths indicated, the diameter growth was still considerably below
normal, and the recovery from the harmful effects of defoliation would
apparently be a slow process.

Another agency of defoliation is that group of needle- cast fungi
that occur on pine, spruce, fir, larch, and cedar. Boyce (2) reports
that the chief damage caused by these needle cast fungi is a reduction
in increment, since defoliation is rarely severe enough to kill any
trees except seedlings.

Artificial Defoliation Experiment

From the foregoing observations it is clear that defoliation
always results in decreased diameter growth. Such a reduction, if
spread over a large area like that covered by the spruce budworm, can
result in a great loss in volume . Since defoliation is so important
in respect to both mortality and growth, several experiments have been
made to measure its harmful effects more accurately.

In experiments on defoliation of longleaf and slash pine by fire
Harper (13-) found that if more than 2f percent of the needles are dec
troyed by the heat of the fire, gum yields will bo slightly decreased,
and tree growth will be retarded. Complete defoliation by burning of
the needles (as in a mild crown fire) caused three times the reduction
in gum yields that 100 percent defoliation by heat Killing (as in a hot
surface fire ) caused. In all cases of complete defoliation by both
the heat and flame treatments, there was subsequent deveioomenu of the
foliage and growth during the first season after the fire.

From his artificial-defoliation experiment on tamarack, Graham
(ll) made the following observations:

-1. Increment is reduced in direct proportion to the amount of
defoliation.

2. Partial defoliation results in a relatively gradual reduction in
increment.

3- Complete defoliation results in increased growth the first year.

followed by a rapid falling-off in increment. Very severe defol-
iation apparently stimulates the use of stored food in an attempt

8

to grow a new set of needles. Wood is also produced Dy this stimul-
ation , most ol the increased increment being added in the lower section
of the stem , the upper section showing little increase or decrease.

An experiment by Craighead (8) to determine the effects of
defoliation on jack pine, Scotch pine, and larch produced the following
results :

1. Early spring defoliation of the new growth on the pines produced a
gradual reduction in the formation of wood. Successive defol-
iation caused a more severe reduction in increment, especially in
the upper section of the tree.

2. Late spring defoliation of the new growth killed the jack pine
within two seasons, while the Scotch pine resisted better. This
treatment drastically affected the production of wood; the jack
pines formed only a trace of wood the following spring and then
died, while one of the Scotch pines failed to produce any wood.

In this study Craighead found that the reduction In wood of recov-
ering trees was relatively more pronounced in the top than at the
base. However, other investigators have observed that the great-
est reduction in growth occurred in the basal part of the tree.

3. Removal of the old foliage on both Scotch pine and jack pine very
early In the season was insufficient to cause mortality, but
resulted in a marked reduction in ring width at the base of the
tree in the years of defoliation. Trees recovered more quickly
from this treatment than from that in which the new growth was
removed.

4. Removal of the old foliage from both species of pine after the
buds had opened resulted in even less wood being produced than
treatment Number 3* The widths of the annual rings were about
one- fourth that of the previous year and remained at that low
level during subsequent years of defoliation. The results of
treatments 3 and 4 Indicate that the old needles play an import-
ant paid, in building up the current year's growth.

5. Complete removal of all the foliage from both Scotch pine and jack
pine resulted in death the following year. Bdsgen and Mdnch (4)
substantiate this observation by pointing out that artificial
defoliation of several young pines at different times of the year
caused death in all cases. However, when defoliation took place
after breaking of the buds, the trees produced some woody growth
until their reserves of starch were used up, then they died.

According to Burke (3) a similar condition exists following the
coincidental attacks of the lodgepole needle tier, which feeds on the
new foliage, and the lodgepole saw-fly, which eats the old foliage,
the result invariably being death to lodgepole pine.

9

Conclusions

In conclusion it may be said that the purpose of this survey ox
literature was to determine whether additional research should be made
on the effects of defoliation on the growth of conifers . As observed
from the literature reviewed, those effects that occurred most fre-
quently can be summed up as follows:

1. Defoliation of the crown by insects, fire, diseases, or other
causes immediately diminishes the increment, the reduction in
growth being directly proportional to the degree of defoliation.

2. Reduction in diameter growth due to defoliation does not take plac
at a uniform rate throughout the stem, In the majority of cases,
the greatest reduction in diameter growth took place in the basal

portion of the tree.

3. Tree vigor prior to defoliation has a definite influence on the
recovery of injured trees.

The literature cited in this report Is evidence of the fact that
a great many field observations and experiments have been made cn
defoliation, and suggests that results of future study to determine
its harmful effects on growth would be largely repetitious.

Ho?jever, there are several aspects of defoliation that might be
worth further study. Among them are these:

1. Do other species have increased growth in the basal portion of the
tree in the year following fire, as was reported for spruce, fir,
and larch?

2. The greatest reduction in growth following defoliation nas gener-
ally been found to occur in the basal portion of the tree. What
treatments and species are exceptions to this, in addition to the
exception reported by Craighead for pine?

3- What other species are capable of withstanding complete defol-
iation by fire, as reported for longleaf and slash pines by
Harper?

Results of future study on such questions will be helpful in
rounding out the present knowledge of how defoliation affects the
growth of conifers.

LITERATURE CITED

1. Baker, V. L.

1941.

Effect of gypsy moth defoliation on certain forest trees
Jour. Forestry 39: 1017-1022, illus.

2. ' Boyce, John Shaw,

1938.

Forest pathology. 600 pp., illus. New York and London.

3 . Burke , H .

1932.

E.

Two destructive defoliators of lodgepole pine in Yellow-
stone National Park. U. S. Dept. Agr. Cir. 224.

20 pp. , Illus .

4. Bllsgen, M. , and Milne h, E,

1929.

The structure and life of forest trees. (Translated by
T. Thomson) 436 pp., illus. New York.

5. Canada Division of Entomology.

1945.

Forest insect survey annual report. Canada Dept, Agr.

6? pp. , illus. Ottawa.

6. Craighead, F. C.

1924. Studies on the spruce budworm. Part II. General bio-

nomics and possibilities of prevention and control.
Canada Dept. Agr. Bui. 37= 28-37, illus.

7.

1927.

Abnormalities in annual rings resulting from fires.
Jour. Forestry 25 : 840-842, illus.

S.

1940.

Some effects of artificial defoliation on pine and larch,
Jour. Forestry 38: 885-888.

9* Cummings, W. H.

. 1942. Early effects of pruning in a young shortleaf pine plant

ing. Jour. Forestry 40: 61-62.

10. Evenden, J. C.

1940.

Effects of defoliation by the pine butterfly upon ponder-
osa pine. Jour. Forestry 38: 949-953* illus.

11 . Graham, S . A .

1931.

The effect of defoliation on tamarack. Jour. Forestry

29: 199-206, illus.

12. Harper, A. G.

■ 1913- Defoliation, its effect on growth and structure of the
wood of larix. Ann. Bot. (London) 27 ‘ 621-642.

library

1022500994

13. Harper, V. L.

1944. Effects of fire on gum yields of longleaf and slash pine
Uo S. Dept. Agr. Gir. ?10. 42 p p. , illus.

14. Holbrook, S.

1945* Loopers in the big timber. Amer. Forests 51: 476-479,
519-520, illus.

A

A

I

,1

.1

•■il

1

15. Show, S. B. , and Kotok, E. I.

1924- The role of fire in the California pine forests.

U. S. Dept. Agr. Bui. 1294- 80 pp., illus.

16. Stone, E. L.

1944. Effect of fire on taper of longleaf pine'.

Jour. Forestry 42: 6O7.

17. and Smith, L. F.

1941 - Hail damage in second growth longleaf pine.

Jour. Forestry 39: 1033-1035, illus.

18. Swaine, J. M. , and Craighead, F. C.

1924. Studies on the spruce budworm. Part I. A general ac-
count of the outbreaks, injury and associated insects
Canada Dept. Agr. Bui. 37: 3-27, illus.

19. Watson, E. B.

1934. An account of the eastern hemlock looper, Ellopia

fiscellaria Gn. , on balsam fir. Sci. Agr. 14: 669-678.
illus .

12

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