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Research Approaches to Understanding the
Roles of Insect Defoliators in Forest
Ecosystems
Karen M. Cl ancy 1
Abstract - Forest insect defoliators have traditionally been viewed as
pests because they damage their host plants, causing reduced growth and
reproduction or even mortality. However, many forest insect defoliators are
endemic species; they have coevolved with their hosts over thousands of
years, ancl they are important components of the forest ecosystem. We
need to develop a better understanding of the roles that insect herbivores
playas recyclers of nutrients, agents of disturbance,: members of food
chains, and regulators of the productivity, diversity and density of plants. I
review some of the empirical approaches that have been used by other
scientists to investigate the roles of insect defoliators as recyclers of
nutrients and regulators of primary production. These include: 1) Simulating
the effects of herbivory on forest biomass production; 2) Estimating
bioelement transfers by insect herbivores; and 3) Testing the effects of
herbivore density on primary production, nutrient turnover, and litter
decomposition in forest ecosystems. I also present some ideas I have on
using greenhouse experiments to investigate these roles for the western
spruce budworm (Choristoneura occidentalis)/Douglas-fir (Psuedotsuga
menziesiJ) model system I am working with. The strengths and weaknesses
of the various research approaches are compared.
INTRODUCTION
forest management activities that created forest conditions
favoring the survival or growth of these "pests" (USDA Forest
Service 1993). Thus, recent epidemics of native pests should be
viewed as symptoms rather than causes of "unhealthy" forests
(Wickman 1992).
Although current epidemics of native forest insect defoliators,
such as western spruce budwonn (Choristoneura occidenta!is),
are symptoms of previous management practices, Wickman
(1992) noted, that presettlement natural forest ecosystems also
suffered major pest outbreaks, implying that long-tenn stable
states for forest communities may be unnatural. The point here
is that native forest insect defoliators have coevolved with their
host trees over thousands of years. They are undoubtedly
important components of the forest ecosystem, functioning as
recyclers of nutrients, agents of distmbance, members of food
chains, and regulators of the productivity, diversity, and density
of plants. Thus, we need to develop a better understanding of
the roles that insect hetbivores play in forest ecosystems in order
to use an ecological approach to forest management.
The new Forest Service philosophy of ecosystem
management requires that we use an ecological approach to
managing our National Forests and Grasslands, so that they
represent diverse, healthy, productive, and sustainable
ecosystems. Accordingly, there is a growing recognition of the
need to understand the roles that insects and diseases play in
these ecological systems (e.g., see Seastedt and Crossley 1984,
Schowalter et al. 1986, Schowalter 1988, Wickman 1992, Haack
and Byler 1993, Schowalter 1993, USDA Forest Service 1993).
Many forest insects and diseases have traditionally been
viewed as pests because they damage their host plants, causing
reduced growth and reproduction or even mortality. It is
generally accepted that current and recent destructive outbreaks
of some native forest insects and diseases are largely due to past
1 Karen M. Clancy is a Research Entomologst and Acting
Project Leader, Rocky Mountain Forest and Range Experiment
Station, USDA Forest Service Research, 2500 S. Pine Knoll Dr.,
Flagstaff, AZ 86001
211
Generation of this new knowledge through research presents
interesting opportunities and significant challenges. I will review
some of the empirical approaches that have been used by other
scientists to investigate the roles of insect defoliators as recyclers
of nutrients and regulators of primaIy production I will also
present some ideas I have on how to address these questions
for the western spruce budwonnIDouglas-fir (Pseudotsuga
menziesii) model system I am working with.
Haack and Byler (1993) review information on the additional
roles of defoliators as agents of distwbance (and drivers of forest
succession), regulators of the diversity and density of plants, and
members of food chains. I will-not discuss research approaches
to quantifying and understanding these roles. Most of what we
know to date is based on observation of historical patterns (e.g.,
vegetation changes following "natural experiments"), and
associations between densities and distributions of insect
defoliators and insectivorous birds (e.g., see papers in Dickson
et al. [1979] and Morrison et al. [1990]). Although a lot of
research has documented a proininent role for birds as predators
of forest insect herbivores (H~lmes 1990), the importance of
herbivores as a food source that regulates the population
dynamics of birds (or insectivorous mammals, reptiles, etc.) is
largely unknown
Time
Figure 1. - The growth cycle (plant biomass) of a forest stand
over time, and the role of insect defoliators in thinning the
stand and recycling foliar nutrients (i.e., biomass reduction
phase). See text for details. Redrawn from Berryman (1986).
spruce-fIr). Wood growth was calculated using equations for
stand diameter growth, plus height growth CUlVes. Likewise,
data from the scientific literature were used to estimate foliage
and caterpillar production, plus the effects of the insect
defoliators on wood production. This calculated information on
annual biomass production was manipulated to map the
insect-plant interactions in a periodic coordinate system, as
shown in Figure 2 for the aspen-forest tent catetpillar system.
The "reference zero" (RO) circle represents null interactions
where systems with and without insects have equivalent biomass
production (insects plus vegetation). The area outside the circle
indicates cases where the system with insects has more total
RESEARCH APPROACHES
The underlying concept of the role that insect herbivores play
as recyclers of nutrients and regulators of primaIy production is
illustrated in Figure 1, redrawn from Berryman (1986). This
shows the growth cycle of a forest stand and the role of insects
in thinning the stand and recycling nutrients. When plants first
get established and during their maximum growth phase, water,
nutrients, and light are not limiting. However, when maximum
biomass is reached, these resources limit plant growth, and the
rate of increase of plant biomass declines. During the biomass
reduction phase, insect herbivores thin weakened trees from the
stand and recycle nutrients that were tied up in foliar and woody
biomass. This enables a growth recovery phase, where nutrients,
water and light are no longer limiting, which leads to a second
maximum biomass phase, which will be followed by another
biomass reduction, and so on
Aspen
RO
/
26,27
Simulating Effects of Herbivory on Forest
Biomass Production
In their seminal 1975 paper in Science, Mattson and Addy
simulated the effects of herbivory on forest biomass production
They used empirically -based simulations to quantify and
compare annual biomass production for aspen (Populus
tremuloides) and spruce and fir (Picea spp. and Abies balsamea)
forests with and without defoliation from one of their major
insect herbivores (forest tent catetpillars [Malacosoma disstria]
for aspen, eastern spruce budworm [CO fumiferana] for
Forest tent
caterpillar
(+) > (-)
/
(+) = (-)
(+) < (-)
~
Figure 2. - Periodic coordinate system map of interactions
between aspen forests and the forest tent caterpillar,
redrawn from Mattson and Addy (197&). See text for details
of how to interpret the insect-plant interactions.
212
production (i.e., there is symbiosis), whereas the area inside the
circle shows less total production with insects (i.e., the insects
parasitize the trees). The numbers near the triangles show the
age of the aspen stand.
The aspen-forest tent caterpillar interaction was
commensalistic at ages 26 and 27, when caterpillars were present
in the infested stand at low densities (Mattson and Addy 1975).
However, it moved into parasitic coaction space at age 28, as
the herbivore population increased. At ages 29 to 31, the
internctions moved inward to a maximum "parasitism" depth;
this was associated with outbreak levels of the forest tent
caterpillar in the infested stand: The interaction intensity
declined after the outbreak subsided (moved back towards RO),
but it remained in parasitic coaction space for ages 32 to 40.
The mapping showed that the forest tent caterpillar affected
forest production most severely in the fIfth and sixth years, but
after this the effect grndually diminished to become nearly zero.
Mattson and Addy (1975) noted that their forest tent
caterpillar-aspen example is typiqI for such outbreaks, which
usually last for 2 to 3 years and then subside. Few if any trees
die from such defoliation, except for suppressed individuals.
When simulating the internctions between spruce budworms
and their host forests, Mattson and Addy (1975) included the
understory response to overstory defoliation (Fig. 3). All
vegetative biomass production data was based on stemwood
increments. Figure 3 shows that whereas the budwonn outbreak
destroyed most of the overstory (note overstory after budwonn
curve), large numbers of understory seedlings and saplings
survived after defoliation and grew (see understory released
curve). By the 15th year following the outbreak, understOly
wood production in the defoliated (released) forest exceeded that
in the undefoliated (no budwonn) forest. Also note that the
overstory trees in the undisturbed (00 budwonn) forest had become
avigorous
inefficient producers due to old age and disease.
(the initial years of budworm population buildup), the
budwonn-balsam fir forest internction was commensalistic. At
age 55-60, during the peak of the outbreak, the internction was
strongly parasitic, and remained so for another 10 years after
the outbreak subsided. But, by age 70-75 (the 15th year after
the outbreak), with understory release, the interaction moved
from parasitism to symbiosis. In other words, wood production
in the defoliated forest exceeded that in the undefoliated forest
in the long run This implies insect-plant relations may be
mutualistic in the long tenn, despite temporary parasitic
coactions (Mattson and Addy 1975).
.
I
(·~6~.
Balsam Fir
240
50-55
commensalism ......:'
6-'6
60-65
'
tD
aj
0
0
Understory
Released
160
~oa:
Schowalter (1993) showed data from studies by Wickman
(1980) and Alfaro and MacDonald (1988) that provide direct
empirical support for such compensatory growth following
defoliation by forest insect herbivores. Figure 3 from Schowalter
(1993) illustrntes trends in the growth index of conifer trees
subsequent to defoliatio~ the initial reduction in growth caused
by herbivory was followed by greater long-tenn incremental
growth for defoliated trees relative to non-defoliated trees.
,-
/
I
I
I
I
I
Understory
No budworm
I
80
I
I
Overstory
After budworm
tD
en
I
i
I
i
/
/
•
.i
I
I
Estimation of Bioelement Transfers by Insect
Herbivores
"::;...(,~------..:-..:"""<--:-~ ,.-'
/
0
0
20
40
60
"
Figure 4. - Periodic coordinate system map of interactions
between balsam fir forests and the spruce budworm,
redrawn from Mattson and Addy (1976). See text for details
of how to interpret the insect-plant interactions.
/
I
11
(+) = (-)
(+) < (-)
I
aj
3: ME E-
,-
(+) > (-)
.~':J~
6
Overstory
'C'
..
Spruce
budworm
55-60
./
..............
65-70 '5.'l>-':J'\
tD
0 tD
.5 >"0 ~
0
RO
/
1:
E
,,6 ; 75-80
! L:z:90-95
\..
95-100
670-75
am
en
80-85
.... 6 .. 85-90
80
100
Age of forest since release (years)
Another way to examine the ecological roles of insect
herbivores in recycling nutrients is to estimate the trnnsfer of
bioelements from the canopy to the soil caused by insect feeding.
Larsson and Tenow (1980) used this approach to describe the
process of consumption by needle-eating insects in a mature (ca
120 years old) stand of Scots pine (Pinus sylvestris) in centrnl
Sweden
Figure 3. - Stemwood production by overstory and understory
balsam fir with and without spruce budworm outbreaks,
redrawn from Mattson and Addy (1976). See text for details.
The spruce budworm and spruce-fir coaction mapping
demonstrntes this positive releasing effect that defoliation had
on understory biomass production (Fig. 4). At stand age 50-55
213
grazing had removed 15.5 kg of needle biomass, which was 0.7
percent of total needle biomass or 2.5 percent of current-year
needle production Of the 15.5 kg removed by grazing, 1.5 kg
was green litter, or needles that were cut off by l3lVae but not
consumed; 14 kg was consumed, with 11 kg (79 percent) being
returned to the litter (soil) as insect feces.
The bioelement transfers of N, P, and K are shown in italics,
and are in grams dry weight per hectare (Fig. 5). When green
litter and feces inputs are combined, the input to the soil was
92 g ofN, 10 g ofP, and 48 g ofK. Larsson and Tenow (1980)
concluded that in 1974, feces plus green litter transferred about
1 percent of the (fatbon, calcium, and sodium, 2 percent of the
nitrogen, phosphorus, magnesium and sulfur, and 4 percent of
the potassium canied annually to the forest floor by total pine
litter. Thus, a part of the bioelement content of this ecosystem
is circulated through the insect herbivore consumer chain,
although Larsson and Tenow (1980) noted that the effect of
these bioelement transfers on soil processes are unknown
Larsson and Tenow (1980) made observations throughout the
season of the available needle biomass, and different age-classes
of needles, plus the abundance (i.e., number of larvae present)
of different insect groups, the grazing damage they cause<L and
their production of feces and green litter (needle litter cut off
by the larvae). Needle biomass and insect abundance and grazing
were measured from samples of canopy foliage taken from a
mobile skylift (the plot had a low density of trees and a level
ground swface). Litter-traps on the ground were used to sample
feces and green litter production The feces data were used in
combination with information on specific assimilation
efficiencies for each group of insect hetbivores to make indirect
estimates of needle biomass consumption In other words, they
conducted feeding studies in the laboratOIY to measure how
much frass the larvae produced when eating a known amount
of needle tissue. This allowed them to predict that if they
collected x amount of frass in their litter-trap, this means that y
biomass of needles was cons~ed. They also measured the
concentrations of bioelements .(including N, P, and K) in the
needles, the needle litter, the green litter, and insect feces. This
yielded calculations of bioelement fluxes from insect feces and
. green litter.
Figure 5 (redrawn from Larsson and Tenow 1980) is a
schematic representation of needle biomass and transfers of dry
matter (on an annual basis) estimated from this study for 1974;
the bold numbers show the biomass measurements in kilograms
dry weight per hectare. For example, there were 626 kg of
current-year needles, 771 kg of l-year-old needles, and 804 kg
of 2-year-old or older needles present in the canopy; 61 plus
674 kg of the l-year-old or older needles were dropped and
became 735 kg of needle litter. At the end of the season insect
Testing the Effects of Herbivore Density on
Primary Production, Nutrient Turnover, and Litter
Decomposition in Forest Ecosystems
Schowalter (1993) emphasized the need to use an ecosystem
framework for experiments that are designed to evaluate the
effects of insect defoliators on integrated forest ecosystem
processes. Figure 6, which is redrawn from Seastedt and
Crossley (1984), illustrates a simplified model of elemental
cycling in a terrestrial ecosystem, where the roles of arthropod
consumers (e.g., insect defoliators) are emphasized. Indirect
Kg d.w./ha
NEEDLES
of 1974
_ 0
626
-
N g/ha
P g/ha
K g/ha
RESPIRATION
~:f:~
3130
of 1973
1.7
~~:~
3855
'older than 1973'
674
-"
14.0
5.2
154
17
70
804
8844
965
4020
,r
11.0
1.5
735
,r
17
2
8
LITTER
..
PRODUCTION
FECES
PRODUCTION
GREEN UlTER
PRODUCTION
NEEDLE UlTER
PRODUCTION
3014
221
588
~
CONSUMPTION
NEEDLE
EATING
INSECTS
r
75
8
40
I
Figure 6. - Schematic representation of needle biomass in October and annual transfers of dry matter and the bioelements N, P, and
K, due to grazing of needle-eating insects and normal needle litter fall in a Scots pine forest in Sweden in 1974. Redrawn from
Larsson and Tenow (1980). See text for details.
214
ARTHROPOD
HERBIVORE
FOOD WEB
~111;",..············~I~ERFAlL
:~~~
. . :. :···:·:·:·:::··:······:·.·.:.:··:.::.OIttRiT!V()FU::.· .....•..
Figure 6. - A simplified model of elemental cycling in a terrestrial ecosystem, where the roles of arthropod consumers (e.g., insect
defoliators) are emphasized. Indirect regulation of elemental flows by arthropods are indicated by dashed lines and open arrows.
Redrawn from Seastedt and Crossley (1984). See text for details.
regulation of elemental flows by arthropods are indicated by
maintained at the 1 ha study site by manually adding or
removing larvae from individual trees, based on biweekly counts
during the feeding period. 1\venty trees were used per defoliation
treatment (low and high defoliator abundance), with equivalent
numbers used for controls. Each tree received the same treatment
for 3 years. Effects on primary production were measured by
estimating foliage and total plant mass from regressions based
on trunk diameters at the litter sunace. Small (1 g) samples of
current and l-year-old needles were collected from each tree in
June and analyzed for N, K, and Ca. Proportional sampler pans
were used to collect throughfalI/sternflow precipitation and
littetfall from 10% of the canopy of each tree. The throughfall
was shunted via plastic tubing to big jugs for storage. Mesh
screens in the collectors retained particulate matter. The
throughfall in the jugs and litterfall on the screens were
collected and measured twice a week, and composite samples
were analyzed for N, K, and Ca content. Finally, litteJ
decomposition rate was measured as mass loss of 10 litteJ
samples under each tree, using litterbags filled witl1
Douglas-fir needle litter. The N, K, and Ca content of the
litter samples was also determined at the start of the
experiment, and after 3-27 months in the field.
Based on their experimental results, Schowalter et al.
(1991) concluded that defoliation by the silver-spotted tiger
moth did not affect Douglas-fIT growth or foliar nutrient
content, suggesting compensatory growth and replacement of
lost nutrients. The decomposition rate of Douglas-fir needle
litter was also unaffected, implying that herbivory does not
"prime" decomposition via throughfall or litter enhancement.
However, the mass of litterfall and the volume and nutrient
content of the throughfall were positively related to defoliator
abundance during the early growing season. Turnover of N,
K, and Ca were also enhanced by the defoliation treatments.
dashed lines and open arrows. Insect heIbivores remove foliage
that contains bioelements from their host trees, but they also
return much of this material to the soil through their feces,
molted exoskeletons, and dead bodies. Nitrogen and other
minerals are more concentrated in this insect-derived material
than in the senescent leaves and needles that trees nonnally drop.
This provides increased nutrients to arthropod detritivores and
microfiora, which could stimulate the activity of decomposer
organisms, and enhance rates of decomposition of plant Iriaterial.
The green litter (partially consumed or clipped leaves and
needles) that results from heIbivore feeding is also a· richer
source of bioelements than normal senescent litterfall.
Furtbennore, accelerated leaching (or throughfall) of nutrients
from grazed foliage may make important contnbutions to the
inorganic mineral pool in the soil, where they could be
reassirnilated by roots.
Crossley et al. (1988) summarized conclusions from studies
at Coweeta that were centered on detennining the impact of
canopy arthropods on forest nutrient cycling. They found that a
partial defoliation in the Coweeta basin by the fall cankeIWonn
(Alsophiia pometaria) resulted in tnaIked changes in nutrient
cycling within the affected watersheds. Nitrate concentrations in
streams increased during the defoliation, there was a net increase
in net primary production, and increases in littetfall, nutrient
inputs from frass and canopy throughfall, and soil nitrogen pools
and associated rnicroflora.
Schowalter et al. (1991) have also tested the effects of
heIbivore density on primary production, nutrient turnover, and
litter decomposition of young (8 years old) Douglas-fir in
western Oregon The defoliator they used was the silver-spotted
tiger moth (Lophocampa argentata), which feeds only on
previous years' foliage. Target densities of the catetpillars were
215
productivity and recycling nutrients. Mattson and Addy (1975)
simulated the effects of defoliators on biomass production of
aspen and spruce-frr forests, using empirical data from the
literature. This was a powerful approach because it allowed
looking at long-tenn effects of hetbivory on primary production,
but it was a simulation rather than direct obselVation or an actual
experiment. Subsequent studies by Wickman (1980) and Alfaro
and MacDonald (1988) were based on direct obselVation of the
effects of different levels of defoliation on growth indices of
trees; the results supported Mattson and Addy's (1975)
hypothesis that insect-plant relations are mutualistic in the long
tenn. A limitatipn of all these studies was that the actual
mechanism for the compensatory growth following defoliation
was not iIwestigated. Recycling of nutrients through defoliation
was suggested, but not proven
Larsson and Tenow (1980) estimated annual nutrient transfers
from defoliators in a Scots pine stand, based on empirical
obselVations of foliar biomass, hetbivore abundance and grazing
damage, and the amount of frass and green litter produced by
defoliators. This study demonstrated that insect defoliators do
circulate a part of the bioelement content of the Scots pine
ecosystem, but it did not identify the long-tenn impacts of these
nutrient transfers on soil processes or primary productivity.
Very few experimental studies have actually tested the effects
of manipulated defoliator densities on primary production,
nutrient turnover, and litter decomposition Schowalter et al.
(1991) did this with young Douglas-fIT trees, and they found
that defoliation enhanced turnover of N, K, and Ca by means
of increased littetfall and throughfall. However, because their
study only lasted for 3 years, they could not address longer tenn
effects of defoliation on ecosystem processes.
I have proposed that greenhouse experiments could be used
to iIwestigate the role of western spruce budwonn defoliation
in recycling nutrients and regulating primary productivity of
Douglas-frr. The strength of this approach is that many of the
system inputs could be readily manipulated; the weakness is that
a grafted Douglas-frr tree in a pot may not respond the same
way a mature tree in the forest would. Also, it would not be
possible to recreate all the ecosystem components and larger
scale effects in a greenhouse environment. Nonetheless, I think
the strongly experimental approach that is possible using potted
plants and budwonn larvae from a laboratory culture could yield
valuable information that would be very helpful in terms of
identifying the key processes to monitor in large scale ecosystem
studies in the field.
Greenhouse Experiments with Western Spruce
Budworm and Douglas-fir
..., ' .
,'
Since 1985, I have been worlcing on a project designed to
detennine physiological mechanisms of Douglas-fir resistance
to western spruce budwonn defoliation. An important result of
this woIk has been the identification of 24 pairs of mature
Douglas-frr trees that are phenotypically "resistant" versus
"susceptible" to western spruce budwonn damage; the
resistance is associated with foliar nutritional chemistry, vigor
of growth, and phenology of budburst (Clancy 1991a, Clancy
et al. 1993).
We are now in the process of vegetatively propagating
cuttings from these 48 genotypes through grafting. This will
provide a pool of ontogenetically mature Douglas-frr "trees" in
pots that can be readily manipulated in greenhouse experiments
to evaluate the role of budwonn defoliation in changing plant
physiology and chemistry, afl!1 in recycling foliar nutrients. I
also maintain a laboratory clllture of non-diapausing western
spruce budwonn (Clancy 1991b), which gives me a continuous
supply of budwonn larvae to achieve prescribed levels of
defoliation on these potted trees. Moreover, these trees can be
manipulated through changing their exposure to day length and
temperatures so that 2 or 3 annual growth and defoliation cycles
can be compressed into a single year. This will enable much
more rapid determination of the long-tenn cumulative effects
that defoliation has on Douglas-frr physiology and productivity.
Another advantage of using a greenhouse experimental
approach will be the ability to manipulate nutrient inputs from
frass, green litter, and throughfalileaching. Screen barriers could
be placed around the base of the plant to intercept the frass and
littetfall, yet allow throughfal1. Or, by not using any overhead
watering system, I could eliminate throughfall. I could also add
frass and green litter from defoliated plants to undefoliated
plants. Furthennore, since I have a diversity of Douglas-fir
genotypes with different physiological characteristics to use, I
can examine variation in responses to defoliation It seems
possible that some genotypes are better adapted to tolerate and
compensate for defoliation than .others, and this may be an
important component of the resistance I have obselVed in the
field (Clancy et al. 1993).
Finally, underground components of the ecosystem could
presumably be manipulated as well. For example, mycorrhizal
associations could be enhanced via inoculations or reduced by
using fungicides. Similarly, soil dwelling detritivore insects
could be added at different densities. Diverse soil types and
nutrient regimes could be created, or soil pH could be varied.
SUMMARY AND CONCLUSIONS
ACKNOWLEDGMENT
Several research approaches have been used successfully to
increase our understanding of the roles that insect defoliators
play in forest ecosystems with regard to regulating primary
I thank Robert A. Haack and Michael R. Wagner for critical
comments.
216
LITERATURE CITED
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dynamics in a mature Scots pine forest in central Sweden
Ecological Bulletins (Stockholm). 32: 269-306.
Mattson, W.J.; Addy, N.D. 1975. Phytophagous, insects as
regulators of forest primary production. Science. 190:
515-522.
Morrison, M.L.; Ralph, C.J.; Verner, I; Jehl, IR., Jr. (eds.).
1990. Avian Foraging: Theory, Methodology, and
Applications. Studies in Avian Biology No. 13. Cooper
Ornithological Society. Allen Press, Lawrence, Kansas. 515
pp.
Schowalter, T.D. 1988. Forest pest management: a synopsis. The
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Schowalter, T.D. 1993. An ecosystem-centered view of insect
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Schowalter, T.D.; Hargrove, W.W., Crossley, D.A., Jr. 1986.
Herbivory in forested ecosystems. Annual Review of
Entomology. 31: 177-196.
Schowalter, T.D.; Sabin, T.E.; Stafford, S.G.; Sexton, 1M. 1991.
Phytophage effects on primary production, nutrient turnover,
and litter decomposition of young Douglas-frr in western
Oregon Forest Ecology and Management. 42: 229-243.
Seastedt, T.R.; Crossley, D.A., Jr. 1984. The influence of
arthropods in ecosystems. BioScience. 34: 157-161.
Wickman, B.E. 1980. haeased growth of white fir after a Douglas-fir
tussock moth outbreak Journal of ForestIy. 78: 31-33.
Wickman, B.E. 1992. Forest health in the Blue Mountains: the
influence of insects and disease. Gen. Tech. Rep.
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USDA Forest Service. 1993. Healthy forests for America's
future - a strategic plan USDA Forest SelVice Misc. Pub.
1513.58 pp
Alfaro, R.I.; MacDonald, RN. 1988. Effects of defoliation by
the western false hemlock looper on Douglas-fir tree-ring
chronologies. Tree-Ring Bulletin 48: 3-11.
Berryman, A.A. 1986. Forest Insects: Principles and Practices
of Population Management. Plenum Press, New YOtic. 279
pp.
Clancy, K.M. 1991a. Douglas-fir nutrients and terpenes as
potential factors influencing western spruce budworm
defoliation In: Baranchikov, Y.N.; Mattson, W.I; Rain, F.;
Payne, T.L. (eds.). Forest Insect Guilds: Patterns of
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