Preliminary Results of the FVS
Gypsy Moth Event Monitor Using
Remeasurement Plot Data from Northern
West Virginia
Matthew P. Perkowski1
John R. Brooks1
Kurt W. Gottschalk2
Abstract—Predictions based on the Gypsy Moth Event Monitor were compared to remeasurement
plot data from stands receiving gypsy moth defoliation. These stands were part of a silvicultural
treatment study located in northern West Virginia that included a sanitation thinning, a presalvage
thinning and paired no-treatment controls. In all cases the event monitor under predicted the initial
mortality for all stands in terms of trees per acre. Prediction errors, with regards to trees per acre,
were largest in stands receiving the heaviest defoliation. In terms of basal area prediction, those
stands receiving the heaviest defoliation had predictions which were too high in the 4- to 5-year
period following initial defoliation, after which prediction error diminished. For stands receiving
light defoliation, predicted basal area was lower than in observed stands and this error increased
with the length of the projection period.
Introduction_______________________________________________________
The Gypsy Moth Event Monitor (GMEM) is a keyword set that modifies growth and
mortality within the Forest Vegetation Simulator (FVS) to simulate the effects of gypsy
moth infestations. The event monitor classifies the relative susceptibility of a stand
based on the percentage of susceptible species present within the stand. Outbreaks occur
stochastically and range in intensity based on stand susceptibility. After an outbreak
occurs, mortality is increased with the use of the FIXMORT keyword. Additionally, basal
area growth increment is reduced using the BAIMULT keyword. These modifiers predominantly impact susceptible species on moderate outbreak levels, but may also impact
resistant species when outbreaks are more severe. A previous silvicultural treatment
study in northern West Virginia was designed to moderate the impact of gypsy moth
infestation (Liebhold and others 1998). The purpose of this study is to compare stand
density predictions, in terms of basal area and trees per acre, from GMEM modified FVS
(GMEM-FVS) simulations using remeasurement data from the previous treatment study
following a gypsy moth outbreak.
Data______________________________________________________________
In: Havis, Robert N.; Crookston,
Nicholas L., comps. 2008. Third Forest
Vegetation Simulator Conference; 2007
February 13–15; Fort Collins, CO. Proceedings RMRS-P-54. Fort Collins, CO:
U.S. Department of Agriculture, Forest
Service, Rocky Mountain Research
Station.
Graduate student and Associate Professor, respectively, Forest Biometrics,
West Virginia University, Morgantown,
WV; e-mail jrbrooks@mail.wvu.edu.
1
Research Forester and Project Leader,
USDA Forest Service, Northern Research Station, Morgantown, WV.
2
The data are from a long-term experimental site located within the West Virginia
University Research Forest (WVURF) in Preston County, West Virginia. The WVURF is
a 7,664 acre forest primarily comprised of oak dominated and cove hardwood stands. In
1989, sixteen stands were selected to investigate the effects of silvicultural treatments
on the impact of gypsy moth as discussed in detail by Liebhold and others (1998). Each
stand was comprised of twenty 0.1-acre plots. Eight stands within the study received
either a sanitation thinning treatment or presalvage thinning treatment. The sanitation thinning treatment was a modified thinning from below, performed on stands with
less than 50 percent of the basal area comprised of susceptible species, to reduce stand
susceptibility by removing highly vulnerable trees regardless of value (Gottschalk 1993).
The presalvage thinning treatment was also a modified thinning from below; however
it was performed on stands having greater than 50 percent of the basal area comprised
of susceptible species, to reduce stand vulnerability while attempting to increase stand
vigor and value. Each of the eight treatment stands were paired with a control comprised
of similar species. The stands were remeasured annually until 1994, after which they
were measured periodically until 2004. At each measurement period, tree species, vigor,
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Perkowski, Brooks, and Gottschalk Preliminary Results of the FVS Gypsy Moth Event Monitor . . .
Table 1—Observed gypsy moth defoliation and tree mortality, by treatment type and stand number, for
stands located in Preston County, West Virginia.
Treatment type
Stand number
Control
Sanitation thinning
Presalvage thinning
2, 4, 5, 8, 10, 12
14, 16
1, 3, 6
7
9, 11
13, 15
Defoliation level
Light
Heavy
Light
Heavy
Light
Heavy
Actual mortality
%
25–35
50–60
20–30
20–30
12–20
30–37
defoliation, and dbh were recorded for every tree. Defoliation varied between stands,
with stands 7 and 8 of the sanitation thinning treatment and stands 13 through 16 of the
presalvage thinning treatment, receiving heavier defoliation than other stands within
the same treatment groups (table 1). In the control stands that received only minimal
defoliation, cumulative mortality ranged between 25 and 35 percent (in terms of trees
per acre). The heavily defoliated control stands experienced higher cumulative mortality
that ranged from 50 to 60 percent. In stands receiving the sanitation thinning treatment,
mortality ranged from 20 to 30 percent with very little difference between the heavily
defoliated and lightly defoliated stands. Two distinct populations existed within the
presalvage treatment stands. The heavily defoliated presalvage treatment stands had
mortality that ranged from 30 to 37 percent, while the presalvage stands that received
light defoliation experienced mortality ranging from 12 to 20 percent.
Methods__________________________________________________________
The 1989 data were input into the northeast variant of FVS and projected until 2004.
The projections were modified by the northeastern key component file of the Gypsy
Moth Event Monitor. A gypsy moth introduction period of 1985 was used for the event
monitor, based on literature (Hicks and Mudrick 1994). Outbreaks were stochastically
determined for each run, using a random seed that was allowed to vary. Output was
generated on a one-year cycle length. For this analysis, mean response was based on
twenty simulations for each stand using FVS. Measured in-growth was removed from
the data set during comparison to reduce confounding results. Simulated stand density,
in terms of trees per acre (TPA) and basal area per acre (BAAC), was compared to actual
recorded conditions.
Results___________________________________________________________
Simulation of Trees Per Acre
GMEM-FVS simulations under predicted mortality, for both the stands receiving
sanitation thinning treatment and their paired control stands (fig. 1). For those sanitation
treatment stands receiving the heaviest defoliation (Stands 7 and 8), over prediction of
stand density increased quickly for three years, after which, both the treated stand and
the paired control leveled off at an over estimation of approximately 38 TPA. Additionally,
GMEM-FVS over predicted TPA for the sanitation treatment stands and their paired
controls which received the lightest defoliation (Stands 1 through 6). Prediction errors
for these stands followed a pattern similar to the heavily defoliated stands; however, the
prediction error was greater for the untreated stands (Stands 2, 4, and 5). The GMEM
modified FVS projection for Stand 1 had the smallest prediction error, approximately
20 TPA.
In most cases, GMEM-FVS simulations over predicted stand density for the stands
receiving the presalvage treatment and their paired controls (fig. 2). This over prediction
ranged from less than 20 TPA, under light defoliation conditions for most treated stands
(Stands 9 and 11), to over 80 TPA for the heavily defoliated control stands (Stands 14
and 16). Prediction errors were slightly higher in the fourth remeasurement period for
presalvage treatment stands receiving heavier defoliation (Stands 13 and 15), but this
error decreased over the rest of the projection period. The largest over prediction of stand
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USDA Forest Service Proceedings RMRS-P-54. 2008
Preliminary Results of the FVS Gypsy Moth Event Monitor . . .
Perkowski, Brooks, and Gottschalk
Figure 1—Residual (simulated–actual) TPA for stands receiving the sanitation thinning. Dark symbols reflect stands receiving the sanitation thinning treatment. Symbols of the same shape, but not filled in, are the paired non-treated controls
Figure 2—Residual (simulated–actual) TPA for stands receiving the presalvage thinning treatment. Dark symbols
reflect stands receiving the presalvage thinning treatment. Symbols of the same shape, but not filled in, are the paired
non-treated controls.
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Perkowski, Brooks, and Gottschalk Preliminary Results of the FVS Gypsy Moth Event Monitor . . .
density was associated with the untreated control stands that experienced the heaviest
defoliation (Stands 14 and 16). Prediction errors for these stands were the greatest at the
fourth remeasurement period and declined over the course of the projection period.
Simulation of Basal Area per Acre
GMEM-FVS simulations generally over predicted BAAC for the sanitation thinning
treatment stand and its paired control that received the heaviest defoliation (Stands 7
and 8, figure 3). The over prediction in these stands was greatest in the second through
fifth remeasurement period, after which this error decreased with time. In all cases, the
simulations under predicted BAAC for the sanitation thinning treatment stands and
paired control stands which received the lightest defoliation (Stands 1 through 6). The
magnitude of this prediction error increased with time.
GMEM-FVS simulations over predicted BAAC for presalvage thinning treatment
and paired control stands by up to 70 ft2/ac. at the fifth remeasurement period, for those
stands receiving the heaviest defoliation (Stands 13 through 15) (fig. 4). The magnitude
of this prediction error decreased with time, with the untreated control stands exhibiting higher prediction errors than the paired treated stands. In all cases, the simulations
under predicted BAAC for presalvage thinning treatment stands and their paired controls
that experienced light defoliation (Stands 9 through 12). This prediction error increased
linearly to an under prediction of approximately 30 ft2/ac.
Figure 3—Residual (simulated–actual) basal area per acre for stands receiving the sanitation thinning. Dark symbols
reflect stands receiving the sanitation thinning treatment. Symbols of the same shape, but not filled in, are the paired
non-treated controls.
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Preliminary Results of the FVS Gypsy Moth Event Monitor . . .
Perkowski, Brooks, and Gottschalk
Figure 4—Residual (simulated–actual) basal area per acre for stands receiving the presalvage thinning treatment.
Dark symbols reflect stands receiving the presalvage thinning treatment. Symbols of the same shape, but not filled in,
are the paired non-treated controls.
Discussion________________________________________________________
GMEM-FVS projections increasingly underestimated TPA mortality over the course
of the projection period, for stands that experienced low defoliation levels. These stands
exhibited an increase in actual mortality over time, which was continuously underestimated by the scheduled outbreaks simulated in FVS.
FVS simulations over predicted TPA at the point of initial mortality, regardless of
stand treatment, for those stands that experienced high defoliation levels. For the heavily
defoliated stand receiving the sanitation thinning treatment and its paired control (Stands
7 and 8), predicted TPA paralleled actual stand development after the initial defoliation
event. This would indicate that equilibrium in overall stand density was achieved, but at
a level higher than that in the observed stands. Actual stand mortality increased slowly
following the initial defoliation event, which the event monitor accurately captured by
scheduling several outbreaks over the course of the FVS projection period. Presalvage
thinned stands and their paired controls that experienced high defoliation (Stands 13
through 16), exhibited little increase in actual mortality after the initial defoliation event.
GMEM scheduled several outbreaks over the rest of the FVS projection period, which
reduced the error in TPA associated with the presalvage thinning treatment.
The Gypsy Moth Event Monitor relies on the classification of stands into susceptibility
levels, which scale the impact of outbreak. Stands 7 and 8 experienced high defoliation,
but were classified as moderately susceptible stands by GMEM. This reduced the impacts
of outbreaks scheduled by the event monitor and resulted in increased error associated
with predicted stand density (TPA). This classification limitation reduces the accuracy
of the event monitor. This suggests that users simulate these stands at multiple susceptibility classes when they are on the border of a classification level in order to obtain
more accurate results.
Basal area per acre was initially over predicted for stands that experienced high
defoliation levels. These prediction errors decreased over time due to multiple scheduled
outbreaks present in the FVS simulations. Stands that experienced light defoliation
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Perkowski, Brooks, and Gottschalk Preliminary Results of the FVS Gypsy Moth Event Monitor . . .
exhibited little reduction in actual basal area growth over time. Basal area was under
predicted by the event monitor, which often scheduled one or more outbreaks over the
FVS simulation period. The outbreaks had a greater effect on the presalvage thinned
stands and their paired controls, which had a higher susceptibility class within the
event monitor. This higher susceptibility class resulted in greater prediction error due
to higher severity outbreaks.
Overall, GMEM-FVS projections performed most accurately on stands where actual
mortality increased slowly over the course of the simulation. The Gypsy Moth Event
Monitor modified Forest Vegetation Simulator projections underestimated mortality for
all stands, especially those that received high defoliation. Additionally, the simulations
underestimated the basal area per acre reductions that accompanied heavy defoliation
events. Further analysis is currently underway to determine whether the trends seen in
this study are also observed for different stands, defoliated by the gypsy moth, throughout
the Appalachian region.
References________________________________________________________
Gottschalk, Kurt W. 1993. Silvicultural guidelines for forest stands threatened by the gypsy moth.
Gen. Tech. Rep. NE-171. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 49 p.
Hicks, Ray R., Jr. Mudrick, Darlene A. 1994. 1993 forest health: A status report for West Virginia.
Charleston, WV: West Virginia Department of Agriculture.
Liebhold, Andrew M.; Muzika, Rose-Marie; Gottschalk, Kurt W. 1998. Does thinning affect gypsy
moth dynamics? Forest Science. 44(2):239–245.
The content of this paper reflects the views of the authors, who are responsible for the facts and accuracy of the information presented herein.
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