United States 

Department of 

Agriculture 

Forest 

Service 

Northern 

Region 

200 East Broadway 

P.O. Box 7669 

Missoula, MT 59807 

Numbered Report 06-06 April 2006

Spread and Intensification of Western Larch Dwarf Mistletoe (Arceuthobium laricis) 

Following Removal of an Infected Overstory and (or) Precommercial Thinning: 

Twelve-Year Remeasurements.

Marcus B. Jackson, Plant Pathologist 

USDA Forest Service, Forest Health Protection 

Missoula Field Office 

Michael A. Marsden, ITX Systems Analysis/Statistician 

Coeur d'Alene, Idaho 

Brennan A. Ferguson, Ferguson Forest Pathology Consulting, Inc. 

 Missoula, Montana. 

Introduction 

This report describes results of the 12-year 

remeasurement of plots established on the 

Flathead Indian Reservation in Montana (Taylor, 

et. al. 1993) in order to: 

1. quantify the spread and intensification of 

larch dwarf mistletoe (Arceuthobium laricis

(Piper) St. John) in western larch (Larix 

occidentalis Nutt.) regeneration following: 

a. overstory removal combined with 

precommercial thinning, 

b. overstory removal only, 

c. precommercial thinning only, 

d. no overstory removal or 

precommercial thinning; 

2. quantify growth impacts of dwarf 

mistletoe on western larch due to the above 

treatments; 

3. provide a visual demonstration of the 

treatment effects on stand growth and 

development; and 

4. validate the dwarf mistletoe model 

extension of the Forest Vegetation Simulator. 

Four replicates (plots) were installed in 1991 for 

each of the four treatments. Remeasurements 

were originally scheduled on a five year cycle, 

but ten-year remeasurements were postponed due 

to staffing changes. Taylor et al. (1993) and 

Taylor and Marsden (1997) reported on study 

objectives, plot establishment, baseline data 

collection, and 5-year remeasurements. 
2 

Methods 

Twelve-year data were collected in July and 

September, 2003, and included the following for 

all live, understory crop trees: 

• stem diameter at breast height (DBH); 

• total tree height; 

• height from base of tree to bottom of live 

crown; 

• Hawksworth (1977) dwarf mistletoe 

rating (DMR) for the lower, middle, and 

upper thirds of the live crown; 

• crown class (suppressed, intermediate, 

codominant, or dominant); 

• up to three additional damage-agents 

such as foliar fungi or snow damage; and 

• height of the highest and lowest live 

mistletoe infections (collected for first 

time in 2003). 

Trees dead since the previous remeasurement 

were noted and cause-of-death determined if 

possible. Dead trees were not included in 

analyses. 

Analysis of variance (ANOVA) was used to 

compare plot means for DMR, DMI (tree ratings 

averaged over infected trees only), height, and 

DBH between plot treatments: overstory removal 

vs. no overstory removal, thinning vs. no 

thinning, and the interaction of overstory 

removal and thinning. Treatments were applied 

at the plot level and 1991 plot means were used 

as covariates when covariates were used. These 

covariates adjusted means so treatments started 

with the same factor rating. 

Results and Discussion 

A total of 1,237 understory trees were tagged and 

measured when plots were installed in 1991. Of 

these, 1,186 were remeasured in 1996 (51 dead 

or missing) and 1,118 were remeasured in 2003 

(116 dead or missing and 3 not used in analyses). 

Cause-of-mortality was difficult to determine in 

many instances since interactions between 

multiple factors affected any given tree. 

Suppression, dwarf mistletoe infection, and (or) 

Armillaria root disease were variously 

implicated. Several trees were not located and 

some non-larch species, particularly subalpine 

fir, had been harvested as Christmas trees. 

Treatment differences are non-significant for plot 

means as measured (Table 1). With addition of 

covariates, DMR, height, and DBH are all 

statistically significant (P<0.1). The covariate 

reduced the error about the means more than it 

changed the means. 

Spread and intensification of larch dwarf 

mistletoe was least in nonthinned plots with no 

overstory removal, but greatest in thinned plots 

with no overstory removal over the first 12 years 

of this study (Figures 1 & 2). While there was 

no change in the percentage of larch infected in 

the nonthinned plots with no overstory removal, 

infection on the thinned plots with no overstory 

removal increased 16 % (Table 1). Increases in 

percent larch infected on the plots with overstory 

removal were 11 % on thinned plots and 8% on 

nonthinned plots. The DMR for nonthinned 

plots with no overstory removal only increased 

an average 0.4, while other treatments increased 

at least one full DMR class. 

Average DMR was significantly higher in the 

thinned versus nonthinned plots where the 

overstory was not removed (Table 1). The 2003 

adjusted ANOVA for DMR which included a 

covariate - plot mean DMR for larch in 1991 - 

showed that the interaction of overstory removal 

and thinning was significant to P = 0.1, while 

thinning itself was significant to P = 0.01 and 

overstory removal was insignificant (P = 0.28). 

Twelve years after treatment, thinning had a 

greater affect on DMR than overstory removal 

(Figure 2). 

Tree height was significantly greater in plots that 

were thinned with overstory removal than in 

thinned and nonthinned plots without overstory 

removal (Table 1). Average height was not 

significant in nonthinned plots with overstory 

removal compared to other treatments. The 2003 
3 

adjusted ANOVA for height, which included the 

covariate plot mean height for larch in 1991, 

showed that overstory removal was highly 

significant (P = 0.01), thinning was significant (P 

= 0.1), and the interaction between logging and 

thinning was significant (P = 0.06). Overstory 

removal appeared to be more important than 

thinning in regards to tree height 12 years after 

treatment (Figure 3). 

Average DBH was significantly greater on 

thinned plots with overstory removal compared 

to other treatments (Table 1). The 2003 adjusted 

ANOVA for DBH, which included the covariate 

plot mean DBH for larch in 1991, showed that 

overstory removal and thinning were each highly 

significant (P < 0.001) and the interaction of 

overstory removal and thinning was significant 

(P = 0.02). 
4 

Table 1. Dwarf mistletoe infection and host data following application of treatments in 1991 and 

remeasurements in 1996 and 2003. For 2003 adjusted-means, values followed by the same letter are not 

significantly different (p=.05). 

All species Western larch

Year Treatment 

No. % 

infected 

No. (% 

all spp.) 

% 

infected 

Avg. 

DMR 

Avg. 

DMI 

Avg. 

Height 

Avg. 

DBH 

Overstory removal 

& PCT 352 63 276 (78) 74 1.0 1.3 11.2 0.9

Overstory removal 

only 322 57 249 (77) 69 0.9 1.3 12.2 1.1 

PCT only 279 59 221 (79) 74 1.1 1.4 13.5 1.1 

1991 

No treatment 284 50 210 (74) 64 1.0 1.1 13.2 1.2 

Overstory removal 

& PCT 331 67 270 (82) 78 1.3 1.7 16.7 2.0 

Overstory removal 

only 304 58 244 (80) 67 1.1 1.6 16.7 1.8 

PCT only 274 70 219 (80) 89 1.8 2.0 18.3 2.0 

1996

No treatment 277 56 207 (75) 69 1.2 1.7 18.9 1.9 

Overstory removal 

& PCT 320 76 267 (83) 85 2.1 2.4 25.4 3.3 

Overstory removal 

only 283 69 237 (84) 77 1.8 2.2 22.4 2.7 

PCT only 256 74 204 (80) 90 2.3 2.6 22.6 2.7 

2003 

(means as 

measured)

No treatment 259 51 196 (76) 64 1.4 1.9 22.8 2.5 

Overstory removal 

& PCT 320 76 267 (83) 85 2.1 a b 2.4 a 27.4 b 3.6 b 

Overstory removal 

only 283 69 237 (84) 77 1.9 a b 2.2 a 22.8 a b 2.7 a 

PCT only 256 74 204 (80) 90 2.2b 2.5 a 21.2 a 2.6 a 

2003 

(means 

adjusted *) 

No treatment 259 51 196 (76) 64 1.4 a 2.1 a 21.7 a 2.3 a 

R Squared (correlation of the 2003 means with the 1991 means) 0.82 0.72 0.81 0.93 

Note: Columns containing "No." in heading show combined number of trees for all four plots with the 

particular treatment. All other values in the columns are means of the plot means for each factor. 

PCT=Precommercial Thin. 

* Means adjusted for the covariate (1991 means). 
5 

0

10

20

30

40

50

60

70

80

90

100

1991 1996 2003 2003 (adj)

Measurement Year

Percent Larch Infected

OSR/PCT

OSR

PCT

None

Figure 1. Infection in western larch by year and treatment. OSR = Overstory removal. PCT = 

Precommercial thin. 

0

0.5

1

1.5

2

2.5

1991 1996 2003 2003 (adj)

Measurement Year

Dwarf Mistletoe Ratin

g

OSR/PCT

OSR

PCT

None

Figure 2. Hawksworth dwarf mistletoe rating of larch crop trees by year and treatment. OSR = 

Overstory removal. PCT = Precommercial thin. 
6 

0

5

10

15

20

25

30

1991 1996 2003 2003 (adj)

Measurement Year

Height (ft)

OSR/PCT

OSR

PCT

None

Figure 3. Height of larch crop trees by year and treatment. OSR = Overstory removal. PCT = Precommercial thin. 

0

0.5

1

1.5

2

2.5

3

3.5

4

1991 1996 2003 2003 (adj)

Measurement Year

DBH (inches)

OSR/PCT

OSR

PCT

None

Figure 4. Diameter-at-breast-height of larch crop trees by year and treatment. OSR = Overstory 

removal. PCT = Precommercial thin. 
7 

Larch height and diameter growth were greater 

on the thinned plots receiving overstory removal 

compared to plots with no overstory removal 

(Figures 3 &4). Tree height on nonthinned plots 

with overstory removal was not significantly 

different than those on the other treatments; 

however, tree diameters on those plots were 

statistically equal to the plots without overstory 

removal and less than the thinned plots with 

overstory removal. 

The larch regeneration was nearing 20 years of 

age by the time this project was initiated and 

treatments applied (Taylor et. al. 1993). Because 

this regeneration had developed under a larch 

overstory with DMRs of mostly 4-5, much of it 

was likely infected with dwarf mistletoe when 

treatments occurred. Mathiasen (1998) showed 

that most western larch regeneration developing 

under an infected overstory will itself be infected 

prior to age 14. Thinning and (or) overstory 

removal occurred in 3 of the 4 treatments, 

allowing infections to occur as barriers to 

mistletoe seed dispersal were removed. In 

addition, latent infections at the time treatments 

were applied were better able to grow as host 

vigor improved. Mistletoe spread is not evident 

in the nonthinned plots without overstory 

removal, most likely because the plots have 

remained relatively undisturbed. 

Conclusion 

Twelve years post-treatment the positive effects 

of overstory removal and thinning on individual 

crop tree growth, and the negative effects of 

increased dwarf mistletoe spread and 

intensification among thinned trees is evident. 

Results of the fifteen and twenty year data 

should clarify the impacts of these treatments on 

crop tree growth and spread and intensification 

of dwarf mistletoe. 
8 

References 

Hawksworth, F. G. 1977. The 6-class dwarf 

mistletoe rating system. General 

Technical Report RM-48. USDA Forest 

Service Rocky Mountain Forest and 

Range Experiment Station, Fort Collins, 

Colorado. 7p. 

Mathiasen, R. L. 1998. Infection of young 

western larch by larch dwarf mistletoe in 

northern Idaho and western Montana. 

Western Journal of Applied Forestry 

13:41-46. 

Taylor, J., and Marsden, M. 1997. Permanent 

plots for studying the spread and 

intensification of larch dwarf mistletoe 

and the effects of the parasite on growth 

of infected western larch on the Flathead 

Indian Reservation, Montana: results 

from the 5-year re-measurement. FHP 

Report 97-2. USDA Forest Service 

Northern Region, Forest Health 

Protection, Missoula, Montana. 5p. 

Taylor, J., Reedy, T., and Corse, T. 1993. 

Permanent plots for studying the spread 

and intensification of larch dwarf 

mistletoe and the effects of the parasite 

on growth of infected western larch on 

the Flathead Indian Reservation, 

Montana. FPM Report 93-2. USDA 

Forest Service Northern Region, Forest 

Health Management, Missoula, Montana. 

13p.