United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

General Technical

Report NRS-2

Commercial Thinning 

in Small-Diameter Aspen 

Stands in Northern Minnesota: 

Study Establishment Report

Daniel W. Gilmore

Jennifer D. Glenn

Michael E. Ostry

John C. Zasada

Michael A. Benedict
Published by: Available online only at

USDA FOREST SERVICE www.nrs.fs.fed.us/pubs/

11 CAMPUS BLVD SUITE 200

NEWTOWN SQUARE PA 19073-3294 

December 2006

Visit our homepage at: http://www.nrs.fs.fed.us/

Abstract

In the spring of 1999, a long-term study was established to examine the physical and 

biological aspects of thinning young aspen stands in Minnesota. Three aspen stands ranging 

in age from 25 to 35 years were selected on lands owned by the State of Minnesota and 

UPM Kymmene. Two thinning treatments (low and high density) and an unthinned control 

were installed at each of the three locations. Permanent plots were installed to measure tree, 

shrub, and herb growth, and to monitor harvesting damage, insect and disease damage, soil 

strength, and fuel loadings.

After 4 years, tree mortality was greater in the unthinned controls. Thinning treatment had no 

significant effect on the incidence of white trunk rot (Phellinus tremulae), Hypoxylon canker, 

or Saperda calcarata. No differences in post-harvest fuel loadings were detected among 

locations and treatments. Thinning increased the amount of light reaching the forest floor that 

resulted in greater herb and shrub biomass in the year immediately following thinning.

Cover Photos

Clockwise from left: harvesting damage on reserve tree, clearcut travel corridor in high 

density thinning treatment, commercial thinning of aspen with a mechanical harvester. 


Introduction

Since the 1940s, aspen fiber shortages in the Lake States 

have been predicted to occur in the early decades of the 

21

st century (Berguson and Perala 1988, Chase 1947, 

David et al. 2001). To avoid such shortages, thinning has 

been suggested as a means to capture fiber volume that 

would otherwise be lost through self-thinning. Ideally, 

thinning would increase the growth of the highest quality 

trees, increase the total yield of merchantable material, 

and increase the net value of the products harvested at 

the end of the rotation. In reality, thinning improves 

the growth of individual trees but reduces total foliar 

biomass. This often offsets any increases in individual 

tree growth at the expense of total stand growth (Smith 

et al. 1997).

Aspen is host to a large number of endemic insect pests 

and pathogens that under some conditions reduce the 

productivity and quality of affected trees. Fortunately, 

few insect pests and pathogens have been serious enough 

to concern forest managers under most management 

systems. However, there is evidence to suggest that 

Hypoxylon canker caused by the fungus Entoleuca 

mammata has greater negative impact in understocked 

or thinned stands than in well-stocked stands (Anderson 

and Anderson 1968, Anderson and Martin 1981, Bruck 

and Manion 1980, Capony and Barnes 1974, Day 

and Strong 1959, Ostry and Anderson 1979, Ostry 

and Anderson 1998, Schreiner 1925). In other studies 

investigators did not observe increases in Hypoxylon 

canker in thinned stands (Anderson 1964, Pitt et al. 

2001). Differences in disease susceptibility among aspen 

clones may have more influence on disease incidence and 

severity than treatment or location effects (Ostry et al. 

2004).

In all the thinning studies reviewed (Bickerstaff 

1946; Gilmore 2003; Perala 1977, 1978; Schlaegel 

1972; Steneker 1964; Steneker and Jarvis 1966; 

Zasada 1952; Zehngraff 1946, 1947), the authors 

recommended that aspen thinning be limited to better 

quality sites to promote stand vigor and health, derive 

maximum growth, and economic benefits. Based on 

the accumulated experience of trials conducted from 

the 1940s through the 1970s, Perala provided the 

following guidelines for thinning for increased wood 

and fiber production: 1) conduct thinning on higher 

quality sites—site index 70 and above; 2) thin in stands 

that are 25 to 35 yr old, preferably 25 to 30 yr. The 

commercial thinning options outlined by Perala (1977) 

were a single thinning to about 240 trees ac-1 (TPA) at 

30-35 yr; or two precommercial thinnings to 550 TPA 

at 10 yr and to 200 TPA at 30 yr. In Perala (1978), more 

intensive thinning schedules are modeled: the lowest 

density recommended is 180-190 TPA at 30 yr, but this 

recommendation follows two precommercial thinnings at 

10 and 20 yr.

The thinning prescriptions are based on older trials using 

individual tree selection as opposed to the new system 

that is basically a row thinning with all trees removed in 

an 8- to 12-ft area. Variable amounts of removal occur 

between rows depending on the desired density/basal 

area. To our knowledge, no aspen thinning trials have 

been established using a cut-to-length harvesting system 

that compare two residual densities and a control. Three 

stands between the ages of 25 and 35 yr were selected 

for study in northern Minnesota to determine the effects 

of thinning with cut-to-length technology in aspen 

forests on 1) mechanical damage; 2) development of 

understory trees, shrubs, and herbs; 3) soil compaction 

as determined via soil strength; 4) insect and disease 

damage to the residual stand; and 5) forest fuel loadings 

immediately following thinning.

The Authors

DANIEL W. GILMORE, Residents’ Committee to Protect the 

Adirondacks, North Creek, NY. Formerly with Department of 

Forest Resources, University of Minnesota, St. Paul.

JENNIFER D. GLENN, Minnesota Department of Natural 

Resources, Sandstone, MN. Formerly with Department of 

Forest Resources, University of Minnesota, St. Paul.

MICHAEL E. OSTRY, U.S. Forest Service, Northern 

Research Station, St. Paul, MN.

JOHN C. ZASADA, U.S. Forest Service, Northern 

Research Station, Grand Rapids, MN. (retired)

MICHAEL A. BENEDICT, USDI Bureau of Indian Affairs, Ft. 

Snelling, MN. Formerly with U.S Forest Service, Northern 

Research Station, Grand Rapids, MN. 
Methods and materials

Study Locations and Thinning 

Treatments

The three study locations were located in Itasca 

County, Minnesota. The legal description for the 

UPM Kymmene site is T58 R23 S35 SW1/4. The legal 

descriptions for the two sites managed by the Minnesota 

Department of Natural Resources are T60 R25 S29 

NE1/4 (site 29), and T60 R25 S33 N1/2 SW1/4 (site 

33). Pre-treatment data were collected using temporary 

plots only in the areas to be thinned. Across all locations, 

pre-treatment basal area (BA) ranged from 100 to 170 

ft2 ac-1, trees per acre (TPA) ranged from 680 to 1,110, 

and average diameter at breast height (d.b.h.) ranged 

from 4.8 to 5.8 in. (Table 1a). Site indices were 80, 75, 

and 60; and ages were 35, 35, and 30 yr for site 29, site 

33, and the UPM Kymmene site, respectively. White 

birch was a significant component of the stand on site 

29 before thinning (Table 2). The UPM Kymmene site 

was harvested in the fall of 2000, and sites 29 and 33 

were harvested in the winter of 2001.

Two thinning treatments (low and high density) and 

an unthinned control were installed at each of the 

three locations. The target BA and density was 40 

ft2ac-1 and 180 to 200 TPA, respectively, for the lowdensity treatment and 60 ft2

 ac-1 and 280 to 300 TPA, 

respectively, for the high-density treatment. Travel 

corridors (8 to 12 ft wide) were spaced approximately 

40 to 45 ft apart and designated before thinning began. 

Using a cut-to-length harvester, the equipment operator 

removed trees from either side of the designated 

travel corridor. The operator was instructed to retain 

apparently healthy trees with good form and no injuries.

Experimental Design and Field 

Procedures

Rectangular (132 ft by 33 ft) one-tenth acre sample 

plots to measure tree data were established the spring 

following treatments (2001) and the corners were 

identified using wooden and metal stakes and an 

identification tag. Plots within the thinning treatments 

were positioned to include three travel corridors. The 

diameter for all living and dead trees greater than 1 in. 

d.b.h. was measured (the tree locations on each plot 

were mapped). Total height was measured on every 

tenth tree. Pre- and post-thinning mechanical, insect, 

and disease damage on boles was examined with the aid 

of binoculars, and the presence and extent of the major 

damaging agents were recorded on all residual trees, 

which were the potential crop trees. 

Twelve 4.1-ft radius plots to measure shrub and tree 

regeneration, and twelve 1.9-ft radius plots to measure 

herbs and forbs were nested within the rectangular 

plots. The total number of shrub species and tree 

regeneration, stem diameter at 6 in. above the soil 

surface, and height of the tallest individual of each 

species were recorded on the shrub and regeneration 

plots. Shrub control plots were not measured in 2001. 

Shrub biomass was estimated using equations extracted 

from the literature by Johnson (2004). Biomass 

for herbs and forbs was estimated by 100 percent 

removal and ovendrying of a sample collected from 

a one-thousandth-ha plot located on 100 percent of 

the subplots before treatment and on a subsample of 

the subplots following treatments. Soil strength was 

measured before and after thinning on semipermanent 

plots with a soil penetrometer. Soil compaction meters 

(often called penetrometers) are used to determine the 

density of soil. An operator pushes a rod with attached 

cone (ASAE standard) into the ground. The resistance 

of the cone as it is pushed in the ground is measured 

and recorded in the memory of the compaction meter. 

The depth of the cone below the soil surface is also 

measured and recorded in the storage memory of the 

penetrometer.
Table 1a.—Pre-thinninga

 (yr 2000), post-thinning (winter 2001-2002), and 2-yr post-thinning basal area (BA), trees per acre 

(TPA), and average diameter at breast height (d.b.h.) measurement for the three treatments in the three stands included in the 

aspen thinning study (English units of measure)

Location and 

treatment

Target

BA

Basal area Target

TPA

Trees per acre Quadratic d.b.h. by location

Pre-thin Post-thin Spring 

2004

Pre-thin Post-thin Spring 

2004

Pre-thin Post-thin Spring 

2004

UPM-Kymmene ------------------ft2 ac-1-------------------- ----------------in.----------------

 Control 99 95 509 427 6.0 6.4

 Low density 40 108 48 37 180-200 680 219 180 5.4 6.3 6.1

 High density 60 100 56 52 280-300 782 339 292 4.8 5.5 5.7

Site 29

 Control 137 133 872 617 5.4 6.3

 Low density 40 143 47 47 180-200 1,110 201 181 4.9 6.5 6.9

 High density 60 129 72 75 280-300 902 301 279 5.1 6.6 7.0

Site 33

 Control 125 121 1,040 768 4.7 5.4

 Low density 40 125 43 44 180-200 816 212 191 5.3 6.1 6.5

 High density 60 170 64 65 280-300 918 276 268 5.8 6.5 6.7

a

 Note: UPM Kymmene thinnings occurred in the fall of 2000.

Table 1b.—Pre-thinninga (yr 2000), post-thinning (winter 2001-2002), and 2-yr post-thinning basal area (BA), trees per acre 

(TPA), and average diameter at breast h eight (d.b.h.) measurement for the three treatments in the three stands included in the 

aspen thinning study (Metric units of measure)

Location and 

treatment

Target Basal area Target Trees per acre Quadratic d.b.h. by location

BA Pre-thin Post-thin Spring 

2004

TPA Pre-thin Post-thi Spring 

2004

Pre-thin Post-thin Spring 

2004

UPM-Kymmene --------------------m2 ha-1------------------ ----------------cm----------------

 Control 23 22 1.258 1,055 15.2 16.2

 Low density 9 25 11 8 445-494 1,680 541 445 13.7 16.1 15.6

 High density 14 23 13 12 692-741 1,932 838 722 12.3 14.0 14.5

Site 29

 Control 0 0 31 31 0 2.155 1,525 0.0 13.6 16.0

 Low density 9 33 11 11 445-494 2,743 497 447 12.3 16.6 17.5

 High density 14 30 17 17 692-741 2,229 744 689 13.0 16.8 17.8

Site 33

 Control 0 0 29 28 0 2.570 1,898 0.0 11.9 13.7

 Low density 9 29 10 10 445-494 2,016 524 472 13.5 15.5 16.5

 High density 14 39 15 15 692-741 2,268 682 662 14.8 16.6 16.9

a

 Note: UPM Kymmene thinnings occurred in the fall of 2000.
Damage from Harvest and Insects and Diseases

Harvesting damage was considered to be removal of 

bark from the tree bole. The affected area and height on 

the tree of all wounds was recorded on all trees in each 

plot. The establishment of this study coincided with a 

forest tent caterpillar (Malacosoma disstria) outbreak in 

2000-2001. Other damaging agents recorded included 

Hypoxylon and Nectria cankers, white trunk rot caused 

by Phellinus tremulae, Saperda calcarata, Agrillus flatheaded wood borers, frost cracks/ sunscald, and open 

scars caused by animal debarking and mechanical wounds 

through the bark.

Forest Fuels Sampling

A planar intersect sampling method was used to 

inventory fuel loadings (Brown 1974). The method is 

rapid, easy to use, and can be applied to naturally fallen 

debris and slash. In brief, 32.8-ft (10 m) fuel sampling 

transects were established across subplots. The 10-m 

transects included smaller nested transects to measure 

diameter classes that correspond to 1-hr, 10-hr, and 100-

hr average moisture time lag classes. One-hr [0 to 0.25 

in. (0.63 cm) diameter] and 10-hr [0.25 to 1.0 in. (0.63 

to 2.54 cm) diameter] fuels were measured in the first 1

m (3.28 ft) of each transect. In the unharvested controls 

and thinned plots, 100-hr [1.0 to 3.0 in. (2.54 to 7.5 

cm) diameter] fuels were tallied along the entire 10 m 

(32.8 ft) of each transect. In the travel corridors, 100-

hr fuels were measured along a 2-m (6.56 ft) transect. 

Heavy fuels greater than 3.0 in. (7.5 cm) are 1,000-hr 

fuels and were measured along the entire length of every 

10-m transect. Diameter and condition class (sound, 

rotten) were recorded for 1,000-hr fuels only. At intervals 

of 5, 6, 10, and 11 m (16.4, 19.7, 32.8, and 36.1 ft) 

along each transect, duff and litter depths were recorded.

Field data were converted to tons ac-1 using techniques 

described by Brown (1974). The equation for fine fuels 

was:

(1) tons ac-1 = (11.64 • n • d2 • Sg • a • c)/(N • l)

where n = total tally of pieces for size class, d2 = 

average diameter class squared in imperial units of 

measure—constant for each size class (metric groupings 

for each fuel size class: 0-0.6 cm = 0.0151; 0.6-2.5 

cm = 0.289; 2.5-7.6 cm = 2.76), Sg = specific gravity, 

a = nonhorizontal angle correction factor, c = slope 

correction factor, N = number of transects, and l = 

transect length in feet.

Table 2.—Percent standing dead trees for all species and percent bircha composition 

by location and treatment in 2000, 2002, and 2004

Location and treatment 2000

pre-thin

2000

% birch (living trees)

2002

post-thin

2004

UPM-Kymmene

 Control 23b 0 no data 21

 Low density 22 1 2 14

 High density 17 1 19 32

Site 29

 Control 1b 18 no data 19

 Low density 22 17 0 10

 High density 23 24 0 7

Site 33

 Control 1b 4 7 32

 Low density 22 5 1 12

 High density 24 3 0 6

a

 Paper birch was the only non-aspen species that was a significant stand component before 

thinning

b

 Data collected for crop tree aspen only.
Tons ac-1

 of heavy fuels were estimated with the equation:

(2) tons ac-1 = (11.64 • Σd2 • Sg • a • c)/(N • l)

where all variables are as previously defined except that 

Σd2 represents a sum of the actual squared diameters. 

Tons ac-1 were converted to tonnes ha-1 using a 

conversion factor of 2.2417.

Data Analyses

Summary data are presented in tabular form by location 

and treatment. Analyses of variance (ANOVA) were 

performed using the general linear model:

y = location + year + treatment + location•treatment 

+ location•year + year•treatment + 

location•year•treatment + e

where y = the measured variable, year of measurement, 

location = site 29, site 33, or the UPM Kymmene site, 

treatment = control, high density thinning, or low 

density thinning, and e = error NID~(0, σ2).

Results

Pre- and Post-thinning Stand Conditions

Pre- and post-thinning stand conditions are presented 

in Tables 1a and 1b. Note that trees were not numbered 

during the collection of pre-treatment data. Therefore, 

it is not possible to match individual tree measurements 

from the pre- and post-treatment time periods. In 

addition, unharvested control plots were measured the 

winter following the thinning of the treated plots. In all 

instances, greater BAs were retained than the target BA 

except at the high-density UMP Kymmene plot where 

residual BA was slightly lower. Target TPAs were nearly 

achieved in all but the UPM Kymmene plots where TPA 

was higher. Average d.b.h. increased in all plots following 

thinning, which is indicative of thinning from below 

where high-quality, larger diameter trees are retained 

(Smith et al. 1997).

Before thinning, standing tree mortality ranged between 

17 and 24 percent; after thinning it was reduced to 

nearly zero (Table 2). Standing tree mortality in the 

unthinned control plots varied considerably during the 

initial measurement period. Average tree mortality in 

the control plots differed by location, ranging between 

19 and 32 percent following two growing seasons (Table 

2). The number of standing dead trees in the thinned 

plots also increased in the two growing seasons following 

thinning. The low-density treatments had greater tree 

mortality than the high-density treatments except on the 

UPM Kymmene site.

Mechanical Damage from Thinning

Wound size averaged slightly larger than 1 ft2, and 

wound height above ground averaged approximately 4 

ft in both the high-density and low-density thinning 

treatments. The size of the wound on the tree and wound 

height above ground were not affected by study location 

or treatment, but there was a marginally significant 

interaction between study location and treatment for the 

height of the wound (Table 3).

Table 3.—Results of ANOVAs performed on the location and extent of mechanical damage from timber harvesting 

using the general linear model: y = location + treatment + location•treatment + e, where y = the measured variable, 

location = site 29, site 33, or the UPM Kymmene site, treatment = high-density thinning or low-density thinning, and 

e = error NID~(0, σ2)

P-value for significance of effect Average (standard error)

Measured variable Location Treatment Location•treatment

High density

n = 93

Low density

n = 74

Surface area of wound (cm2) 0.261 0.766 0.737 78.01 (10.12) 87.55 (10.80)

Height of wound (m) 0.249 0.747 0.021 1.24 (0.07) 1.22 (0.12)

Surface area of wound (in.2) 12.09 13.57

Height of wound (ft) 4.06 4.00
Table 4.—Summary of percentage mechanical and insect and disease damage by location, treatment, and year

a

 Control plot measurements are the same for 2000 and 2001.

Mechanical Frost/sunscald Phellinus Hypoxylon Saperda Dead

UPM

2000 - Pre-treatment

 Controla 0 0.01 2.12 0.28 1.45 23.0

 Low density 0 0 5.88 0 0.70 14.48

 High density 0 0 3.21 0.51 1.61 17.03

2001 - Post-treatment

 Controla 0 0.01 2.12 0.28 1.45 23.0

 Low density 29.95 0 4.50 1.18 0 18.48

 High density 26.74 0 6.21 0 0 25.83

2004 - Post-treatment

 Control 0 0.01 1.06 0.56 0.72 15.66

 Low density 9.65 0 8.08 1.30 0.23 20.24

 High density 8.92 0 8.01 0.81 1.18 24.23

Site 29

2000 - Pre-treatment

 Controla 0 0 0 0 0 1.16

 Low density 0 0 0 0.24 0.09 0.89

 High density 0 0 0 0.10 0 0.65

2001 - Post-treatment

 Controla 0 0 0 0 0 1.16

 Low density 10.45 0.01 0.92 0.48 0 10.32

 High density 12.62 0 0.65 0 0 8.30

2004 - Post-treatment

 Control 0 0.01 0.38 0.40 0 0.79

 Low density 3.48 0.06 1.56 0.24 0.32 8.83

 High density 4.21 0.07 1.01 0.03 0 8.52

Site 33

2000 - Pre-treatment

 Controla 0 0 0.27 0.66 0 0.86

 Low density 0 0 0.53 1.65 0.28 2.84

 High density 0 0 0 1.08 0.37 1.98

2001 - Post-treatment

 Controla 0 0 0.27 0.66 0 0.86

 Low density 10.63 0.01 0.85 0 0 10.82

 High density 7.49 0.01 1.45 0 0.38 9.89

2004 - Post-treatment

 Control 0 0 0 0 0 0.20

 Low density 0 0.01 11.73 1.01 0 19.00

 High density 0 0.01 8.44 0.38 0.40 15.21
Damage from insects and diseases

The percentage of trees affected by 

frost/sunscald, Phellinus, Hypoxylon, 

Saperda, and the number of dead 

potential aspen crop trees greater than 

12 cm within each treatment at each 

location are provided in Table 4. Table 

5 presents ANOVAs on pre-treatment, 

immediately post-treatment, and 3-yr 

post-treatment evaluations for frost/

sunscald, Phellinus, Hypoxylon, Saperda, 

and the number of dead potential aspen 

crop trees greater than 5 in. d.b.h. 

testing the significance of location, 

treatment, and the interaction between 

location and treatment. Pre-treatment 

ANOVAs detected an interaction 

between location and treatment for frost/

sunscald and the number of dead trees. 

Location had a significant impact on all 

measured attributes before treatment. 

Not surprisingly, treatment was not 

significant for all measured variables, 

except for frost/sunscald. The majority 

of trees affected by damaging agents were 

removed during thinning. Thus, results of post-treatment 

ANOVAs detected few difference between locations or 

treatments. Location and treatment effects became more 

evident after 3 yr because additional trees became affected 

by damaging agents. Some trees also died, which had an 

effect on the results.

The prevalence of mechanical damage was greatest 

at the UPM site immediately post-treatment where 

the percentage damage was nearly triple the 1-yr 

post-treatment amount for sites 29 and 33 (Table 4). 

There was a greater amount of frost/sunscald at site 29 

3 yr post-treatment. In general, a greater incidence of 

Phellinus was present at the UPM site except 3 yr posttreatment when there was a greater incidence of Phellinus

at site 33. The incidence or prevalence of Hypoxylon 

canker and damage by Saperda decreased immediately 

post-treatment and then increased slightly 3 yr posttreatment. The percentage of dead trees decreased 

immediately following treatment and increased 3 yr 

post-treatment.

Herb Biomass

A comparison of pre-treatment aboveground herb 

biomass and 1- and 2-yr post-treatment aboveground 

herb and forb biomass for each location is provided 

in Table 6. Analyses of variance showed the effects of 

location (P < 0.001), year (P < 0.001), and treatment 

(P < 0.001) were significant. Due to missing data 

(Table 6), the location•year interaction term could 

not be tested. The treatment•location (P = 0.188) and 

year•location•treatment (P = 0.093) interaction terms 

were not significant. Herb and forb biomass was the 

same across all locations before treatments. The forest 

tent caterpillar outbreak in 2001 and 2002 increased the 

amount of light reaching the forest floor and resulted in 

greater herb and forb biomass for the control treatments 

as well as the high- and low-density thinning treatments. 

The 2-yr forest tent caterpillar outbreak likely negated 

any differences that would have been detected between 

the controls and thinning treatments.

Table 5.—Summary of analyses of variance using the model y = location + 

treatment + location•treatment + e, where y = the measured damage metric, 

location = site 29, site 33, or the UPM Kymmene site, treatment = control, 

high-density thinning, or low-density thinning, and e = error NID~(0, σ2)

Attribute Location Treatment Location•treatment

2000 – Pre-treatment

 Frost/sunscald 0.015 0.012 0.004

 Phellinus 0.000 0.256 0.457

 Hypoxylon 0.000 0.322 0.159

 Saperda 0.012 0.728 0.811

 Dead 0.001 0.102 0.006

2001 – Post-treatment

 Mechanical 0.001 0.001 0.004

 Frost/sunscald 0.505 0.908 0.657

 Phellinus 0.017 0.593 0.829

 Hypoxylon 0.375 0.095 0.375

 Saperda 0.474 0.422 0.474

 Dead 0.001 0.410 0.108

2004 – Post-treatment

 Mechanical 0.018 0.080 0.444

 Frost/sunscald 0.000 0.058 0.035

 Phellinus 0.117 0.019 0.592

 Hypoxylon 0.035 0.490 0.473

 Saperda 0.508 0.191 0.110

 Dead 0.001 0.001 0.190
Table 6.—Average (standard error in parentheses) herb and shrub biomass values by year, location, and 

treatment 

Treatment

Herb biomass Shrub biomass

n kg ha-1 lbs ac-1 n tonnes ha-1 tons ac-1

Year - 2000

UPM site

 Control 5 183.9 164.0 44 3.8 1.7 

(16.8) (15.0) (0.7) (0.3)

 HD 5 129.7 115.8 

(12.0) (10.7)

 LD 5 166.1 148.2 44 3.1 1.4 

(30.1) (26.9) (0.4) (0.2)

Site 29

 Control 6 25.4 22.7 54 2.4 1.1 

(5.2) (4.6) (0.3) (0.1)

 HD 9 39.7 35.4 81 3.2 1.4 

(7.0) (6.3) (0.3) (0.1)

 LD 9 36.4 32.5 78 11.4 5.1 

(5.3) (4.8) (0.3) (0.1)

Site 33

 Control 6 32.2 28.7 52 2.4 1.1 

(9.1) (8.1) (0.2) (0.1)

 HD 9 49.1 43.8 81 4.8 2.1 

(9.3) (8.3) (0.3) (0.1)

 LD 9 35.0 31.2 80 4.6 2.0 

 (6.3) (5.6) (0.3) (0.1)

Year - 2001

UPM site

 Control 5 1,239.0 1,105.5 

(260.4) (232.4)

 HD 10 1,487.3 1,327.0 60 360.6 160.9 

(227.7) (203.2) (31.5) (14.1)

 LD 10 1,180.4 1,053.1 60 585.8 261.3 

(241.2) (215.2) (38.3) (17.1)

Site 29

 Control 6 837.9 747.6 

(98.8) (88.1)

 HD 18 555.2 495.4 108 1,080.8 482.2 

(54.6) (48.7) (70.4) (31.4)

 LD 18 656.3 585.6 108 1,060.7 473.2 

(94.2) (84.1) (66.8) (29.8)

Site 33

 Control 6 682.3 608.8 

(93.4) (83.3)

 HD 18 538.3 480.3 108 1,519.9 678.0 

(78.5) (70.1) (70.1) (31.3)

 LD -- -- -- 108 1,289.7 575.3 

(55.6) (24.8)

Continued
Shrub Biomass

A comparison of pre-treatment aboveground shrub 

biomass and 1- and 2-yr post-treatment aboveground 

shrub biomass for each location is provided in Table 

6. Analysis of variance showed that location, year, and 

treatment effects along with all combinations of their 

interaction terms were significant (P < 0.001 for all 

effects). Due to these significant interactions, it is not 

possible to comment extensively on the individual effects. 

The 2-yr forest tent caterpillar outbreak likely influenced 

the shrub aboveground biomass results. Note, however, 

that there was less of a detectable effect over time with 

the control treatments at sites 29 and 33. At the UPM 

Kymmene site, however, there was an increase in shrub 

biomass in the control.

Soil Compaction

A major source of yield loss in agriculture is soil 

compaction. Soil compaction prevents moisture 

penetration and hinders plant root growth, and it is 

becoming increasingly important in forest management 

(Smith et al. 1997). Penetrometer readings at depths 

of 4, 8, 12, and 18 in. were highly variable, and the 

variation in the density of forest soils overwhelmed any 

statistical analyses. In some instances, greater compaction 

was detected before treatment (Table 7). Separate twofactor analyses of variance (ANOVAs) conducted on 

pre-treatment and post-thinning penetrometer data at 

the above soil depths did not detect any pre- or posttreatment similarities in the data among locations and 

treatments or suggest any trends.

Herb biomass Shrub biomass

Treatment n kg ha-1 lbs ac-1 n tonnes ha-1 tons ac-1

Year - 2002

UPM site

 Control 5 1,570.5 1,401.2 802.9 358.2 

(431.2) (384.7) (104.9) (46.8)

 HD 10 2,422.8 2,161.6 60 864.1 385.5 

(244.8) (218.4) (110.0) (49.1)

 LD 10 2,073.7 1,850.2 60 996.1 444.4 

(301.2) (268.7) (82.2) (36.7)

Site 29

 Control 6 831.8 742.1 

(59.5) (53.1)

 HD 16 888.4 792.6 107 2,960.7 1,320.7 

(115.3) (102.8) (231.9) (103.4)

 LD 18 867.1 773.6 107 5,122.6 2,285.2 

(111.2) (99.3) (352.8) (157.4)

Site 33

 Control 6 447.0 398.8 

(159.0) (141.8)

 HD 18 1,042.1 929.7 106 2,432.2 1,085.0 

(129.4) (115.4) (200.0) (89.2)

 LD 18 1,432.2 1,277.8 108 2,027.1 904.3 

(170.3) (151.9) (157.0) (70.0)

Table 6.—continued 
10

Table 7.—Minimum, maximum, average (standard errors in parentheses), and probability values for a t-test of the 

hypothesis of pre- and post-thinning differences in soil penetrometer readings averaged for depths of 10, 20, 30, 

and 40 cm by location and treatment. Soil compaction data are presented in metric units only. Note that one cm 

equals 0.39 in.

Treatment n Pre-thin Post-thin Difference P-value of t-test

10 cm depth ----------------------------------kilopascals----------------------------------

UPM Kymenee

 Control 60 425.42 (13.75) 

251.0 – 718.9

350.99 (13.52) 

178.5 – 762.8

74.43 (12.54) 

-154.4 – 246.3

< 0.001

 Thin 63 529.95 (27.61) 

156.9 – 1246.0

514.36 (19.34) 

262.6 – 1011.9

15.60 (23.14) 

-370 – 575.47

0.502

 Travel corridor 57 510.80 (27.88) 

246.2 – 1018.5

591.65 (22.54) 

274.2 – 1131.7

-80.85 (29.61) 

-424.8 – 638.6

0.008

Site 29

 Control 72 616.71 (18.47) 

344.4 – 1081.8

398.14 (11.17) 

226.5 – 705.7

218.57 (22.75) 

-161 – 711.0

< 0.001

 Thin 116 646.76 (22.50) 

281.3 – 1722.9

405.87 (9.44) 215.3 

– 903.3

240.89 (21.01) 

-145.9 – 1019.6

< 0.001

 Travel corridor 100 654.82 (21.67) 

234.67 – 1,511.67

508.03 (15.35) 

242.57 – 1,027.07

146.78 (25.62) 

-430.5 – 780.4

< 0.001

Site 33

 Control 72 490.76 (15.26) 

222.1 – 1028.1

638.22 ( 20.00) 

348.4 – 1290.3

-147.46 (17.22) 

-608.5 – 88.5

< 0.001

 Thin 142 722.07 (21.79) 

269.1 – 1636.6

535.5 (11.36) 262.0 

– 1004.8

186.57 (23.63) 

-539.8 – 1018.2

< 0.001

 Travel corridor 74 729.44 (30.81) 

226.0 – 1939.2

662.32 (28.91) 

306.9 – 1544.9

67.12 (37.20) 

-951 – 679.1

0.075

20 cm depth ---------------------------------- kilopascals ----------------------------------

UPM Kymenee

 Control 60 796.18 (52.17) 

328.3 – 2123.7

572.35 (25.64) 

262.1 – 1333.9

223.82 (55.15) 

-590.8 – 1345.6

< 0.001

 Thin 63 733.20 (36.10) 

178.7 – 1731.2

758.6 (37.46) 332.3 

– 1692.7

-25.35 (35.82) 

-600.5 – 817.8

0.481

 Travel corridor 57 714.07 (38.39) 

346.5 – 1764.4

1,028.07 (42.78) 

396.4 – 1923.0

-314.00 (48.63) 

-1468.9 - 292

< 0.001

Site 29

 Control 72 974.04 (35.15) 

491.0 – 1864.9

634.38 (29.28) 

235.7 – 1717.9

339.65 (44.21) 

-1,121.3 – 1370.5

< 0.001

 Thin 116 1,051.84 (47.90) 

474.7 – 2715.9

657.06 (22.09) 

258.4 – 1814.5

394.78 (43.69) 

-591 – 2360.1

< 0.001

 Travel corridor 100 995.70 (39.79) 

279.3 – 3012.5

867.95 (28.35) 

257.7 – 1741.1

127.74 (43.50) 

-828.6 – 2136.0

0.004

Site 33

 Control 72 774.87 (32.99) 

284.0 – 1939.2

880.97 (43.19) 

421.6 – 2990.3

-106.10 (28.56) 

-1051.06 – 510.3

< 0.001

 Thin 142 987.57 (30.80) 

469.2 – 2522.9

802.41 (23.14) 

379.8 – 1841.1

185.17 (33.86) 

-1,092.5 – 1406.9

< 0.001

 Travel corridor 74 1,021.26 (42.87) 

341.4 – 2447.0

1,084.23 (45.84) 

455.3 – 2227.6

-62.97 (50.47) 

-1,005.6 – 902.9

0.216

Continued
11

Table 7.—continued

30 cm depth ---------------------------------- kilopascals ----------------------------------

UPM Kymenee

 Control 60 1,567.10 (116.75) 

447.6 – 3846.7

1,143.40 (82.43) 

421.1 – 3080.3

423.70 (105.53) 

-1,353.7 – 2,735.9

< 0.001

 Thin 63 1529.25 (82.32) 

363.5 – 2867.4

1,228.48 (57.44) 

566.8 – 2726.9

300.77 (78.62) 

-790.4 – 1,766.07

< 0.001

 Travel corridor 57 1,326.83 (64.93) 

624.87 – 2,406.07

1,650.48 (73.23) 

659.3 – 3,446.1

-323.64 (78.67) 

-1,740.9 – 977.4

< 0.001

Site 29

 Control 72 1,324.57 (52.94) 

538.9 – 2,976.3

879.15 (43.11) 

363.7 – 1932.1

445.42 (58.43) 

-1,148.1 – 1294.7

< 0.001

 Thin 116 1,575.74 (81.29) 

458.1 – 4291.8

939.83 (39.42) 

415.2 – 2527.4

635.91 (73.26) 

-988.9 – 3,139.1

< 0.001

 Travel corridor 100 1,513.10 (69.09) 

496.1 – 4,265.6

1,189.63 (40.86) 

283.6 – 2,314.1

323.47 (66.83) 

-847.9 – 3120.3

< 0.001

Site 33

 Control 72 988.59 (41.98) 

271.1 – 2495.0

1,047.02 (43.21) 

526.7 – 2,918.1

-58.42 (34.64) 

-736.9 – 893.9

0.096

 Thin 142 1,312.02 (42.04) 

564.8 – 3143.5

1,006.8 (26.51) 

194.86 – 1965.1

305.19 (41.28) 

-1,117.3 – 1,953.1

< 0.001

 Travel corridor 74 1,339.00 (54.81) 

539.4 – 2,725.5

1,277.74 (42.79) 

662.7 – 2,424.5

61.26 (63.23) -

1,108.2 – 1,464.9

0.335

40 cm depth ---------------------------------- kilopascals ----------------------------------

UPM Kymenee

 Control 60 2,318.16 (116.13) 

812.7 – 3984.2

1,826.69 (98.04) 

611.7 – 3,758.7

504.94 (120.09) 

-1,838.3 – 2,480.7

< 0.001

 Thin 63 2,571.00 (95.90) 

1,067.6 – 4,393.6

2,209.63 (69.86) 

1,070.2 – 3,396.7

361.37 (103.98) 

-1,430.2 – 2,379.0

< 0.001

 Travel corridor 57 2461.61 (100.37) 

1,215.0 – 4,157.2

2,557.27 (78.89) 

1,370.3 – 3,830.5

-95.67 (102.23) 

-1,682.2 – 1,296.9

< 0.001

Site 29

 Control 72 1,850.90 (83.00) 

684.1 – 3,900.9

1,295.24 (59.18) 

362.5 – 2,892.8

555.65 (77.42) 

-1,528.1 – 1,855.1

< 0.001 

 Thin 116 2,116.93 (85.49) 

575.3 – 4432.0

1,305.71 (53.14) 

401.1 – 3,575.9

811.21 (79.72) 

-1,663.4 – 3,395.1

< 0.001

 Travel corridor 100 1,995.75 (80.02) 

670.3 – 4,376.7

1,473.46 (58.08) 

375.8 – 3,719.7

522.28 (77.03) 

-940.3 – 2,892.9

< 0.001

Site 33

 Control 72 1,245.62 (47.78) 

253.9 – 2,503.1

1,328.08 (46.94) 

628.4 – 2,492.0

-82.46 (55.19) 

-1,204.1 – 1,259.6

0.144

 Thin 142 1,805.68 (61.08) 

675.4 – 3954.1

1,335.72 (42.18) 

413.6 – 4,033.4

469.96 (58.54) 

-1,390.3 – 2,896.1

 < 0.001

 Travel corridor 74 1,823.47 (80.58) 

730.0 – 3,602.6

1,583.95 (49.24) 

846.8 – 2,786.1

239.52 (84.08) 

-1,301.9 – 2,202.7

0.005

Treatment n Pre-thin Post-thin Difference P-value of t-test
12

Fuel loadings

Tables 8a and 8b present summary statistics of 1 hr, 

10 hr, 100 hr, total fine fuels, 1,000 hr sound, 1,000 

hr rotten, total heavy fuels, total fuels, fuel depth, and 

duff depth by location. Results of separate two-factor 

ANOVAs performed on total fine fuels, total heavy fuels, 

total fuels, fuel depth, and duff depth are presented in 

Table 9.

Fine fuels were affected by treatment (Table 9) with lesser 

amounts of fine fuels observed in the control plots than 

in the thinned plots (Table 8). No effect of location or 

location by treatment interaction was detected for fine 

fuels. Heavy fuels and total fuels were not affected by 

location or treatment (Table 9). There was an interaction 

between location and treatment for fuel depth and duff 

depth (Table 9). This suggests that site quality, season of 

harvest, or the type of harvesting equipment used may 

have impacted slash density and forest floor disturbance.

Discussion

We attempted to select sites that had a high site index 

for inclusion in the study. Numerous authors (Bickerstaff 

1946; Gilmore 2003; Perala 1977, 1978; Schlaegel 

1972; Steneker 1964; Steneker and Jarvis 1966; Zasada 

1952; Zehngraff 1946, 1947) have suggested that aspen 

thinning be conducted on only the highest quality sites 

to maximize tree and stand growth. In our study, there 

was a light-on-the-land harvest and minimal harvesting 

damage to the residual stand. The percentage of trees 

damaged through harvesting and via insects and diseases 

decreased over time as these trees died. However, the 

UPM-Kymmene site had a very low site index (60) for 

aspen, and tree mortality was greater than on other sites 

(Table 2). It will be interesting to follow the development 

of this stand over time relative to the other two sites that 

had substantially higher (75 and 80) site indices.

The purpose of this establishment report is to provide 

scientists with baseline information that can be used to 

follow the growth, development, and change in these 

stands over time. Much of the information on growth 

and yield and on insects and diseases for aspen can be 

inferred or predicted from past studies. Two additional 

lines of important research can also be conducted on 

these sites. First, biomass production as an energy source 

is becoming increasingly important. Our baseline data 

would be useful to researchers studying the development 

of an understory in an aspen forest following thinning. 

Understory growth is being explored by the industry as a 

possible source of biomass fuel.

Second, aspen is an important food source for numerous 

wildlife species—most notably ruffed grouse. Ruffed 

grouse require a varied habitat that includes, in part, 

young regenerating aspen for cover and older, flowering 

aspen for food (Gullion 1984). Because there has been 

relatively little thinning of young aspen stands, we do 

not know the quality of grouse habitat that would be 

produced from thinning. Our thinning treatments 

produced linear corridors that could increase the level 

of predation by raptors and owls. On the other hand, 

the forest habitat requirements for an important grouse 

predator, the goshawk, do not overlap strongly with 

ideal grouse habitat (Zimmerman et al., in press). 

Grouse habitat in thinned aspen stands may either be 

poor or good depending on the interactions of the cover 

produced, the food available, and the suitability of the 

habitat for predators. We realize the limitations of our 

study sites for monitoring trends in wildlife populations, 

but it may be possible to monitor grouse habitat quality 

over time.

In conclusion, a careful operator using cut-to-length 

equipment results in minimal damage. Thinning from 

below increased stand diameter. Thinning did not 

appreciably affect insect and disease prevalence. Thinning 

did not affect fuel loadings. We were unable to measure 

soil strength but soil compaction did not appear to be 

a problem. Aspen stands growing on a low site index or 

poor site are not good candidates for thinning. It would 

be beneficial to explore the effects of thinning across a 

greater range of densities.

Acknowledgments

We thank John Elliof, Glenn Erickson, Crystal Kern, 

UPM Kymmene, and the Minnesota Department 

of Natural Resources for their assistance in plot 

establishment and logistics. We also thank Jana Albers, 

Mike Albers, Gary Wyckoff, and Guthrie Zimmerman 
13

Table 8a.—Average (standard errors in parentheses) summary of post-treatment fuel loadings by location and treatment 

(English units of measure)

Treatment n 1 hr 10 hr 100 hr Fine 

fuels

1000 hr 

sound

1000 hr 

rotten

Heavy 

fuels

Total 

fuels

Fuel 

depth

Duff 

depth

-------------------------------------------------tons ac-1------------------------------------------------- in. in.

Site 29

 Control 18.00 0.07 0.38 1.31 1.75 18.46 5.04 23.51 25.26 2.49 2.05 

(0.01) (0.09) (0.21) (0.23) (14.54) (2.31) (14.40) (14.37) (0.31) (0.30)

 High density 18.00 0.13 0.75 1.38 2.26 19.98 7.20 27.19 29.45 5.19 1.38 

 Travel corridor (0.04) (0.12) (0.26) (0.35) (3.25) (2.36) (3.90) (4.08) (0.72) (0.26)

 High density 27.00 0.17 1.14 2.93 4.24 8.31 3.08 11.39 15.64 6.67 1.86 

 Thin (0.03) (0.31) (0.39) (0.65) (2.08) (1.14) (2.39) (2.49) (0.78) (0.14)

 Low density 29.00 0.22 1.38 1.66 3.25 30.61 4.55 35.17 38.42 4.91 1.24 

 Travel corridor (0.04) (0.33) (0.33) (0.57) (5.37) (3.39) (7.01) (7.09) (0.48) (0.16)

 Low density 27.00 0.16 0.99 3.66 4.82 19.54 3.16 22.70 27.51 6.41 2.02 

 Thin (0.20) (0.19) (0.45) (0.62) (7.16) (2.12) (8.96) (9.08) (0.84) (0.16)

Site 33

 Control 17.00 0.06 0.26 1.04 1.36 0.59 2.26 2.84 4.21 2.39 2.25 

(0.01) (0.05) (0.12) (0.15) (0.58) (1.31) (1.61) (1.62) (0.34) (0.16)

 High density 18.00 0.18 1.35 1.03 2.56 24.87 10.98 35.86 38.42 9.19 1.91 

 Travel corridor (0.03) (0.27) (0.15) (0.42) (3.93) (4.20) (5.35) (5.26) (2.65) (0.10)

 High density 25.00 0.18 1.70 2.67 4.56 11.54 4.28 15.82 20.37 9.27 2.07 

 Thin (0.02) (0.46) (0.43) (0.88) (3.95) (2.68) (4.43) (4.63) (1.03) (0.09)

 Low density 17.00 0.28 0.93 1.39 2.60 15.02 5.26 20.28 22.89 6.53 1.81 

 Travel corridor (0.06) (0.19) (0.23) (0.34) (2.60) (1.60) (3.34) (3.49) (0.72) (0.16)

 Low density 27.00 0.25 1.14 2.58 3.97 13.42 6.46 19.88 23.84 7.49 2.38 

 Thin (0.02) (0.24) (0.22) (0.36) (2.98) (2.08) (3.45) (3.65) (0.76) (0.14)

UPM Kymenee

 Control 15.00 0.06 0.55 1.49 2.10 11.45 5.65 17.10 19.20 5.16 0.30 

(0.01) (0.17) (0.17) (0.28) (3.07) (2.07) (4.47) (4.50) (1.31) (1.20)

 High density 10.00 0.09 0.72 1.43 2.25 39.11 4.43 43.54 45.79 3.37 0.99 

 Travel corridor (0.02) (0.20) (0.17) (0.35) (7.83) (3.54) (7.77) (7.84) (0.72) (0.23)

 High density 15.00 0.08 0.73 2.95 3.76 10.89 0.00 10.89 14.65 5.66 1.36 

 Thin (0.01) (0.15) (0.39) (0.47) (3.68) (3.68) (3.72) (0.55) (0.22)

 Low density 10.00 0.10 0.73 0.81 1.64 25.51 0.49 26.00 27.64 5.66 0.69 

 Travel corridor (0.01) (0.23) (0.14) (0.32) (5.62) (0.49) (5.93) (5.77) (0.55) (0.12)

 Low density 15.00 0.17 1.05 3.12 4.34 37.59 167.46 205.05 209.40 7.79 0.54 

 Thin (0.04) (0.28) (0.51) (0.75) (28.96) (165.43) (194.31) (194.53) (0.78) (0.07)
14

Table 8b.—Average (standard errors in parentheses) summary of post-treatment fuel loadings by location and 

treatment (Metric units of measure)

Treatment n 1 hr 10 hr 100 hr Fine 

fuels

1000 hr 

sound

1000 hr 

rotten

Heavy 

fuels

Total 

fuels

Fuel 

depth

Duff 

depth

---------------------------------------tonnes ha-1--------------------------------------- cm cm

Site 29

 Control 18.00 0.16 0.85 2.94 3.92 41.38 11.30 52.70 56.63 6.39 5.25 

(0.02) (0.21) (0.46) (0.52) (32.59) (5.17) (32.27) (32.20) (0.80) (0.75)

 High density 18.00 0.29 1.68 3.09 5.07 44.79 16.14 60.95 66.02 13.32 3.55 

 Travel corridor (0.08) (0.28) (0.58) (0.77) (7.29) (5.29) (8.74) (9.14) (1.84) (0.67)

 High density 27.00 0.38 2.56 6.57 9.50 18.63 6.90 25.53 35.06 17.09 4.77 

 Thin (0.07) (0.69) (0.88) (1.46) (4.66) (2.55) (5.36) (5.59) (1.98) (0.35)

 Low density 29.00 0.49 3.09 3.72 7.29 68.62 10.20 78.84 86.13 12.59 3.17 

 Travel corridor (0.09) (0.73) (0.74) (1.27) (12.04) (7.60) (15.71) (15.89) (1.22) (0.42)

 Low density 27.00 0.36 2.22 8.20 10.80 43.80 7.08 50.89 61.67 16.43 5.19 

 Thin (0.45) (0.41) (1.00) (1.40) (16.06) (4.75) (20.09) (20.35) (2.14) (0.40)

Site 33

 Control 17.00 0.13 0.58 2.33 3.05 1.32 5.07 6.37 9.44 6.12 5.76 

(0.02) (0.11) (0.28) (0.33) (1.31) (2.94) (3.61) (3.62) (0.87) (0.40)

 High density 18.00 0.40 3.03 2.31 5.74 55.75 24.61 80.39 86.13 23.56 4.90 

 Travel corridor (0.08) (0.60) (0.34) (0.93) (8.81) (9.42) (12.00) (11.79) (6.78) (0.26)

 High density 25.00 0.40 3.81 5.99 10.22 25.87 9.59 35.46 45.66 23.78 5.31 

 Thin (0.05) (1.03) (0.97) (1.98) (8.86) (6.00) (9.94) (10.38) (2.65) (0.23)

 Low density 17.00 0.63 2.08 3.12 5.83 33.67 11.79 45.46 51.31 16.74 4.63 

 Travel corridor (0.13) (0.43) (0.50) (0.77) (5.83) (3.58) (7.49) (7.83) (1.86) (0.42)

 Low density 27.00 0.56 2.56 5.78 8.90 30.08 14.48 44.56 53.44 19.21 6.10 

 Thin (0.05) (0.54) (0.50) (0.80) (6.68) (4.67) (7.74) (8.19) (1.96) (0.35)

UPM Kymenee

 Control 15.00 0.13 1.23 3.34 4.71 25.67 12.67 38.33 43.04 13.22 0.78 

(0.02) (0.38) (0.39) (0.64) (6.88) (4.65) (10.01) (10.08) (3.36) (3.07)

 High density 10.00 0.20 1.61 3.21 5.04 87.67 9.93 97.60 102.65 8.63 2.55 

 Travel corridor (0.05) (0.44) (0.39) (0.77) (17.55) (7.93) (17.41) (17.57) (1.85) (0.60)

 High density 15.00 0.18 1.64 6.61 8.43 24.41 0.00 24.41 32.84 14.52 3.48 

 Thin (0.02) (0.34) (0.87) (1.05) (8.25) (8.25) (8.33) (1.41) (0.58)

 Low density 10.00 0.22 1.64 1.82 3.68 57.19 1.10 58.28 61.96 14.52 1.78 

 Travel corridor (0.03) (0.52) (0.32) (0.71) (12.60) (1.10) (13.30) (12.93) (1.41) (0.31)

 Low density 15.00 0.38 2.35 6.99 9.73 84.27 375.40 459.66 469.41 19.98 1.38 

 Thin (0.09) (0.64) (1.15) (1.67) (64.92) (370.84) (435.58) (436.09) (2.01) (0.17)
15

for their thoughtful pre-publication reviews. 

Funding was provided by the Creating Research 

OPportunitieS (CROPS) program of the U.S. 

Forest Service; the U.S. Forest Service, Northern 

Research Station; the Department of Forest 

Resources, the North Central Research and 

Outreach Center, and the Undergraduate Research 

Opportunities Program of the University of 

Minnesota.

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unsalvaged windthrow in two forest cover-types of 

Table 9.—Results of ANOVAs performed on total fine fuels, total 

heavy fuels, total fuels, fuel depth, and duff depth y = location 

+ treatment + location•treatment + e, where y = the measured 

fuel metric, location = site 29, site 33, or the UPM Kymmene site, 

treatment = control, high density thinning, or low density thinning, 

and e = error NID~(0, σ2)

P-values for factors

Attribute Location Treatment Location•treatment

Fine fuels 0.696 0.001 0.535

Heavy fuels 0.323 0.181 0.350

Total fuels 0.325 0.169 0.355

Fuel depth 0.094 0.001 0.015

Duff depth 0.001 0.398 0.001
16

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on aspen. Res. Pap. NC-331. St. Paul, MN: U.S. 

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of Agriculture, Forest Service, Lake States Forest 

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press. Integrating grouse habitat and forestry: an 

example using the ruffed grouse in Minnesota.

Wildlife Biology. 
Printed on Recycled Paper

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on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, 

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and employer. 

Gilmore, Daniel W.; Glenn, Jennifer D.; Ostry, Michael E.; Zasada, John C.; Benedict, 

Michael A. 2006. Commercial thinning in small-diameter aspen stands in northern 

Minnesota: study establishment report. Gen. Tech. Rep. NRS-2. Newtown Square, 

PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 16 p.

In the spring of 1999, a long-term study was established to examine the physical and 

biological aspects of thinning young aspen stands in Minnesota. Three aspen stands 

ranging in age from 25 to 35 years were selected on lands owned by the State of 

Minnesota and UPM Kymmene. Two thinning treatments (low and high density) and an 

unthinned control were installed at each of the three locations. Permanent plots were 

installed to measure tree, shrub, and herb growth, and to monitor harvesting damage, 

insect and disease damage, soil strength, and fuel loadings.

After 4 years, tree mortality was greater in the unthinned controls. Thinning treatment had 

no significant effect on the incidence of white trunk rot (Phellinus tremulae), Hypoxylon 

canker, or Saperda calcarata. No differences in post-harvest fuel loadings were detected 

among locations and treatments. Thinning increased the amount of light reaching the 

forest floor that resulted in greater herb and shrub biomass in the year immediately 

following thinning.

KEY WORDS: biomass, cut-to-length harvesting system, silviculture, Populus tremuloides
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