United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

Research Paper 

NRS-20

80 Years of Thinning Research 

on Northern Hardwoods in the 

Bartlett Experimental Forest, 

New Hampshire

William B. Leak

Mariko Yamasaki
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Abstract

Commercial and noncommercial thinning studies in northern hardwoods on the 

Bartlett Experimental Forest, New Hampshire, began in 1932. One of the studies, still 

maintained today, consisted of several precommercial treatments at age 25 (1959) 

and a commercial treatment in 2003. Although economic returns from precommercial 

work appear somewhat marginal and require additional research, commercial thinning 

in northern hardwoods is clearly advisable—almost necessary because evenaged 

northern hardwoods offer a unique thinning opportunity: a component of shadeintolerant aspen and paper birch maturing in 50 to 70 years coupled with a component 

of long-lived species that will mature at age 100 to 120 years.
1

INTRODUCTION

Th inning is an intermediate partial cutting in an evenaged stand, beyond the 

sapling stage, designed to increase diameter growth, quality and health, and 

stand-level production without excessive opening of the canopy (Nyland 2002). 

Th e overall objective in northern hardwood stands is to produce quality saw 

logs in the shortest possible time. Regeneration is not an objective of thinning, 

although an understory often develops following a thinning operation.

Th inning research in northern hardwoods on the Bartlett Experimental Forest 

began in 1932, and one of the well-designed thinning studies, initiated in 

1959 (Marquis 1969), still is maintained today. Results from these studies 

covered a range of topics: stand and tree growth responses, species and quality 

changes, economic returns, eff ects on rotation age, and damage from wind and 

harvesting. Th ese topics are organized in this paper into two main sections: (1) 

the early studies, and (2) the 1959 study, separated into the early results, the 

followup, and the commercial harvest phase.

THE EARLY STUDIES

In 1932, the fi rst Bartlett thinning study was initiated consisting of a thinned 

plot and an unthinned plot in a 60-year-old northern hardwood stand (Jensen 

1940). Plot size was unrecorded, probably 1 acre. Th e purpose was to study 

the responses of yellow and paper birch (See Appendix Table 1 for common/

scientifi c names) to the environmental changes caused by thinning; these two 

species often exhibit symptoms of decline following harvesting. Th e thinning 

was heavy, removing nearly half the volume; this would reduce the basal area 

per acre from an estimated 120 square feet/acre down to perhaps 60 square feet. 

Th e better stems of the two birches were left in the residual stand. Th e thinning 

resulted in higher maximum air and soil temperatures, slightly lower minimum 

temperatures, lower relative humidity, 50 percent greater wind movement, and 

more than 60 percent greater evaporation rates. Th ese eff ects declined rapidly 

in the years following the thinning. During the second growing season after 

thinning, yellow birch showed signs of stress: epicormic branching on nearly 

half the trees and dieback of twigs and branches coupled with some evidence of 

shoestring fungus (Armillaria mellea). Other species appeared healthy. However, 

by 1937, 5 years after thinning, the yellow birch apparently had recovered 

from the thinning impacts, and the mixed-species understory was vigorous. 

Th e growth results from this study were expressed as basal area growth percent, 

which is diffi cult to evaluate. However, annual growth in volume was somewhat 

greater in the thinned plot (about 1 cord) as compared to the unthinned (about 

1⁄2 cord), and the species/quality was much better. But the thinning apparently 

increased susceptibility to wind damage: the 1938 hurricane destroyed 15 

percent of the volume on the thinned plot as compared to 1 percent on the 

unthinned.

Th e Authors

WILLIAM B. LEAK is a research 

silviculturist and MARIKO 

YAMASAKI is a research wildlife 

biologist with the U.S. Forest 

Service, Northern Research 

Station, Durham, NH.
2

A second, more ambitious, thinning study began in 

1936 (Wilson 1953), consisting of 40 contiguous 1⁄4-acre 

thinned plots plus four controls (Compartment 16). 

However, six plots were heavily damaged by the 1938 

hurricane and eliminated from the study. Th e stand was 

a 60-year-old northern hardwood stand on an excellent 

site: 20 percent of the basal area was in white ash. 

Percentages of basal area for the other species were red 

maple (32), paper birch (13), aspen (12), sugar maple 

(9), yellow birch (9), beech (3), and other species (2). 

Th e thinning (a commercial one), coupled with light 

hurricane damage, left a residual basal area of 80 square 

feet basal area per acre in 1941 out of an original stocking 

of 122 square feet. In the 10-year period from 1941 to 

1951, the thinned plots produced an annual average of 

2.6 square feet of basal area per acre while the unthinned 

plots averaged 0.7 square feet.

Average annual diameter growth in the thinned plots 

averaged 0.13 inches/year; sugar maple and beech grew a 

little faster: 0.15 and 0.16 inches, respectively (Table 1). 

D.b.h growth of the unthinned stand averaged between 

0.05 and 0.10 inches annually depending upon tree 

diameter.

One of the more signifi cant contributions from this 

thinning study was that it showed how evenaged stands 

develop a reverse J-shaped distribution (no. trees over 

d.b.h) due to the layering of short-lived shade-intolerant 

species over longer lived species that are intermediate or 

shade tolerant (Fig. 1). Th e less tolerant species present 

a somewhat normal distribution, but the tolerant species 

(beech, sugar maple, and hemlock) provide an understory 

that forms the reverse J-shape. Th e implication was 

that such stands can readily be handled by unevenaged 

methods, provided that the understory exhibits an 

acceptable species composition, but this is often not the 

case in northern hardwood stands.

Another message was that thinning maintains an 

increasing mean annual basal area (and cubic- volume) 

growth rate, thus extending the rotation age as compared 

to an unthinned stand where mean annual growth begins 

to decline at age 50 to 60. Th is theme was reevaluated 

some years later showing that thinning could extend 

the rotation age (based on mean annual cubic-foot 

Table 1.—Average diameters in 1941 (trees 4.6 inches 

plus) and average annual diameter growth of the thinned 

stand, 1941-51 (Wilson 1953)

Species Average d.b.h. Average d.b.h. growth

---------------------- (in.) ------------------------

Red maple 7.8 0.13

White ash 9.4 0.12

Paper birch 10.9 0.10

Yellow birch 7.2 0.11

Beech 6.1 0.16

Sugar maple 6.9 0.15

All 8.1 0.13

increment) to 85 to 120 years (Leak 1999). Side benefi ts 

include larger, more valuable products, as well as lower 

regeneration acreages/costs/impacts. It is a common belief 

that thinning shortens the rotation; however, this would 

only be true for a rotation based on growing a certain-size 

tree—a so-called technical rotation.

THE 1959 STUDY: EARLY RESULTS

In 1933-35, a 22-acre tract (Compartment 22) was 

completely clearcut, essentially a whole-tree harvest 

because about 60 cords of split fuelwood were removed 

per acre. At age 25, the stand was the site of a designed 

thinning (release) study. As described by Marquis (1969), 

twenty 1⁄4-acre plots were established with 1-chain 

isolation strips between. Up to 100 crop trees were 

selected per plot. About half were paper birch and the rest 

were other species. Before thinning, the entire stand had 

more than 4,000 stems (0.5 inches and over) per acre, a 

basal area of 99 square feet, and an average diameter of 

1.8 inches. Th e crop trees numbered 385 per acre with 33 

square feet and an average diameter of 3.7 inches (Table 

2). It is noteworthy that clearcutting on a fairly good 

northern hardwood site produces a signifi cant proportion 

of long-lived, valuable species (sugar maple, yellow birch, 

and some white ash) in addition to paper birch and aspen.

To implement the thinning treatments, fi ve blocks of four 

plots each were established based on plot characteristics 

of species composition and basal area per acre. Th en four 

treatments were assigned at random within each block:

1. Heavy crop-tree: All trees that competed directly 

with each crop tree were removed, leaving a 
3

residual basal area (0.5 inches d.b.h and larger) of 

56 square feet per acre.

2. Light crop-tree: A maximum of one competing 

tree was removed around each crop tree; there were 

a few cases (crop trees that were strong dominants 

or when there was one competing tree between 

two crop trees) where less than one competitor was 

removed. Residual basal area was 72 square feet.

3. Species removal: All aspen, pin cherry, and striped 

maple were removed as well as red maple sprouts 

not selected as crop trees. Residual basal area was 

66 square feet.

4. Control: Residual basal area was 100 square feet.

Th inning was done in late August 1959 with sodium 

arsenite in frills; red maple sprout clumps with a retained 

crop tree were thinned by hand.

At the outset of the study, crown class (including crown 

vigor) and bole condition were recorded for the crop 

trees. Th e bole conditions were:

1. Good: No live (including epicormic) branches in 

the fi rst 17 feet. Only slight lean, crook, sweep, 

or wounds.

2. Medium: Up to one live branch and 5 

epicormics in the fi rst 17 feet. Slight to moderate 

lean, crook, or sweep. Slight wounds.

Figure 1.—Diameter distribution 

of the 60-year-old stand in 1936 

(Wilson 1953).
4

3. Fair: Any number of live or epicormic branches 

in the fi rst 17 feet. Slight to moderate lean, 

crook, sweep, or wounds.

4. Poor: Any number of defects.

Results of the thinnings over a 5-year period showed 

that annual basal area growth of the entire stand ranged 

from 2.2 square feet per acre in the control plots to 4.9 

square feet for the species thinning (Table 3). Mortality 

in number of stems over the 5-year period ranged from 

about 1,000 stems in the thinned plots to about 1,500 

stems in the controls. Annual basal area growth of the 

crop trees ranged from about 2.1 square feet in the 

control to 3.2 square feet following the heavy thinning; 

the comparable rates of diameter growth were 0.11 

inches and 0.18 inches, respectively. In general, growth 

responses indicated that the thinnings were eff ective.

Responses in quality were less clear. Briefl y, the thinnings 

resulted in much higher proportions of crop trees in 

dominant and strong codominant positions, as expected 

(Table 4). Th ere was very little change in bole quality 

classes. However, a high proportion of crop trees (40 to 

60 percent) showed increases in epicormic branching, 

mostly very small epicormics, following all thinning 

treatments. Sugar maple and yellow birch were the most 

prolifi c epicormic sprouters; however, at this stage, it is 

doubtful whether epicormic branching would result in 

long-term changes in quality.

Table 2.—The 25-year-old stand before thinning (Marquis 1969)

Entire stand Crop trees

Species Stems/acre Basal area/acre Mean d.b.h. Stems/acre Basal area/acre Mean d.b.h.

(no.) (sq. ft.) (in.) (no.) (sq. ft.) (in.)

Beech 1,029 11 1.3 24 1 2.7

Sugar maple 1,041 11 1.3 73 3 2.8

Striped maple 126 2 1.4 -- -- --

Conifers 58 1 1.8 -- -- --

Yellow birch 474 10 1.7 67 4 3.2

White ash 93 3 2.3 31 3 3.9

Red maple 176 5 2.0 21 2 3.5

Paper birch 401 24 2.7 169 20 4.4

Aspen 54 9 5.1 -- -- --

Pin cherry 636 23 2.4 -- -- --

All 4,088 99 1.8 385 33 3.7

Table 3.—Five-year growth responses to thinning of the entire stand and of the crop trees only 

(Marquis 1969)

Entire stand Crop trees

Treatment 5-year mortality Annual basal area 

growth

Annual basal area 

growth

Annual diameter 

growth

(no. stems/acre) (sq. ft./acre) (sq. ft./acre) (in.)

Heavy 1,019 4.0 3.2 0.18

Light 1,129 3.3 2.7 0.15

Species 977 4.9 2.7 0.15

Control 1,502 2.2 2.1 0.11
5

THE 1959 STUDY: FOLLOWUP

Th e long-term eff ects of these early precommercial 

treatments were evaluated by a remeasurement in 1990, 

31 years after the initial thinning (Leak and Solomon 

1997).

Comparisons of crop-tree development showed that the 

response varied by species. White ash and yellow birch 

showed little long-term diameter response to the 1959 

treatments; i.e., the thinned crop trees were about the 

same size as the unthinned. Paper birch, sugar maple, 

beech, and red maple showed increased crop-tree 

diameters of up to 2 inches larger than controls as a result 

of the early treatments. For paper birch, red maple, and 

beech (to a lesser extent) all of the thinning treatments 

produced about the same response. For sugar maple, the 

light and species-removal treatments produced about a 

1-inch gain; the heavy treatment, about a 2-inch gain.

A broader analysis of the 1990 remeasurement (Leak 

and Smith 1996) showed that the 1959 treatments 

had only minimal eff ects by age 56 on stems numbers, 

stand structure, and species mix except for a moderate 

proportion (about 10 percent) of aspen in the control 

and light crop-tree treatments. Pin cherry and striped 

maple were absent and minimal, respectively, by age 56.

Two economic evaluations provided a basis for evaluating 

the effi cacy of these early treatments. One evaluation 

was made by projecting the 5-year growth results over a 

20-year period to stand age 45 (McCauley and Marquis 

1972). Th is evaluation estimated that the species-removal 

and light thinning treatments could produce return rates 

on the initial investment of 9 to 10 percent. Th e heavy 

treatment was too costly to provide a positive economic 

benefi t. Th e authors suggested that lower numbers of 

crop trees—200 or less—would boost the economic 

returns due to lower initial costs.

Based on a remeasurement in 1990 (stand age 56), a 

second economic analysis provided return-rate estimates 

of 3.5 percent for the light thinning and a negative return 

for the heavy. Based on a 2003 remeasurement, the 

comparable rates of return were 2.3 and 0.2 percent for 

light and heavy thinnings, respectively (Sendak and Leak 

2008). Again, the authors suggested that greater returns 

would likely be achieved by releasing no more than 200 

crop trees and by applying a commercial harvest at about 

age 45.

THE 1959 STUDY: COMMERCIAL 

HARVEST

In the spring of 2003 (stand age 69 years), all the plots 

and buff er zones except for fi ve control plots were 

commercially thinned by removing most of the aspen 

and paper birch. Th ese species were the largest in the 

stand, so the operation was referred to as a dominant-tree 

thinning (Leak 2007). Basal area per acre was reduced 

to an average of 74 square feet per acre (4-inch class and 

larger) ranging from 54 to 126 square feet. Th e goal 

was to leave at least 50 square feet on each plot, which 

necessitated leaving a few paper birches (4.6 square feet/

acre). Th e controls averaged 141 square feet.

Logging damage from this commercial operation was 

higher than expected or desirable. Light damage was 

defi ned as a wound that extended less than 1⁄4 the way 

around the girth at the point of damage; heavy damage 

was defi ned as more than 1⁄4 girth. Damage levels (light 

and heavy combined) ranged from 8 percent for white 

Table 4.—Selected quality responses of the crop trees by treatment over the 5 years after the 1959 thinning 

(Marquis 1969)

Response Heavy Light Species Control

-------------------------------- Percent ----------------------------------

Percent dominants/strong codominants 49 40 40 27

Percent change in trees with bole class 1 and 2 +3 +4 +3 +1

Percent change in trees with no epicormics -9 -27 -18 -9

Percent of trees with increased numbers of 

epicormics

41 59 49 57
6

ash up to 30 percent for beech (Table 5). Th e majority 

of the wounds were in smaller trees—below 7 to 8 inches 

d.b.h. Th is level of damage could be reduced by more 

attention to layout of the skid trails, choice of equipment, 

and supervision of the operation.

After the thinning, 84 randomly selected residual sample 

trees (63 on thinned plots and 21 on the controls—

roughly equal numbers of each major species) were 

designated for diameter growth measurements. Results 

after 7 years were compared with the 31-year growth 

rates from the 1959 precommercial treatments (all 

thinning treatments combined excluding the control) 

and the 10-year rates from the 1936 study (Table 6). 

Following the commercial operation in 2003, thinning 

increased diameter growth of white ash and yellow 

birch as compared to the unthinned, and maintained 

or slightly increased growth of the other species. Th e 

rates are appreciably greater (except for white ash) than 

the 31-year rates following the 1959 precommercial 

treatments and moderately better than the rates following 

the commercial thinning in 1936. Th e conclusion is 

that the dominant tree removal in 2003 was a successful 

operation. A second conclusion is that current diameter 

growth rates are at least as good now as in the past 

despite atmospheric/environmental concerns.

Although diameter growth following the 2003 thinning 

was somewhat better than rates after the 1936 thinning, 

annual basal area growth per acre was poorer: 1.5 square 

feet as compared to 2.6 square feet, respectively (Table 

7). Sugar maple, beech, and white ash were the most 

productive in basal area growth. Growth on the 2003 

control plots was negative. Th e reason for the diff erences 

in the thinned rates in 2003 vs 1936 is the species 

composition (Table 8). After thinning, the 1936 stand 

had less yellow birch, much more red maple, and the 

paper birch in the slightly younger 1936 stand had not 

quite reached the point of overmaturity and decline.

Table 6.—Annual d.b.h growth (inches) for four thinning scenarios

Scenario Growth period Beech Yellow birch Sugar maple Red maple White ash No. samples

(yrs.) (in.) (in.) (in.) (in.) (in.)

Thinned 2003 7 0.17 0.13 0.20 0.19 0.17 59

Unthinned 2003 7 0.16 0.06 0.17 0.19 0.13 15

PCT 1959 31 0.09 0.08 0.15 0.14 0.18 128

Thinned 1936 10 0.16 0.11 0.15 0.13 0.12 --

Table 7.—Annual net basal area growth (sq. ft.) per acre (trees 4.5 inches plus) over a 7-year period following 

the 2003 commercial thinning and the 1936 thinning

Regime All species Beech Yellow 

birch

Sugar 

maple

Red 

maple

Paper 

birch

White ash Aspen Other

--------------------------------------------------------- Sq. ft. -------------------------------------------------------------

Thinned 2003 1.5 0.8 -0.2 0.8 0 -0.4 0.4 -- 0.1

Unthinned 2003 -1.9 0.4 0.1 0.8 0.1 -3.9 0.1 0.5 -- 

Thinned 1936 2.6 0.4 0.1 0.7 0.7 0.1 0.5 -- 0.1

Table 5.—Percent of trees (4 in. d.b.h and over) with light and heavy logging damage from the 2003 thinning

Damage Beech Yellow birch Sugar maple Red maple Paper birch White ash Other

--------------------------------------- Percent ----------------------------------------

Lighta 12 9 9 13 9 4 12

Heavy 18 8 14 12 4 4 17

No. samples 208 192 209 89 23 95 24

a Light damage was a wound less than 1⁄4 girth; heavy was more than 1⁄4 girth.
7

CONCLUSIONS

1. Based on the 1936 and 2003 thinnings at Bartlett in 

60- to 70- year-old stands, commercial thinning proves 

to be a productive operation in the northern hardwoods. 

Both thinnings, the 2003 thinning especially, removed 

large volumes of mature/overmature paper birch and 

aspen, still retaining suffi cient stocking of long-lived 

species. Diameter growth remained high, basal area 

production was adequate, and mean annual increment 

was maintained. At about age 70, paper birch and—to 

a lesser degree—aspen are becoming overmature and 

should be removed. Yellow birch did not respond 

very well and individual trees of this species should 

be carefully screened for vigor and quality. Th e one 

downside to the commercial operation was the excessive 

damage—up to 30 percent for beech, much of it in the 

smaller trees. Better layout, equipment, and supervision 

could readily reduce the damage.

2. Precommercial release treatments, as applied during 

the 1959 study in a 25-year-old stand, produced 

adequate growth responses on the retained crop trees 

(385 per acre). However, the economic returns at ages 

56 and 69 were only 3.5 percent or less. To make 

precommercial work more feasible, it would be advisable 

to identify less than 200 crop trees—possibly 50 to 100 

per acre would be suffi cient. Th is would allow for an 

adequate release at much less cost. Th e one downside is 

that fewer crop trees allow less opportunity to mold the 

species composition and less room for poor choices—a 

real possibility in young stands.

Northern hardwood stands provide a somewhat unique 

opportunity for commercial thinning since most 

evenaged stands, originating from a clearcutting, contain 

a signifi cant proportion of intolerant species, mainly 

aspen and paper birch. Th ese species mature in about 50 

to 70 years and provide a profi table commercial thinning 

for pulpwood and small logs (depending on local 

markets). Usually there is suffi cient residual stocking 

(50 square feet/acre plus) of longlived species (sugar and 

red maples, yellow birch, white ash) to continue stand 

management until a fi nal regeneration harvest at age 

100-120 years. Without this commercial thinning of 

intolerant species, total stand productivity declines, and 

the opportunity to release the long-lived species is lost.

LITERATURE CITED

Jensen, V.S. 1940. Results of thinning and its eff ect 

on residual yellow birch and associated hardwoods 

species. Stn. Tech. Note 33. New Haven, CT: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 4 p.

Leak, W.B. 1999. Short versus long rotations.

Northern Journal of Applied Forestry. 16(4): 200-

202.

Leak, W.B. 2007. Th inning northern hardwoods in 

New England by dominant-tree removal—early 

results. Northern Journal of Applied Forestry. 24(4): 

312-313.

Leak, W.B.; Smith, M.L. 1996. Long-term species and 

structural changes after cleaning young even-aged 

northern hardwoods in New Hampshire. Forest 

Ecology and Management. 95: 11-20.

Leak, W.B.; Solomon, D.S. 1997. Long-term growth 

of crop trees after release in northern hardwoods.

Northern Journal of Applied Forestry. 14(3): 147-

151.

Table 8.—Basal areas per acre (sq. ft.) after thinning (trees 4.6 inches plus) by species for the 2003 and 

1936 thinning studies

Regime All 

species

Beech Yellow 

birch

Sugar 

maple

Red 

maple

Paper 

birch

White 

ash

Aspen Other

---------------------------------------------------- Sq. ft. ----------------------------------------------------------

2003 thinning 69.7 7.5 13.0 13.9 10.9 5.0 18.4 -- 1.0

1936 thinning 79.7 2.5 3.2 10.9 24.8 11.6 25.0 -- 1.7
8

Marquis, D.A. 1969. Th inning in young northern 

hardwoods: 5-year results. Res. Pap. NE-139. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 22 p.

McCauley, O.D.; Marquis, D.A. 1972. Investment 

in precommercial thinning of northern 

hardwoods. Res. Paper NE-245. Upper Darby, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 13 p.

Nyland, R.D. 2002. Silviculture concepts and 

applications. 2d ed. Long Grove, IL: Waveland Press 

Inc. 682 p.

Sendak, P.E.; Leak, W.B. 2008. Early crop-tree release 

and species cleaning in young northern hardwoods: 

a fi nancial analysis. Res. Pap. NRS-6. Newtown 

Square, PA: U.S. Department of Agriculture, Forest 

Service, Northern Research Station. 13 p.

Wilson, R.W. 1953. How second-growth northern 

hardwoods develop after thinning. Stn. Pap. No. 62. 

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 12 p.

Appendix Table 1.—Common and scientifi c names of 

tree species referred to in this report

Common Name Scientifi c Name

Beech Fagus grandifolia

Yellow birch Betula alleghaniensis

Sugar maple Acer saccharum

Red maple Acer rubrum

Paper birch Betula papyrifera

Eastern hemlock Tsuga canadensis

Red spruce Picea rubens

Pin cherry Prunus pensylvanica

Striped maple Acer pensylvanicum 

Aspen Populus spp.
Printed on Recycled Paper

Leak, William B.; Yamasaki, Mariko. 2012. 80 Years of Thinning Research on 

Northern Hardwoods in the Bartlett Experimental Forest, New Hampshire.

Res. Pap. NRS-20. Newtown Square, PA: U.S. Department of Agriculture, Forest 

Service, Northern Research Station. 8 p.

Commercial and noncommercial thinning studies in northern hardwoods on the 

Bartlett Experimental Forest, New Hampshire, began in 1932. One of the studies, still 

maintained today, consisted of several precommercial treatments at age 25 (1959) 

and a commercial treatment in 2003. Although economic returns from precommercial 

work appear somewhat marginal and require additional research, commercial 

thinning in northern hardwoods is clearly advisable—almost necessary because 

evenaged northern hardwoods offer a unique thinning opportunity: a component of 

shade-intolerant aspen and paper birch maturing in 50 to 70 years coupled with a 

component of long-lived species that will mature at age 100 to 120 years.

KEY WORDS: commercial thinning, precommercial thinning, northern hardwoods 

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