United States 

Department of Precommercial Crop-Tree Agriculture 

Forest Service Thinning in a Mixed Northern 

Northeastern Forest Hardwood Stand 

Experiment Station 

Research Paper NE-640 

Nancy G. Voorhis 
Abstract 

The effectiveness of precommercial crop tree thinning was investigated by thinning 

crop trees of paper birch, yellow birch, and sugar maple to two intensity levels at 8 

years of age. Analysis of covariance on post thinning growth measurements taken 

7 years later indicated a significant increase in diameter and crown-diameter 

growth, with the pattern of response varying by species. The study suggests that a 

uniform crop-tree thinning treatment for a stand composed of species with varying 

tolerances will rarely produce the best response for all the species. 

The Author 

Nancy G. Voorhis formerly was a research scientist in timber management for the 

Forest Service, Northeastern Forest Experiment Station, Durham, New Hampshire. 

She holds a Masters of Science in Mathematics from the University of New 

Hampshire and a Bachelors of Science in Forest Management from the University 

of Vermont. She is currently a programmer analyst for a small business firm. 

Acknowledgment 

The publication of the results of this study has only been accomplished with the 

substantial contributions of Russ Walters who designed and established the study 

while working as a research scientist at the George D. Aiken Laboratory in 

Burlington, Vermont. Thanks also to the many foresters and technicians at the 

Rochester District, Green Mountain National Forest, the Aiken Laboratory, 

Burlington, and at the Louis C. Wyman Laboratory, Durham, New Hampshire, who 

helped with the thinnings and measurements. Special thanks also to Carl Tubbs 

and Bill Leak for their thoughtful reviews, contributions, and patience. 

Manuscript received for publication 28 September 1988 

Northeastern Forest Experiment Station 

100 Matsonford Road, Radnor, PA 19087 

June 1990 
Introduction 

The current condition of the typical northern hardwood 

forest in Vermont indicates the long-term effects of past I 

cutting practices. These practices have increased the 

I amount of cull in an old-growth forest by as much as onethird of the gross volume (Gilbert and Jensen 1958). This 

undesirable increase has focused attention on management 

tools that concentrate the growing stock into fewer and 

better stems. For young even-aged northern hardwood 

stands, the best method for accomplishing this is either 

crown-tree thinning or crop-tree thinning (Smith 1962). This 

study investigates crop-tree thinning performed on an 

8-year-old stand of mixed northern hardwoods. The effects 

on the stand and its structure are examined by analyzing 

diameter, height, and crown growth, and by relating these 

measures to tree quality. 

Study Area 

The area is in a 231-acre northern hardwood stand on the 

eastern side of the Green Mountains in central Vermont at 

an elevation of 1,800 feet. The site index is 55 for sugar 

maple and the major species are paper birch, sugar maple, 

yellow birch, and red maple with many minor component 

and understory species including beech, striped maple, and 

white ash. The stand had been cut as part of the Green 

Mountain National Forest Boyden Brook Sale. Five years 

later, the stand had an average basal area of 40 square feet 

per acre, an average stand diameter of less than 1 inch, 

and was treated with a prescribed TSI release that yielded 

the sapling stand thinned for this study. The thinning was 

performed when the stand was 8 years old. The stand was 

densely stocked with average diameters of 1.3 inches for 

yellow birch, 1.1 inches for sugar maple, and 1.7 inches for 

paper birch with average heights of 16.9, 13.9, and 20.0 

feet, respectively. 

The soils are about 50 percent Berkshire and 40 percent 

Marlow: extremely stony, well-drained loams which are 

formed in glacial till derived from quartzite and schistose 

rocks. They often have a large amount of organic surface 

accumulation, a gravelly surface layer, and the Marlow soils 

often have a fragipan at 15 to 30 inches below the surface. 

The average temperature in the area for the study period, 

I 1977-83, was 41.3 degrees Fahrenheit with average highs 

of 93 degrees and average lows of -22 degrees. The 

average precipitation was 46.5 inches, close to 1 inch above 

normal. 

Methods 

A total of 308 crop trees were selected at 20-foot intervals 

along parallel lines spaced 20-feet apart. At each 20-foot 

interval a stem meeting the crop-tree criteria was chosen: a 

tree of good quality appearing to have the potential for 

developing into a sawtimber-quality tree. To ensure 

selection of young trees, the tree diameter had to be less 

than 2 inches at breast height. The goal was to choose 100 

trees for each species, with species determined by 

availability of suitable stems and perceived value of the 

species. Priority was given to choosing 100 stems of the 

highest valued species. Approximately 100 stems each 

then, of yellow birch, sugar maple, and paper birch were 

chosen. Each stem was randomly assigned to control, light, 

or heavy treatment. The crowns were released by thinning 

from stem outward to a radius of 4 feet for the light 

treatment or 8 feet for the heavy treatment. The thinning 

treated all stems two-thirds or more the height of the crop 

tree and any other overtopping trees with major limbs 

extending into the thinning radius. 

Measurements were made at the time of thinning and 7 

years later on d.b.h., total height, crown diameter (average 

of N-S and E-W measurements), height to crown base, and 

height to the first branch longer than 24 inches. Diameter 

measurements were also taken at 1.2 and 4 years after 

thinning. The data were analyzed as a completely random 

factorial design, with factors species and treatment. 

Analysis of covariance was performed on the 7-year 

postthinning diameter growth using initial diameter as the 

covariate, and on change in crown width using initial crown 

size as the covariate. Total height growth, change in height 

to base of crown, and change in height to the first major 

limb were analyzed using analysis of variance. 

Results 

The average diameter growth increased by treatment for all 

species, but each species responded differently to the 

treatments (Table 1). Analysis of covariance showed that 

overall, treatment was significant. For individual species, it 

showed that yellow birch had a significant increase 

(p = 0.05) from control to light treatment and from light to 

heavy treatment; paper birch had a significant increase from 

control to light treatment and from control to heavy 

treatment, but not from light to heavy treatment; and sugar 

maple did not respond significantly to either treatment. 

Intermediate d.b.h. growth data indicated that the increase 

in growth continued longer as thinning level increased. For 

example, the paper birch trees in the heavy thinning grew 

36.7 percent of the total growth increase during the last 

Table 1 .-Average 7-year diameter growth at breast 

height adjusted by initial diameter (inches) 

Treatment 

Species Control Light Heavy 

Paper birch 1.33a 1.68b 2.05b 

Yellow birch 1.44a 1.79b 2.15~ 

Sugar maple l.lla 1.46a 1.83a 

Means within rows followed by the same letter are not 

significantly different (p = 0.05). 
study period (years 5 through 7), as compared with the 

control group that grew 30.5 percent of the total growth 

increase during the same period (Table 2). 

The average crown diameter growth increased as thinning 

intensity increased. One exception was sugar maple which 

had an average of 1.88 feet for the light treatment group as 

compared to 2.14 for the control group, but the control 

value was exceptionally high as compared to averages of 

1.06 and .94 feet for yellow and paper birch, respectively. 

The analysis of covariance indicated that overall treatment 

was significant, but as with diameter growth, each species 

responded differently (Table 3). Yellow and paper birch had 

significant increases from control to light treatment and 

from light to heavy treatment, while sugar maple had a 

significant increase only from light to heavy treatment. The 

lack of significance for this difference could be attributed to 

the unusually large value for control. 

The average total height growth was not significantly 

different by treatment for any species and was about 12 to 

13 feet for all treatment and species combinations. So, 

despite thinning, the canopy structure was similar to what 

was present at the onset of the study. 

The effect of thinning on stand quality was indicated by 

analyzing change in height to base of crown and change in 

height to the first major limb. Treatment was significant for 

both of these measures, with trends showing a general 

decline as thinning intensity increased (Figs. 1 and 2). The 

change in height to base of crown showed the most decline 

in the heavy thinning for all three species. An increase was 

found in the light paper birch thinning. The amount of 

Table 2.-Average diameter growth by intermediate periods 

(percentage of the total growth for the '/-year period) 

Treatment 

Species Year Control Light Heavy 

1 16.7 15.4 15.1 

Paper birch 2 22.1 20.1 16.9 

3,4 30.7 29.4 31.3 

Yellow birch 2 19.6 18.6 19.7 

3,4 28.2 33.4 31.1 

decline in height to the first major limb was more dramatic 

for both yellow and paper birches, which started with longer 

clear boles at the time of thinning, than for sugar maple, 

which showed less decline but also had less clear bole at 

the outset. These trends indicate there may be a more 

negative impact on the quality of less tolerant species when 

the thinning treatment prescribed is one, like the heavy 

treatment described here, that maximizes d.b.h. and crown 

diameter growth. 

Somewhat comparable results were obtained from the 

16-year remeasurement of a previously reported thinning 

study (Marquis 1969) in a 25-year-old northern hardwood 

stand on the Bartlett Experimental Forest, New Hampshire. 

At Bartlett, the heavy thinning treatment removed all trees 

that competed with the crowns of the crop trees and the 

light treatment removed the one most severe competitor. 

Residual stand basal areas for trees 0.5 inches d.b.h. and 

greater were 56, 72, and 100 square feet per acre for the 

heavy and light treatments and the control group, 

respectively. 

As in the Boyden Brook study, there were no significant 

differences in the diameter growth response of paper birch 

to the light and heavy thinning treatments. Paper birch's 

small-sized crown may not need any more than a light 

release to completely free it from competition, or its fast 

growth may allow it to take advantage of a newly thinned 

area more quickly than other species. In contrast to the 

results of this study, sugar maple at Bartlett showed 

response to both light and heavy thinnings. In addition, the 

growth response from the heavy thinning at Bartlett did not 

persist any longer than the response from the light thinning. 

Table 3.-Average 7-year crown diameter growth 

adjusted by initial crown diameter size (feet) 

Treatment 

Species Control Light Heavy 

Paper birch .94a 2.56b 3.37~ 

Yellow birch 1.06a 2.48b 4.60~ 

Sugar maple 2.14a 1.88a 3.22b 

Means within rows followed by the same letter are not 

significantly different (p = 0.05). 

Sugar maple 2 21.6 18.1 18.3 

3, 4 27.8 28.1 28.0 
paper birch 

U Contrc 

E3 Light 

yellow birch sugor maple 

Figure 1 .-Change in height to crown base, by species and 

treatment. 

Conclusions 

Many thinning studies have been conducted to answer the 

question: Is it worth it? It is well understood that crop-tree 

thinning does increase growth (Erdmann and Godman 

1981, Hannah 1978, Marquis and others 1969, Stoeckeler 

and Arbogast 1947). The worthiness of crop-tree thinning in 

this study focuses on two issues: were the trees released at 

the appropriate age, and did the methods produce the best 

possible response for all three species? 

Any thinning should begin after crown closure. Successful 

crop tree thinning also begins after good choices for crop 

trees can be made. Applying these guidelines in a mixed 

stand may present an ambiguous situation. Faster growing 

species may appear ready for thinning while slower growing 

species do not. This study showed that age 8 was an 

appropriate time for choosing paper birch crop trees, but 

was too early for choosing yellow birch and sugar maple 

crop trees. At the time of final measurements (age 15), 

many stems of yellow birch and sugar maple were observed 

that were more vigorous and of better quality than those 

chosen as crop trees at age 8. 

One simple solution is to wait until the slower growing 

species have differentiated in stem quality, but not to wait 

so long that the faster growing species have begun to 

dominate the stand. Other researchers have found success 

in choosing crop trees in pure stands of sugar maple at 11 

years of age (Godman 1968, Stoeckler and Arbogast 1947) 

and in choosing yellow birch crop trees at 10 years of age 

(Erdmann and others 1981). The total height data from this 

study showed that thinned and unthinned paper birch did 

not grow significantly faster than any of the other species, 

so thinning between the ages of 10 and 14 would increase 

10 ( El Contrc 

E3 Light 

I Heavy 

paper birch yellow birch sugor maple 

Figure 2.-Change in height to first major limb, by species 

and treatment. 

success in choosing crop trees and would not change the 

prethinning canopy structure. Site index relations show that 

the difference in height between paper birch, sugar maple 

and yellow birch will increase as the quality of the site 

increases (Marquis and others 1969), thus thinning at age 

10 to 11 is ideal for better sites and age 13 to 14 is ideal for 

poorer sites. - 

Discussion of the second issue pivots on the definition of 

the best response to thinning. Ideally, the best response is 

that which produces the most increase in growth. 

Realistically, it is also the method which produces the least 

decrease in quality. Ambiguity in a mixed stand can arise 

when the best thinning method varies between species, and 

choosing one method for the whole stand will involve 

compromises. For example, the diameter growth results 

indicate that yellow birch responded most to the heavy 

thinning, while paper birch responded equally to the light 

and heavy thinnings, and sugar maple did not respond to 

either. Taken individually, the conclusion might be to thin 

yellow birch heavily, paper birch lightly, and sugar maple to 

an even larger radius than was used. But it is unclear if one 

thinning method is appropriate. The prescription of thinning 

to a fixed radius in a mixed stand with species of varying 

growth rates and shade tolerances may not be the best 

method. The pattern of growth response found in this study 

shows that faster growing species require a smaller thinning 

radius, while slower growing species require a larger 

thinning radius. This may be emphasized by the indirect 

release of faster growing species on the edge of the thinned 

area around a slower growing species, which reduces the 

effective size of the thinned area (Leak and others 1987). 

One possible technique is to leave only edge trees of the 

same species. For example, begin at the stem of the 

chosen crop tree, thin outward to a radius of 8 feet, then 
continue to thin outward until a tree of the same species as 

the crop tree is found and can be left as the edge tree. It is 

clear that more studies will be needed to substantiate 

whether this method will work. 

Despite so many thinning studies, there is still subjectivity 

involved in determining whether a particular thinning 

method is "worth it". The data from this study indicate that 

a precommercial crop-tree thinning to an &foot radius in a 

mixed stand will produce a significant increase in diameter 

and crown growth in paper and yellow birches without 

severely decreasing quality. But, the data also show that 

this may not be the most efficient or effective way to thin in 

a mixed stand. 

Literature Cited 

Erdmann, G. G.; Godman, R. M. 1981. Tending young 

northern hardwood stands. In: Proceedings, national 

silvicultural workshop-hardwood management; 1981 

June 1-5; Roanoke, VA. Washington, DC: US. 

Department of Agriculture, Forest Service, Timber 

Management Division. 124-150. 

Erdmann, G. G.; Peterson, Jr., R. M.; Godman, R. M. 1981. 

Cleaning yellow birch seedling stands to increase 

I survival, growth and crown development. Canadian 

Journal of Forest Research. 1 1 : 62-68. 

Gilbert, A. M.; Jensen, V. S. 1958. A management guide 

for northern hardwoods in New England. Station 

Paper No. 112. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 22 p. 

Godman, R. M. 1968. Culture of young stands. In: 

Proceedings, sugar maple conference; 1968; 

Houghton, MI. Houghton, MI: Michigan Technical 

University. 82-87. 

Hannah, P. R. 1978. Growth of large yellow birch 

saplings following crop tree thinning. Journal of 

Forestry. 76: 222-223. 

Leak, W. B.; Solomon, D. S.; Debald, P. S. 1987. 

Silvicultural guide for northern hardwood types in the 

northeast (revised). Res. Pap. NE-603. Broomall, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 36 pi 

Marquis, D. A. 1969. Thinning 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. 21 p. 

Marquis, D. A.; Solomon, D. S.; Bjorkbom, J. C. 1969. A 

silvicultural guide for paper birch in the northeast. 

Res. Pap. NE-130. Upper Darby, PA: US. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 47 p. 

Smith, D. M. 1962. The practice of silviculture. 7th ed. 

New York: John Wiley & Sons. 578 p. 

Stoeckler, J. H.; Arbogast, C. F. 1947. Thinning and 

pruning of young second-growth hardwoods in 

northeastern Wisconsin. In: Proceedings, Society 

American Foresters meeting; 1947 December 17-20; 

Minneapolis, MN. Washington, DC: Society of American 

Foresters. 328-346. 
Voorhis, Nancy G. 1990. Precommercial Crop-Tree Thinning in a Mixed 

Northern Hardwood Stand. Res. Pap. NE-640. Radnor, PA: US. 

Department of Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 4 p. 

Analysis of growth measurements taken 7 years after thinning an 8-year-old 

hardwood stand showed significant diameter and crown-diameter growth 

increases. Further analysis showed dissimilarities in the pattern of response 

of the three species observed: yellow birch, sugar maple, and paper birch. 

Keywords: crop-tree; thinning 

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