Department of 

Agriculture 

Forest Service 

Northeastern Forest 

Experiment Station 

Research Paper NE-618 

Effect of Thinning on 

Mixed-Oak stemWQuality 

~Avid L. Sonderman 

Everette D. Rast 
Abstract 

Changes in limb-related stem defects were studied from 1977 to 1982 on 595 

mixed-oak trees following a schedule of thinnings dating back to 1962. All of the 

thinnings were controlled by stocking goals rather than predetermined time intervals. Results show that heavy and moderate thinnings, as opposed to light thinning~, had an adverse effect on the number and size of live and dead limbs. The 

light thinnings resulted in fewer live and dead limb defects per square foot of surface area but much slower diameter growth. 

The Authors 

DAVID L. SONDERMAN, research forest products technologist, joined the Northeastern Forest Experiment Station in June 1962 and was on the staff of the Eastern Softwood Timber Quality project until June 1972. From 1972 to 1987 he was 

located at Delaware, Ohio, with the Northeastern Station's project on management and utilization alternatives for nonindustrial private forests. He is currently 

working at the Station's Forestry Sciences Laboratory, Princeton, West Virginia, on 

the development of the new advanced hardwood processing and technical 

resource center. 

EVERETTE D. RAST, research forest products technologist, received a B.S. 

degree in forestry from the University of Missouri in 1960 and an MS. degree in 

agricultural economics from The Ohio State University in 1970. He joined the 

Forest Service in 1960 as a forester on the Mendocino National Forest and was 

located at the Northeastern Forest Experiment Station's Delaware, Ohio, laboratory from 1966 to 1987. He is currently with the Station's Forestry Sciences Laboratory at Princeton, West Virginia, working on the advanced hardwood processing 

and technical resource center. 

Manuscript received for publication 9 May 1988 

Northeastern Forest Experiment Station 

370 Reed Road, Broomall, PA 19008 

September 1988 
I Introduction 

The upland oak forest type is the most extensive hardwood 

timber type found in the United States. Growing-stock 

t volumes of white and red oak in the Eastern United States 

exceed 38 billion cubic feet (USDA Forest Service 1982). To 

better understand the resource, a series of studies were 

initiated to determine potential quality in young oak trees. 

By regulating the stand density, researchers wanted to 

determine whether they could grow trees faster while maintaining wood quality. To accomplish this, a measure of the 

I stem-related defects and their change over time under various cultural treatments was needed. 

Research on bole-quality changes following cultural treatment has been neglected, though not cohpletely ignored. 

An early study by Brinkman (1955) reporjed that individual 

trees in the oak type tend to inherit epico'rmic branch traits, 

but that the amount of branching is influenced by the crown 

position and degree of thinning. Smith (1966) found that 

oak trees are more prone to develop epicormic branches 

than yellow-poplar or white ash trees. Della-Bianca (1983) 

found that white oaks apparently produce and retain more 

branches and epicormics than most other species, even 

other oaks. In a more recent study, Sonderman (1984) 

1 found that the number of defects per square foot of surface 

area on oak species increased significant1,y at the heaviest 

thinning level, lowering potential stem quality. In a study 

describing changings in provisional tree grades, Sonderman 

and Rast (1987) also found that light thinning and crop-tree / thinning produced higher quality and better grade trees 

than heavy thinning. 

In this study, changes in limb-related sterri defects were analyzed from 1977 to 1982 as a result of initial tree thinnings 

followed by subsequent thinnings that maintained the stocking. This situation is typical of many wood'lot ownerships 

throughout the central hardwood region where subsequent 

thinnings, such as cleanings for firewood, wre common. Particular attention was given to the residual dtocking and its 

relationship to individual defect changes over time. This 

type of information is needed to select thebest thinning 

methods for growing trees as defect free as possible. 

The Stand 

The plots selected for this study are located in southeastern 

Ohio. The original stand was made up of well-stocked, polesize, even-aged, 31-year-old mixed-oak trees. Before initial 

thinning of the stand in 1962, basal area averaged 74 to 89 

ft* and the stand diameter range (diameter lat breast 

height-d.b.h.) was 3.8 to 4.5 inches. lnitialthinnings were 

made on sixteen %-acre plots at six stockinb levels: 40, 50. 

60, 70, 80, and 100 percent. Stand-density levels in this 

study were expressed as percent of full stokking. Stocking 

plots were grouped by three thinning methods: heavy (40 to 

50 percent stocking), moderate (60 to 70 pdrcent), and light 

(80 to 100 percent). Isolation strips were established around 

each plot and treated as the included plot. 

Following the original thinnings in 1962, subsequent thinnings were scheduled and controlled by stocking goals 

rather than predetermined time intervals. The stocking 

goals allowed the selected plots to increase to various levels 

of percent stocking before being thinned back to the 

intended stocking level. The stocking goals were as follows: 

(1) short thinning interval that allowed the selected plots to 

be thinned to initial stocking after an increase of 10 percent; 

(2) an increasing stocking level that allowed plots to be 

thinned to 10 percent following an increase of 20 percent; 

(3) long thinning interval that allowed the plots to increase 

to 80 or 90 percent before thinning to initial stocking; and 

(4) no thinning after the initial treatment. These treatments 

generally were free thinning with consideration for species, 

size, form, and spacing. 

In 1977, when the stand was first measured for stem quality, 

species composition consisted primarily of mixed oaks and 

a few other species (yellow-poplar, red maple, aspen, black 

cherry, and hickory). The diameter (d.b.h.) range at that 

time was between 3.6 and 11.9 inches. Only the oaks were 

included in the study because of the limited number of 

other species. There were 595 white, red, and chestnut oak 

sample trees. 

Defect information from the study trees following the 5-year 

period after 1977 demonstrates the effects of heavy, moderate, and light thinnings on tree quality. Prior to and including thestudy period, the plots were maintained according to 

prescribed thinning methods that closely controlled residual 

stocking. These young hardwood trees, and the way they 

were thinned, were typical of many found throughout the 

central hardwood region. 

Methods 

Detailed measurements of the external tree characteristics 

were taken on each sample tree for quality determination in 

1977 and 1982. Only trees 3.6 inches and larger in d.b.h. 

were included in the study. All branches measuring 0.3 inch 

in diameter and larger were tallied as limbs on the first and 

second &foot sections of the butt 16-foot log of the tree. 

Branches smaller than 0.3 inch were counted as epicormic 

branches. The variables measured and recorded were: 

D.b.h. 

Height 

Crown class 

Crown ratio 

Sweep and crook 

Number of live limbs on the first and second 8-foot section of the butt 16-foot log 
Number of dead limbs on the first and second &foot section of the butt 16-foot log 

Diameter of the largest live limb on the first and second 

8-foot section of the butt 16-foot log 

Diameter of the largest dead limb on the first and second 

8-foot section of the butt 16-foot log 

Number of epicormic branches on the first and second 

8-foot section of the butt 16-foot log 

The number of epicormic branches was broken down into 

three classes: None, 1 to 6, and 7 or more. The three classes 

of epicormic branch measurements were grouped together 

for the analysis. 

For comparative purposes, a relative quality index (Sonderman 1979) was computed for each tree by assigning a 

numerical value to individual characteristics: 

Crown Class 

Dominant 

Codominant 

Intermediate 

Suppressed 

Sweep and Crook 

(Deviation in inches for 

16-foot butt log) 

1 

2-4 

5-6 

7-8 

9+ 

Value 

4 

3 

2 

1 

Value 

4 

3 

2 

1 

0 

If the tree has forks, rot, or seams, then the sweep and 

crook numerical value = 0. 

Limb Count 

(Limbs 2 0.3 inch) 

1-2 

3-4 

5-8 

9-1 6 

17 + 

Value 

4 

3 

2 

1 

0 

By adding the numerical values for crown class, sweep and 

crook, and limb count, the relative quality index and class of 

a tree can be determined as follows: 

Relative Quality Index 

(Range of individual values) Relative Quality Class 

1-7 Poor 

8-9 Medium 

10-1 2 Good 

Results and Discussion 

Although considerable research has been conducted on the 

effects of timber management practices on tree-growth 

rates, there has been limited work in this area on timberquality development. In this study we have described the 

response of several tree-quality characteristics common to 

young oak stands at various levels of thinning. In dealing 

with these young, potentially high-quality trees, quality 

change over time is as important, if not more important, 

than growth rate. 

Diameter 

Differences in diameter growth due to thinning treatments 

were small overall with the greatest change taking place in 

the moderate thinning (Table 1). Increases in diameter 

growth for heavy, moderate, and light thinning were, respectively: 1.2, 1.3, and 0.9 inches. Given more time, the heavy 

thinning should have resulted in the largest increase in 

diameter growth. 

Table 1.-Summary of average mixed-oak tree characteristics by thinning method, 

1977-82 

Tree 

characteristic 

Thinning method 

Heavy Moderate Light 

1977 1982 1977 1982 1977 1982 

D.b.h. (inches) 9.6 10.8 8.3 9.6 7.7 8.6 

Crown ratio (%) 36.4 36.5 32.4 32.4 29.8 32.5 

Defects/ft2 surface area (no.) ,290 .290 ,340 ,400 ,420 ,320 

Relative quality index (no.) 8.1 8.5 7.5 7.6 7.5 7.8 
Crown Class 

All of the thinning methods affected thecrown classes of 

the study trees (Table 2) in different ways. The heavy thinning resulted in an increase in the percentage of dominant 

trees. The medium thinning increased both the dominants 

and codominants, while the light thinning increased the 

percentage of codominants only. This is important to young 

growing-stock trees because of the number of defects that 

are associated with different crown classes. Normally, 

genetically inferior trees with intermediate and suppressed 

crowns tend to sprout more prolifically tpan genetically 

superior trees with dominant and codomlinant crown 

classes. By creating changes in crown class through thinning~, stem quality can be modified over time. 

Crown Ratio I 

Crown ratio is the ratio of the length of tqe live crown to the 

total height of the tree and usually is a good indicator of 

tree growth and vigor. Because of thinning, the combination 

of additional limbs and epicormic branchles that can 

develop sometimes causes a downward extension of the 

crowns on young trees, resulting in additional defects and 

lower quality of the section. Also, thinning methods that 

reach stocking goals through subsequent thinnings usually 

allow large amounts of light to reach the Stand at different 

times, causing additional crown growth ahd large crown 

ratios. Therefore, response to light created by openings in 

the crown canopy as a result of thinningd will cause an 

increase in the crown ratio (Table 1). ~hisl usually means 

larger live limbs, additional epicormic sprouts, and a change 

in the crown dominance. However, results did not verify this 

completely because of the short study time of only 5 years. 

Subsequent longer-term crown ratio studies are needed to 

substantiate or reject this statement. 

Relative Quality Index 

The relative quality index is a gradient that expresses a tree 

or stand in terms of a numerical index of stem qualityrelated characteristics-the higher the index the higher the 

tree quality. In this study, the index increased the most in 

the heavy thinning, reflecting more dominant crown classes 

(Table 1). The index also increased in the light thinning as a 

result of a decrease in the number of live and dead limbs. 

Moderate thinning resulted in little change. None of the 

thinning methods resulted in a shift from one relative quality 

class to the next. 

Moderate thinning (60 to 70 percent residual stocking) 

resulted in the poorest quality index, reflecting changes in 

both crown class and the number of live and dead limbs. 

Over the 5 years, heavy thinning (40 to 50 percent) had the 

best relative quality index, but given more time the index 

would have been lower because of the sprouting of the 

trees with epicormic branches and small live limbs. Also 

given more time, the opposite would have resulted in the 

light thinning (80 to 100 percent)-there would be fewer 

epicormic branches and live limbs over time because of the 

dense stocking maintained. 

Table 2.-Mixed-oak crown classes, by thinning method, 1977-82 

Crown 

class 

Thinning method 

Heavy Moderate Light 

1977 1982 1977 1982 1977 1982 

I Percent .............................. 

Dominant I 17 30 18 26 24 22 

Codominant I 

69 67 54 55 43 48 

Intermediate 14 3 18 14 20 18 

Suppressed I - - 10 5 13 12 
Live and Dead Limb Defects per Square Foot 

of Surface Area 

Limb defects per square foot of surface area (Table 1) 

include live and dead limbs and overgrowths, but not epicormic branches. The expansion of the surface area 

through increased diameter growth Usually results in a 

decrease in the number of defects per square foot of surface area if no new defects develop. Even though the 

number of defects per square foot of surface area in the 

heavy thinning appeared to remain constant from 1977 to 

1982, the actual number of defects increased, offset by the 

large d.b.h. growth increase. For the same reasons, there 

was a slight increase in the number of defects per square 

foot of surface area in the moderate thinnings. By contrast, 

the light thinning showed a reduction in the number of 

defects per square foot of surface area, probably due to 

natural pruning and smaller d.b.h. growth. 

Live Limbs 

The number of live limbs was reduced in all thinning 

methods from 1977 to 1982 (Table 3). The heavy and moderate thinnings each reduced the number of live limbs by 26 

percent and the light thinning by 15 percent. The high 

reduction in the number of live limbs in the heavy and moderate thinnings resulted from many of the live limbs dying 

over the 5-year period. At the same time, the remaining live 

limbs on the trees persisted and grew larger from the exposure to more light. In fact, increases in live-limb size in the 

second 8-foot section were, respectively: 0.30 inch for the 

heavy, 0.1 1 inch for the moderate, and 0.09 inch for the light 

thinning. We believe that the variations in limb size caused 

by different thinning methods resulted mainly from differences in the amounts of light. 

Table 3.-Characteristics of mixed-oak limbs in butt and second &foot sections, by 

thinning method, 1977-82 

Thinning method 

Section Heavy Moderate Light 

1977 1982 1977 1982 1977 1982 

First &foot 

Second &foot 

First &foot 

Second &foot 

First &foot 

Second 8-foot 

First &foot 

Second &foot 

LIVE LIMBS (Number) 

0.35 0.30 0.60 0.50 0.41 0.37 

5.50 4.08 3.86 2.86 3.38 2.88 

DEAD LIMBS (Number) 

3.94 3.55 4.10 4.54 2.77 2.29 

8.61 10.61 10.47 13.07 8.75 8.25 

LIVE-LIMB SlZE (Inches) 

54 .70 58 54 .56 .64 

.81 1.1 1 56 .77 .62 .71 

DEAD-LIMB SlZE (Inches) 

.76 .73 .62 .67 .58 .61 

.77 .92 .73 .91 .69 .79 

EPlCORMlC BRANCHES (Per~ent)~ 

First &foot 27.5 74.2 31.3 68.5 21.5 64.0 

Second 8-foot 44.8 77.3 47.1 84.9 37.5 87.7 

aPercentage of trees with epicormic branches. 
Dead Limbs 

The number of dead limbs in the second 8-foot section 

increased by 23 and 25 percent, respectively, in the heavily 

and moderately thinned plots, and decreased by less than 1 

percent in the lightly thinned plots (Table 3). The increase in 

the number of dead limbs in the heavy and moderate thinnings resulted from a large number of the live limbs dying. 

The slight reduction in the number of dead limbs in the light 

thinning resulted from the natural pruning of the smaller 

dead limbs. The smaller pruned dead limbs, if given enough 

time, will result as a defect (overgrowth) while the larger 

dead limbs will persist for years. Both are considered 

defects until the scar or overgrowth is covered with enough 

clear wood to place them out of the quality zone (Rast et al. 

1 973). 

Epicormic Branches 

Epicormic branches are less likely to develop on dominant 

and codominant trees than on intermediate or suppressed 

trees. Our study results (Table 3) show fewer epicormic 

branches on the heavy to moderate thinnings because they 

included mostlv dominant and codominant trees known to 

sprout less. In comparison, the light thinning treatment had 

a substantial mix of all crowns that included intermediate 

and suppressed trees that are known to be more prolific 

sprouters. From 1977 to 1982, epicormic branching in the 

second &foot section increased by 33 percent in the heavy 

thinning, 38 percent in the moderate thinhing, and 50 percent in the light thinning. 

The mixed-crown classes, along with the lvarying amount of 

light from the thinnings, were responsibld for a major portion of the epicormic branching. Becauseof these conditions, some new epicormic sprouts resulted and some of the 

existing live limbs continued to grow faster and better. 

Summary and Conclusion 

Limb-related defects are the greatest degraders to potential 

stem quality. The results show that the moderate and heavy 

thinnings, as opposed to the light thinnings, adversely 

affected the number and size of the stem defects. Moderate 

to heavy thinning in mixed-oak stands redhced stem quality 

by increasing the defects per square foot of surface area. In 

comparison, light thinning maintained stem quality by 

reducing the number of limb-related defecfs. Dale and 

Sonderman (1984) reported similar findings on 33-year-old 

white oak trees that showed poorer stem duality after heavy 

to moderate thinnings. 

reflect a biological sequence of events that include epicormic branches changing to live limbs, live limbs changing to 

dead limbs, and dead limbs changing to overgrowths. All of 

this happens simultaneously but at different rates among 

the various thinning levels. Age and site are a few of the 

stand variables that affected the changes in this biological 

sequence. The full effects of thinning, whether heavy, moderate, or light, were combined and grouped by plots and 

treatments for this study. This, plus the fact that a subsequent thinning scheme was used, requires caution when 

applying this information to trees and stands in other geographic areas. 

We believe that land managers are interested in knowing 

and recognizing the quality of their hardwood timber and 

that increasing profits from their investment by proper thinning is a goal. At best, full utilization of the land while maintaining good growth and quality are requirements. Other 

important findings were: 

D.b.h. increased as the thinning intensity increased. 

The proportion of dominant trees increased in the heavy 

thinning. 

The proportion of dominant and codominant trees 

increased in the moderate thinning. 

The proportion of codominant trees increased in the light 

thinning. 

The relative quality index was highest in the heavy 

thinning. 

Live and dead limbs were substantially larger and more 

numerous on the second 8-foot section of the butt 

16-foot log of the tree. 

There were more epicormic branches on the second 

8-foot section of the butt 16-foot log than on the first 

8-foot section. 

There were fewer and smaller live and dead limbs in the 

light thinnings than in the heavy thinnings. 

This study was based on trees that had be$n initially 

thinned and then subsequently thinned using stocking 

goals to reach specific stocking percentagbs. The fact that 

the plots were maintained over time at spehific stocking 

levels and grouped accordingly provided aIgood base for 

comparison of limb-related defects. The tr4nds in this study 
Literature Cited 

Brinkman, K. A. 1955. Epicormic branching on oaks in 

sprout stands. Tech. Pap. 146. Washington, DC: U.S. 

Department of Agriculture. 8 p. 

Dale, Martin E.; Sonderman, David L. 1984. Effect of thinning on growth and potential quality of young white oak 

crop trees. Res. Pap. NE-539. Broomall, PA: U.S. 

Department of Agriculture, Forest Service, Northeastern 

Forest Experiment Station. 12 p. 

Della-Bianca, Lino. 1983. Effect of intensive cleaning on 

natural pruning of cove hardwoods in the Southern 

Appalachians. Forest Science. 29: 27-32. 

Rast, E. D.; Sonderman, D. L.; Gammon, G. L. 1973. A guide 

to hardwood log grading. Gen. Tech. Rep. NE-1. Upper 

Darby, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 32 p. 

Smith, H. Clay. 1966. Epicormic branching on eight species 

of Appalachian hardwoods. Res. Note NE-53. Upper 

Darby, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 4 p. 

Smderman, David L. 1979. Guide to the measurement of 

tree characteristics important to the quality classification 

system for young hardwood trees. Gen. Tech. Rep. NE54. Broomail, PA: US. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 12 p. 

Sonderman, David L. 1984. The quality response of evenaged 80-year-old white oak trees after thinning. Res. Pap. 

NE-543. Broomall, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment Station. 

6 P. 

Sonderman, David L.; Rast, Everette D. 1987. Changes in 

hardwood growing-stock tree grades. Res. Pap. NE-608. 

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

Service, Northeastern Forest Experiment Station. 8 p. 

U.S. Department of Agriculture, Forest Service, 1982. An 

analysis of the timber situation in the United States 1951- 

2030. For. Resour. Rep. No. 23. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 499 p. 
*U. 5. GOVERNMENT PRINTING OFFICE: 1988/548-066/60022 
Sonderman, Davi? L.; Rast, Everette D. 1988. Effect of thinning on mixedoak stem quality. Res. Pap. NE-618. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest Experiment Station. 6 p. 

Changes in limb-;elated stem defect were studied from 1977 to 1982 on 595 

mixed-oak trees fbllowing a schedule of thinnings dating back to 1962. All of 

the thinnings were controlled by stocking goals rather than predetermined 

time intervals. Results show that heavy and moderate thinnings, as opposed 

to light thinnings,lhad an adverse effect on the number and size of live and 

dead limbs. The light thinnings resulted in fewer live and dead limb defects 

per square foot ofisurface area but much slower diameter growth. 

ODC 228.1 :242:85/1:852.11 

Keywords: Stand density; thinning; stem quality 
Headquarters of the Northeastern Forest Experiment Station are in Broomall, Pa. 

Field laboratories are maintained at: 

a Amherst, Massachusetts, in cooperation with the University of Massachusetts. 

a Berea, Kentucky, in cooperation with Berea College. 

a Burlington, Vermont, in cooperation with the University of Vermont. 

a Delaware. Ohio. 

a Durham, New Hampshire, in cooperation with the University of New Hampshire. 

a Hamden, Connecticut, in cooperation with Yale University. 

a Morgantown, West Virginia, in cooperation with West Virginia University, 

Morgantown. 

a Orono, Maine, in cooperation with the University of Maine, Orono. 

a Parsons, West Virginia. 

a Princeton, West Virginia. 

a Syracuse, New York, in cooperation with the State University of New York 

College of Environmental Sciences and Forestry at Syracuse University, 

Syracuse. 

a University Park, Pennsylvania, i? cooperation with the Pennsylvania State 

University. 

a Warren, Pennsylvania. 

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