FOREST RESEARCH NOTES 

NORTHEASTERN FOREST EXPERIMENT STATION 

Upper Darby, Pennsylvania 

No, 58 

1956 

a 

0 

1 

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Thinning Young Oak Stands On Poor Sites 

In Southern New Jersey Does Not Px 

Between 1933 and 1940 the Civilian Conservation Corps 

undertook many cultural operations in the state forests of 

southern Mew Jersey; one of the most common was thinnings in 

predominantly oak stands on upland sites. The thinnings 

varied from very light--removing only trees that would soon 

die--to very heavy. They were usually justified on the prevalent notion that growth of the residual stems would be 

stimulated. 

This same justification is still being used to some 

extent today for thinning oak stands on poor sites in the 

Northeast. But is the growth of residual stems markedly 

stimulated? Let's look at the results from a thinning study 

started in the Lebanon Experimental Forest near New Lisbon, 

M. J., in 1935. 

Experimental Methods 

The site selected for the study was typical of many 

upland soils in the Pine Region of southern New Jersey. It 

had a well-drained Evesboro soil. Moore states that in this 

section the oaks are generally 40 to 50 feet tall when 40 to 

50 years old,l thus indicating a site index of about 50 feet. 

The stand selected was also typical in many ways. It 

had about 1,950 stems per acre larger than 0.5 inch in diameter (breast high), all but 28 of which were oaks. The 

hardwoods vere chiefly black, white, and chestnut oaks, with 

a few scarlet and post oaks. All were sprouts, usually from 

large stools; and most of them had started after a wildfire 

-- 

?Moore, E. B. Forest management in New Jersey. N. J. 

Dept. Conserv. and Devlpmnt. 55 pp., illus. 1939. 
13 years before. Thus the majority of oaks were less than 

3.5 inches d.b.h. and less than 25 feet tall when the study 

started. The largest oak in the study area was 6 inches in 

diameter, 32 feet tall, and had lived through the last mildfire. A few shortleaf and pitch pines of various sizes were 

scattered through the stand. 

A &-acre block of plots was established for this 

study--sixteen 1/16-acre plots, each of which mas surrounded 

by a 3/16-acre isolation strip that received the same treatment. These plots were arranged in a Latin Square. 

Three thi~ing treatments and an untreated control 

were used, each in 4 plots. The light, medium, and heavy 

thinnings removed 13, 29, and 46 percent respectively of the 

original basal area; or 33, 52, and 72 percent of the original number of stems. 

D.b.h. of all stems, both those cut and those left, 

and total height of each stem left were measured in 1935. 

Individual trees were tagged, and individual-tree records 

werekept. In1955thediameters ofalltrees wereagain 

measured to the nearest 1/10 inch. And total heights were 

measured on a- sample of 5 oaks, if available, in each of the 

following 1935 diameter classes in 'each plot: 1.1-1.5 inches, 

1.6-2.0 inches, 2.1-2.5 inches, and 2.6 inches or more. 

Results 

Thinning greatly reduced the mortality of oaks during 

the 20-year period. In the unthinned controls 54 percent of 

the oaks died during that interval. Where light thinning 

was used, 35 percent died; medium thinning 19 percent; and 

heavy thinning 4 percent. 

This mortality and the initial effect of the thi~ing 

on the number and size of oaks, of course, make the results 

confusing. For example, if we consider the 20-year change 

in basal area of all oaks in the stand, theref s an appreciable difference in results between figuring this on the 

basis of trees living in 1935 and using only those living in 

1955 (table 1). Thinning increased net growth in basal area, 

but the basal-area growth per acre of all oaks living in 

1955 was greatest in the unthinned and lightly thinnedplots. 

Since thinning greatly reduced the initial basal area, as 

well as losses through mortality, it greatly increased 

growth percent: the difference between heavy and no thinning 

is 5.9 percent for net annual growth in basal area, 2.3 percent when based on trees living in 1955. 
Thinning from below of course changed the average 

diameter of the oaks in these plots in 1935--from 1.7 inches 

in the controls to 2.3 inches in the heavily thinned areas. 

But by 1955 natural dying had also eliminated most of the 

smaller stems in the lightlythinned or unthinned areas. 

For example, in the unthinned controls 85 percent of the 

oaks that were 1 inch in diameter in 1935 had died. Thus 

the average diameters in 1935 of the oaks living in 1955 

differed only slightly among the treatments (table 1). 

Table 1.--20-year effects of thinning on the chaxes in 

basal area and diameter of oaks 

'~irst-column values are less than those in the second because they are net values, 

having been reduced by the basal area in trees that died since 1935. 

Average diameter (b.h.) 

1935 I 

I 

20-year increase in 

All oaks 

Some of the data in table 1 might be interpreted as 

showing that thinning did affect the diameter growth of all 

oaks living in 1955. Those in the heavily thinned plots 

grew 1.9 inches in the 20 years, compared to 1.6 or 1.4 

inches in the other treatments, And the difference between 

heavy thinning and the other treatments was statistically 

significant. 

Treatment 

But remember that in 1955 the thinnings still have a 

great effect on the number of oaks per acre. In the controls there are 916 oak stems per acre, in the lightly thinned plots 868, medium thinning 688, and heavy thinning 512. 

Perhaps the slow growth of the smaller stems dragged down 

the values for all living oaks in the more lightly thinned 

or unthinned areas. 

basal area per acre1 

Sq.ft. Sq.ft. Inches Inches Inches 

Heavy thinning 34.8 35.1 2.3 2 3 4.2 

Medium thinning 34.9 36.9 2.1 2.2 3.8 

Light thinning 33.9 39.6 2.0 2.2 3.6 

No thinning 31.2 39.6 1.7 2.1 3.5 

Oaks living 

in 1955 

Consequently the growth in basal area of the 20 fastest growing oaks per plot was summarized and analyzed. Expressed in terms of d.b.h., this amounted to 2.2 inches per 

tree in the heavily thinned plots, 2.1 under medium thinning 

and in the controls, and 2.0 in the lightly thinned plots. 

The analysis of the basal-area data from which those diameAll oaks Oaks living 

in 1955 

1955 
ter values were obtained indicated that the treatments produced no statistically significant differences. 

Height growth was also unaffected by the thinning 

treatments, although it was affected by size class. Of the 

stems measured, those 1.1-1.5 inches in diameter in 1935 

grew 6 feet in height (average) in the 20-year period; those 

1.6-2.0 inches, 9 feet; those 2.1-2.5 inches, 13 feet; and 

those larger, 14 feet. Only among stems larger than 2.0 

inches were there no significant differences between size 

classes. 

By 1955 the largest oaks in all plots were between 5 

and 8 inches in diameter, and between 35 and 52 feet tall. 

Evidently the noncommercial thinnings of small oaks 

'tested in this study did not appreciably stimulate the 

growth of the residual stand, and thus produced no practical 

benefits. Similar results should probably be expected elsewhere from thinning similar sprout stands of oaks growing on 

relatively poor sites. 

--S. LITTLE and H. A. SOMES 2 

Northeastern Forest Experiment Station 

Forest Service, U.S. Dept. Agriculture 

;?Stationed at the Lebanon Experimental Forest, New 

Lisbon, N.J., which is maintained by the Northeastern Forest 

Experiment Station in cooperation with the New Jersey Department of Conservation and Econonic Development, Bureau of 

Forestry, Parks, and Historic Sites.