No. 95 

1960 

a THINNING SWEETGUM STANDS 

IN SOUTHERN NEW JERSEY 

In the past 40 years, many poorly drained fields and 

pastures in southern New Jersey have been abandoned; and on 

some of them even-aged stands of sweetgum have developed. 

These stands seem to lend themselves well to management because (1) they are accessible, (2) sweetgum is an economically desirable species, and (3) the individual stems are 

well formed, with good lengths clear of branches. 

But just how fast do- these sweetgum stands grow? Are 

there any indications that thinnings would stimulate their 

growth or improve their quality? 

Partial answers to these question? are now available 

from three plots of about 0.1 acre each that the New Jersey 

Department of Conservation and Economic Development estab0 lished in one stand in 1928. The stand then was about 11 

years old. At that time one of the plots was given a low 

thinning; the other two plots, which have had no treatment, 

provide growth data on the unthinned stand. The three plots 

were remeasured in 1958 when the stand was 41 years old. 

The Site 

The soil is very poorly drained, even though the site 

had been ditched earlier and the drainage somewhat improved 

when the land was under cultivation. From measured tree 

heights, Trenk's tables (3) indicate a site index at 50 

years of about 85 feet. 

Results 

The unthinned stand had made good growth: despite 

losses from natural mortality, growth had averaged more than 

a cord per acre annually, and the stand at 41 years of age 
Table1.--Development of a pure even-aged sweetgum stand without thinning 

* 

Of stems larger than 1.6 inches. ** To a top diameter (0.b.) of 4.0 inches. 

contained 4,896 cubic feet of merchantable volume per acre 

(table 1). 

Stand 

age 

(years) 

However, the growth rate had declined markedly with 

increasing age, particularly during the last 15 years. 

Growth between 21 and 26 years was 1,274 cubic feet, but in 

the last 5 years it was only 264 cubic feet (table 1). 

- No. No. - - In. - In. -- Square Square -- Cubic Cubic 

- feet feet - - feet feet - 11 1764 100 3.0 4.7 85 12 124 62 

2 1 1121 100 5.1 8.0 160 35 1915 643 

2 6 994 100 6.0 9.3 195 47 318.9 1088 

31 734 100 6.9 9.9 187 54 3836 1474 

36 610 100 7.5 10.3 188 58 4632 1793 

41 502 100 8.2 10.9 181 65 4896 2137 

Merchantable 

volume 

per acre** 

Growth of crop trees--i.e., the 100 largest trees per 

acre--had declined less spectacularly. However, their fastest growth also was made prior to 26 years, when they grew 

1.3 to 1.6 inches in diameter, or about 12 square feet in 

basal area per 5-year period. In each subsequent 5-year 

period, their growth was only about 0.5 inch in diameter and 

6 square feet in basal area. 

Total 

stand 

Growth on the plot thinned when the stand was 11 

years old has not differed appreciably from that on the untreated plots. Since the thinning removed 49 percent of the 

stems, mortality was reduced and stand growth was increased 

for a few years (1). However, over the 30-year period the 

low thinning has had a negligible effect on crop-tree growth. 

Crop 

trees 

St ems 

per acre 

Discussion 

Total 

stand 

In the 41-year old sweetgum stand studied, growth 

without benefit of any management has been relatively fast, 

producing a total merchantable volume of 4,896 cubic feet 

per acre. In contrast, 40-year oak stands on 80-foot sites 

contain only 2,610 cubic feet (2). - 

Crop 

trees 

Average 

diameter* 

Basal area 

per acre 

Total 

stand 

Total 

stand 

Crop 

trees 

Crop 

trees 
With their high growth potential, sweetgum stands ,/' 

such as these theoretically should respond well to thinning. ' 

However, the thinning made at 11 years on one plot of this //' 

stand evidently was done too early to exert much lasting 

effect. When, then, should the first thinning be made? 

White (4) recommends that thinning should be started 

in Louisiana sweetgum stands at 15 to 20 years of age. In 

the present case, data from the unthinned plots (table 1) 

show that the diameter growth rate of the crop trees began 

to fall off most markedly after about age 25. This would 

imply that thinning in this stand could have been delayed, 

without appreciable sacrifice of growth, until age 20 to 25, 

or about 5 years later than White recommended for Louisiana. 

Conclusions regarding New Jersey stands in general 

cannot be drawn from observations on one stand, of course. 

However, since local growth rates probably are somewhat lower than those in Louisiana, New Jersey stands could be expected to go a few years longer to the first thinning; and 

the inference from our growth data that the first thinning 

be done at 20 to 25 years is reasonable in the light of 

White's recommendation. 

The above remarks are oriented to silvicultural considerations; whether or not thinnings at 25 years would pay 

their way will not be dealt with here, except to point out 

that the answer would vary with local market conditions for 

such products as pulpwood and treated fence posts. Where 

the thinnings cannot be sold or used, non-commercial thinning by use of herbicides might merit consideration. 

Literature Cited 

1. Moore E. B., and Waldron, A. F. 

1940. The growth of thinned and unthinned sweet gum in 

New Jersey. N. J. Dept. Conserv. & Devlpmnt ., Div. Forests and Parks Tech. Note 9. 4 pp. . 

2. Schnur, G. L. 

1937. Yield, stand, and volume tables for even-aged 

upland oak forests. U.S. Dept. Agr. Tech. Bul. 

560. 88 pp. 

3. Trenk, F. B. 

1929. Sweetgum in Maryland. State Dept. Forestry, 

Univ. Md. 75 pp ., illus. 

4. White, L. E. 

1958. Methods of thinning of sweet gum stands. 

La. State Univ. 7th Ann. Forestry Symposium 

Proc. : 42-45. 

--H. B. TEPPER 

Northeastern Forest Experiment Station 

Forest Service, U .S . Dept . of Agriculture 

and G. T. BAMFORD 

Forest Management Section, N. J. 

Department of Conservation and Economic Development