SO-252

Five Years’ Growth of Pruned and Unpruned Cottonwood

Planted at 40- by 40-Foot Spacing

R. M. KRINARD

Four pruning treatments have been applied for

5 years on cottonwood (Populus deltoides Bartr.)

select clone Stoneville 66, planted at 40- by 40-ft

spacing. As pruning severity increased, average

diameter and maximum crown width decreased.

Diameters ranged from 9.2 inches for trees

pruned half of height yearly to 11.4 inches for

unpruned trees; crown widths ranged from 16.5

to 24.6 ft.

Addltional keywords: Populus deltoides.

trees. And resulting diameter growth would serve

as an upper limit for comparison to diameter

growth at other spacings and would also tell us

how fast sawtimber and veneer logs can develop.

Widely planted cottonwood trees would need to

be pruned for quality growth. A comparison of

several pruning intensities would tell us how

crown length influences diameter growth. This

paper gives data on 5-year growth of .widely

spaced cottonwoods whose crown length has

been controlled by pruning (fig. 1).

INTRODUCTION

Cottonwood (Populus deltoides Bartr.) grows

fast, but its potential maximum diameter increment has not been determined. If wide enough

spacing were used at planting on site index

120 + , crown development would not be restricted by competition from other cottonwood

The g-acre study area (on Chicago Mill and

Lumber Company land at Huntington Point, Bolivar County, Mississippi) was cleared in 1971, fallowed in 1972, and planted in 1973. Because of

a 1973 flood, the study was replanted in January

1974. A matrix of 16 rows by 16 columns contained 256 planting spots. Spacing was 40 by 40

ft on Commerce silt loam soil. Three 18-inch cuttings of select Stoneville clone 66 (Land 1974)

Southern Forest Experiment Station/T-10210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La 70113

Forest Service, U.S. Department of Agriculture.

Servmg Alabama. Arkansas, LouIslana, Mississippi, Eastern Oklahoma, Tennessee, Eastern Texas
Figure 1 .-General view of cottonwood trees, planted at 40-by-40 ft spacing, leafing out at start

of sixth growing season. Tree nearest observer has been pruned to half its height,

the next four frees in the row are pruned to a third of their height, and the next

group of trees are unpruned.

were planted at each spot and thinned to one

tree per spot the first week of June.

A randomized complete block design was used,

with four replications. Each plot per block was

4 rows by 4 columns. Data were analyzed at the

0.05 level of significance (Duncan’s Multiple

Range Test).

Disking controlled weeds during the five growng seasons.

Pruning treatments were: 1) no pruning (control); 2) pruning the bottom third of the tree yearly

(third-height pruning); 3) pruning the bottom half

of the tree yearly (half-height pruning); and 4)

pruning the bottom 17 ft of the tree (17-ft pruning)

when d.b.h. became ~8.5 inches, as happened

in the 4th year. Trees were pruned in the dormant

season: January 1975, February 1976, March

1977, and March 1978. Trees were examined

monthly during the growing seasons and any

sprouts were removed. But the first examination

was delayed until June 1975 because high water

made trees inaccessible.

Height and d.b.h. of each tree were measured

yearly. Cut limbs of the inside four trees in each

pruning treatment plot were counted after the 1 st

year, and diameters of newly cut limbs were measured at the end of each year thereafter. Maximum

crown width of all trees in both a north-south and

east-west direction was measured after the 5th

year. And, at that time, diameter outside bark at

17 ft of each plot’s four inside trees was also

measured.

Two replicates of clone 66 had also been

planted in January 1974 at 10 by 10 ft in an

adjoining study. At the end of the 5th year, the

inside 8 trees per 1 O-tree replicate were measured

for d.b.h., height, crown width, diameter outside

R. M. Krinard is Mensurationist at the Southern Hardwoods Laboratory, maintained at Stoneville, Mississippi, by the Southern

Forest Experiment Station, Forest Service-USDA, in cooperation with the Mississippi Agricultural and Forestry Experiment

Station and the Southern Hardwood Forest Research Group.

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able 1 .-Average height and height growth and d.b.h. and d.6.h. growth by year and pruning

treatment

Treatment

Check

One-Third

One-half

17-A

Height Height growth

Year Year

1 2 3 4 5 2 3 4 5

.._....._................. feet .._.____........ .._____...._... feet .

10.5 23.0 36.9 49.9 62.5 13.2 13.2 12.8 12.6

10.8 23.6 36.0 49.9 62.3 12.9 12.4 13.9 12.3

10.6 23.0 34.8 48.1 59.6 12.4 11.8 13.3 11.6

10.9 24.3 36.5 .50.0 62.3 13.4 12.2 13.3 12.4

Average 10.7 23.7 36.0 49.5 61.7 13.0 12.4 13.3 12.2

D.b.h. D.b.h. growth

Treatment Year Year

1 2 3 4 5 2 3 4 5

_.............____________ inches .._____................. inches

Check 1.1 4.0 7.2~~ 9.6c 11.4d 2.9b 3.2~ 2.4b 1.8d

One-Third 1 .l 4.1 6.9b 8.9b 10.3b 3.0b 2.8b 2.0a 1.5c

One-half 1 .l 3.8 6.3a 8.la 9.2a 2.7a 2.4a 1.8a l.la

17-ft 1 .l 4.1 7.3c 9.6c 10.9c 3.0b 3.2~ 2.4b 1.3b

Average 1 .l 4.0 6.9 9.0 10.4 2.9 2.9 2.2 1.4

’ Treatment applied after 4th year.

’ Means appearing with same letter not significantly different within each column at 0.05 level

by Duncan’s Multiple Range Test.

bark at 17 ft, and length of live crown (Kennedy

1979). Because lo-by-1 0 spacing is common,

these data were compared to 40-by-40 spacing

results.

All measured trees were incuded in analyses

except four trees replanted the 1 st year and trees

with broken tops. Where only inside trees were

measured and one of these was damaged, an

adjacent outside-row tree was measured instead.

RESULTS

Through 5 years no difference between treatments in either total height or height growth occurred in any year (table 1). To the nearest foot,

and averaged over all treatments, height mean

annual increment (m.a.i.) was 12 ft for the 2d

through 5th years. Current annual increment

(c.a.i.) was 13 ft in the 2d and 4th years and 12

ft in the 3d and 5th years. Mean height after 5

years was 62 ft.

d.b.h. first differed after the 3d year (when unpruned trees were larger than pruned trees). Differences as small as 0.3 inch were significant

(table 1). Trees pruned to 17-ft after the 4th year

had less d.b.h. growth in the 5th year and smaller

total d.b.h. after 5 years than unpruned trees had.

Diameters at breast height of all treatments differed after 5 years, when average d.b.h.‘s were

9.2 inches (half-height pruning), 10.3 inches

(one-third pruning), 10.9 inches (17-ft pruning),

and 11.4 inches (unpruned). Diameter at breast

height m.a.i. peaked in the 3d year for third- and

half-height pruned trees and in both 3d and 4th

years for unpruned trees. Current annual increment peaked in the 2d year for pruned trees and

3d year for unpruned trees.

As a measure of form, ratio of diameter at 17

ft to d.b.h. was determined. The ratio was higher

for third- and half-height pruning (0.73 and 0.74)

than for 17-ft pruning and check (both 0.70).

Taper decreased as pruning intensity increased

(fig. 2).

Differences first occurred in d.b.h. growth dur- Maximum crown width for unpruned trees (24.6

ing the 2d year (when half-height pruned trees ft) was larger than for pruned trees; crown widths

grew less than did trees in other treatments). Total for 17-ft (19.2 ft) and third-height (19.9 ft) pruned

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Table 2.-Average values of pruned limbs by year and pruning treatment

Pruned

Pruned l/3 ht Pruned l/2 ht 17 tt

Year Year Year

1 2 3 4 1 2 3 4 4

No. limbs cut/tree 13.0 13.7 14.5 13.5 17.8 18.4 14.1 19.5 46.2

Limb diameter (in) * 1.2 1.5 1.5 * 1 .o 1.2 1.1 1.8

Limb area (in’)/tree * 16.6 28.2 28.7 * 15.1 18.1 22.7 129.7

No. limbs cut/ft pruned 3.8 3.3 3.5 2.8 3.3 3.0 2.4 2.7 2.7

Bole length pruned (tt) 3.5 4.2 4.2 4.8 5.4 6.1 5.8 7.2 17.0

* Not measured.

trees were larger than for half-height pruned trees

(16.5 ft). For all treatments, crown width in a

north-south direction was larger than in an eastwest direction, where, mean differences between

directions were 4.9 ft for unpruned, 3.6 ft for 17-ft,

3.8 ft for third-height, and 2.9 ft for half-height

treatments.

Pruning third and half heights is more severe

than it might seem. Third-height pruning cut 56

percent of the branches, and half-height pruning

cut 79 percent at the end of the 1st year.

Throughout the study, third-height prunmg removed fewer, larger limbs and so produced

greater wound area per tree than did half-height

pruning. The 17-ft pruning removed largest

average limb size and produced largest aggregate

wound area (table 2).

Figure P.-Average diameteratgiven stem heights forpruning

treatments and for lo- by lo-ft spacing at age 5.

Top breakage started in April of the 3d year.

Proportion of trees with top breakage through 5

years was 0 for unpruned, 5 percent for thirdheight and 17-ft, and 23 percent for half-height

pruning treatments. Storms caused breakage

during the growing season. Stems snapped off

from just below pruning height to 20 ft above.

When one limb of a fork was pruned, the remaining limb was more likely to break. No signs were

found of canker or borer damage on the broken

tops.

ln the lo- by 1 0-ft spacing, cottonwood

averaged 1.2 inches d.b.h. and 11.5 ft tall after

1 year and 3.6 inches d.b.h. and 29.3 ft tall after

2 years. At age 5, trees averaged 6.5 inches d.b.h.

(from 5.3 to 7.1 inches), 60.6 ft in height (from

56 to 65 ft), and 8.7 ft in maximum crown width

(from 5 to 11 ft in either direction, 6.0 to 10.0

ft average of two directions). Average ratio of

diameter at 17 ft to d.b.h. was 0.76 (from 0.72

to 0.79). All limbs to 17 ft were dead, and no

natural pruning had occurred. Average ratio of

live crown to total height was 0.32 (from 0.18 to

0.47).

DISCUSSION

As in earlier studies (Krinard and Johnson

1975) planted cottonwood has shown fast early

diameter growth followed by a rapid drop in

growth. For unpruned trees, diameter growth the

4th year was 75 percent of 3d-year growth; 5th

year growth was 75 percent of 4th-year growth

and 56 percent of 3rd-year growth.

Where there was no pruning, trees at 40- by

40-ft spacing averaged 0.1 inch smaller d.b.h. and

0.8 ft shorter after 1 year, and 0.4 inch larger

and 5.5 ft shorter after 2 years than trees at lo4
by lo-ft spacing averaged. After 5 years, the 1 Oby lo-spaced trees were 4.9 inches smaller in

d.b.h. and 1.9 ft shorter in height than were trees

in the 40- by 40-spacing. Maximum tree measurements were 12.6 inches in diameter and 67 ft in

height for the 40- by-40, 6.5 inches in diameter

and 65 ft in height for the lo-by-lo.

To determine cubic-foot volume, I used mean

values of trees whose upper stem diameters were

measured, and d.b.h.* times height times form as

a cubic-foot volume indicator. Third-height

pruned trees had 84 percent as much cubic-foot

volume as unpruned trees had. Half-height

pruned trees had 67 percent as much, 17-ft

pruned trees had 88 percent as much, and trees

at lo-by lo-ft spacing had 33 percent as much

cubic-foot volume as unpruned trees had.

At the end of the 5th year, third- and half-height

pruned trees had 73 percent and 57 percent as

much crown as unpruned trees had, if crown

length is considered crown size. If crown shape

is considered conical, with the base an ellipse,

then the lateral crown surface areas of third- and

half-height pruned trees are only 59 percent and

39 percent as large as unpruned trees. Conical

crown volumes are only 48 percent and 26 percent as large.

Crown length required for a given diameter

growth cannot be determined from this study. For

the 3d through 5th years, however, diameters of

third-height pruned trees were 96, 93, and 90

percent as large as diameters of unpruned trees,

while diameters of half-height pruned trees were

88, 84, and 81 percent as large.

Wide-spaced trees need pruning for quality

growth. Pruning reduces diameter growth but not

height growth. Because nearly a fourth of the

half-height pruned trees suffered broken tops,

half-height pruning is apparently too severe.

Third-height pruning is more successful and results in only 1 inch (10 percent) less diameter

in 5 years than half-height pruning produces. The

17-ft pruning produced results mathematically

equal to those of third-height pruning. But boles

and limbs pruned in the 17-ft treatment were

larger than those pruned in the third-height treatment. So, 17-ft pruning produced a larger, lower

quality core than did third-height pruning. Pruning

was not effective in keeping branches off tree

boles.

Although only one clone-type was used, and

other types may have different crown shapes,

unpruned trees of clone 66 maintained live limbs

to the ground through 5 years. The widest part

of an unpruned tree crown was generally near

the bottom of the tree, and the crown tapered

to the top. Maximum crown width measured in

one direction was 30 ft for check and 29 ft for

third-height pruned trees. Mean north-south

crown widths were 27 ft. for check and 22 ft for

third-height pruned trees. Cottonwood crowns do

not mingle; so, as this study shows, a minimum

spacing of 26-30 ft square is needed for maximum

diameter growth through 5 years for trees receiving third-height pruning.

Cottonwood planted at 40 by 40 ft and pruned

to a third of its height yearly can average 2 inches

annual diameter growth through 5 years. Maintaining two-thirds of a crown has produced less

diameter and stem volume than an unpruned tree

would have but has improved tree form and quality. Growth over the next 5-l 0 years will determine

the feasibility of producing sawlogs on a lo- to

15-year rotation. With wide spacing, intercropping

seems a reasonable method of acquiring an early

return to help offset clearing, planting, cultivating,

and pruning costs.

LITERATURE CITED

Kennedy, Harvey E., Jr.

1979. Planting season for cottonwood can be

extended. South. J. Appl. For. 3(2):54-55.

Krinard, R. M., and R. L. Johnson.

1975. Ten-year results in a cottonwood plantation spacing study. U. S. Dep. Agric. For.

Serv. Res. Pap. SO-106,lO p. South. For. Exp.

Stn., New Orleans, La.

Land, Samuel B., Jr.

1974. Forest tree improvement: Mississippi certifies Nation’s first “blue tag.” J. For.

72(6):353.

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