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U4S Forest Service ,111 Southern Forest

Research Note

Experiment Station

SO-31 6

September 1965

Ten Years’ Growth of Pruned and Unpruned Cottonwood

Planted at 40- by 40-Foot Spacing

Roger M. Krinard

SUMMARY

Diameters at age 10 of cottonwood (Pop&s delfoides

Bartr. ex Marsh.) select clone Stoneville 66 planted at

40- by 40-foot spacing varied from 13.4 to 16.4 inches

depending on height of pruning. Pruning treatments, applied from the 2nd through 8th years, were (1) unpruned,

(2) pruned to a third of height yearly, (3) pruned to half

of height yearly, and (4) pruned to 17 feet when d.b.h.

>8.5 inches. Total tree heights did not differ among treatments and averaged 94 feet.

Additional keywords: Popuhs deltoides, pruning.

INTRODUCTION

Early diameter growth of cottonwood (Popuhs de/-

toides Bartr. ex Marsh.) is spacing-dependent. Thus,

spacing is an important consideration in plantation management in determining tree sizes and thereby rotation

length, whether cubic volume or board-foot volume production is desired. Previous research has shown that

maximum diameter growth of trees planted at four spacings, ranging from 4 by 9 to 16 by 18 feet, occurred in

the first 4 years (Krinard and Johnson 1975). Even at a

nearly open-grown spacing of 40 by 40 feet the mean

annual increment (mai) for diameter peaked by the 4th

year (Krinard 1979). Trees were planted at the 40- by

40-foot spacing for the purpose of using their growth

characteristics as an indicator of ultimate cottonwood

growth under conditions of minimal competition. Several

pruning intensities were applied to follow the influence

of crown length on growth and for quality growth. This

paper provides stand information over 10 years for cottonwood planted at 40- by 40-foot spacing (fig. 1) and

subjected to four pruning treatments, together with a

comparison of unpruned trees planted at lo- by 1 O-foot

spacing.

METHODS

The study was planted in January 1974 on cleared

Commerce silt loam soil at Huntington Point in Bolivar

County, Mississippi, on Chicago Mill and Lumber Company land. Commerce, a member of the thermic family

of Aeric Fluvaquents, has an estimated site index range

of 105 to 125 feet at age 30 for natural cottonwood stands

(Broadfoot 1976). The 40- by 40-foot spacing consisted

of 16 rows by 16 columns. Each of the 256 planting spots

was planted with three 18-inch cuttings of select Stoneville clone 66 (Land 1974) and thinned to one tree per

spot in June 1974.

A randomized complete block design was used with

four replications. Each pruning treatment 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 was used to control weeds during the first five

growing seasons. In July of the 6th year, the area was

rototilled to help eliminate ruts; thereafter, yearly weed

control was by mowing.

Pruning treatments to control crown length were applied from the 2nd through 8th years. Treatments were:

control or no pruning (C); pruning the bottom third of the

tree annually (l/3); pruning the bottom half of the tree

annually (l/2); and pruning the bottom 17 feet of the tree

(17) when d.b.h. exceeded 8.5 inches, which occurred

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

season (January to March) for the 2nd through 5th years,

delayed until the first week of July because of high water

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.

Serving Alabama, Arkansas, Louisiana, Mississippi, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
Figure 1. - Cottonwood planted at 40- by 40-foot spacing in July of the 11 th year (the tractor is mowing

a dense johnson grass ground cover that still develops because of open stand conditions).

in the 6th year, and during the first week of May in the

7th and 8th years. From the 2nd through 5th growing

seasons, trees were examined monthly and any sprouts

were removed. After the 5th year, epicormic branches

were removed only at time of pruning.

Height and d.b.h. of each tree were measured annually. Measurements taken after the 5th and 10th years

were maximum crown width in the north-south and eastwest directions for all trees and diameters outside bark

at 9, 13, and 17 feet on four trees per plot. Cut limbs of

the inside four trees in each pruning treatment plot were

counted through age 4.

Board-foot estimations for the 40 by 40 spacing were

obtained from Barr and Stroud dendrometer measurements taken on two trees per plot in the 9th growing

season. Equations used for trees 212.5 inches d.b.h. to

a lo-inch top were:

BF (Int. l/4) = -440.5 + 31.60 + 0.98H, R* = 0.96,

s,, = 7.9

BF (Doyle) = -269.2 + 19.7D + 0.55H, R* = 0.94,

s,, = 5.9

where D = d.b.h. and H = total height.

All measured trees were included in analyses except

four trees replanted the first year and trees with broken

tops.

For comparison with the 40 by 40 spacing, two 1 O-tree

replications of clone 66 in an adjacent study that were

also planted in January 1974 but at lo- by 1 O-foot spacing were measured after the 5th and 10th years. The

interior eight trees per replication were measured for

d.b.h., height, diameter at 17 feet, length of live crown,

and maximum crown width (Kennedy 1979). Statistical

inferences are not implied in comparing the two spacings.

RESULTS

D.b.h. growth differences between pruning treatments

began in the 2nd year with the first pruning and continued

through the 6th year and during the 8th year (table 1).

There were no differences in the 7th year or in the 9th

and 10th years. D.b.h. growth generally declined from

the 2nd through 8th years and was nearly constant during the last 3 years. Diameters differed among treatments by the end of the 3rd year and the differences

continued through 10 years. D.b.h. growth of the 17-foot

treatment differed from the unpruned trees only in the

year of pruning and following year (5th and 6th years).

Yearly height growth never differed by treatments. The

trend was for (l/2) to reduce height, but only after the 7th

year were trees in that treatment shorter than other treatments (table 1).

The first pruning removed 56 percent of the branches

Roger 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.

2
Table 1 . - Average tree size and growth increment for height and d.b.h. by year and pruning treatment

Year

Treatment 1 2 3 4 5 6 7 6 9 10

Control

One-third

One-half

17 feet

Average

Control

One-third

One-half.

17 feet

Average

Control

One-third

One-half

17 feet

Average

Control

One-third

One-half

17 feet

Average

---- .-----. -------- __________-_____--______ height (feet) ____________.___________________________-------

10.5 23.6 36.9 49.9 62.5 71.6 60.9b’ 83.6 90.8 94.6

10.6 23.6 36.0 49.9 62.3 71.6 80.5b 84.4 92.1 95.7

10.6 23.0 34.6 48.1 59.6 67.2 76.8a 80.7 88.1 91.8

10.9 24.3 36.5 50.0 62.3 70.5 80.2b 82.2 91.1 95.6

10.7 23.7 36.0 49.5 61.7 70.3 79.6 82.8 90.5 94.5

---------------------- ___________----__._ height growth (feet) _________.__._________________________I_.

13.2 13.2 12.8 12.6 9.1 9.4 3.0 6.9 4.0

12.9 12.4 13.9 12.3 9.4 8.8 3.9 7.7 3.6

12.4 11.8 13.3 11.6 7.6 9.6 3.8 7.4 3.7

13.4 12.2 13.3 12.4 8.2 9.7 3.0 7.9 4.5

13.0 12.4 13.3 12.2 8.6 9.4 3.4 7.5 4.0

-- -------------------------------------.--- diameter (inches) ________._______________________________---

1.1 4.0 7.2~ 9&c 114d 129d 14.Od 14&t 155d 16.4d

1.1 4.1 6.9b 8.9b 10.3b 11.6b 12.5b 13.2b 13.9b 14.7b

1.1 3.8 6.3a 8.la 9.2a 10.5a 11.4a 12.0a 12.6a 13.4a

1.1 4.1 7.3c 9.6c 10.9c 12.2~ 13.2~ 14.Oc 14.7~ 15.5~

1.1 4.0 6.9 9.0 10.4 11.8 12.8 13.5 14.2 15.0

---- ___---_________---_______________ diameter growth (inches)---- ______________ ___ .________________

2.9b 3.2~ 2.4b l&l 1.5b 1.0 0.9b 0.7 0.8

3.0b 2.8b 2.0a 15c 1.2a 0.9 0.7a 0.7 0.8

2.7a 2.4a 1.8a l.la 1.2a 0.9 0.6a 0.6 0.8

3.0b 3.2~ 2.4b 1.3b 1.2a 1.0 0.8b 0.7 0.8

2.9 2.9 2.2 1.4 1.3 1.0 0.8 0.7 0.8

Weans appearing with same letter not significantly different within each column at 0.05 level by

Duncan’s Multiple Range Test; if no letters, overall test was not significant.

with (l/3) and 79 percent with (l/2). Average number of

branches cut per tree through age 4 were 55 for (l/3), 70

for (l/2), and 46 for (17). How limbs cut relate to photosynthetic area removed is unknown, as average limb size

was inversely related to number (Krinard 1979).

The ratio of diameter at 17 feet to d.b.h., a measure

of form, did not differ among pruning treatments at age

10 and ranged from 0.82 for (C) and (17) to 0.84 for (l/3)

and (l/2). After 5 years, the ratios of (C) and (17) were

0.70, which were significantly different from the ratios of

0.73 and 0.74 for (l/3) and (%), respectively.

Diameter growth at 17 feet (d) was significantly greater

than diameter growth at 4.5 feet (D) in all pruning treatments from years 6 through 10: (C)d = (17)d > (C)D =

(17)D = (%)d > (%)d = (l/3)0, and (%)D = (%)D. Average diameter growth for the last 5 years tended to be

numerically larger at 9 and 13 feet than at breast height

(fig. 2).

Average maximum crown width at age 10 ranged from

29 feet for unpruned trees to 23 feet for one-half pruned

trees. Crown width increase in 5 years varied from 4 feet

for (C) to 8 feet for (17). For all treatments, crown width

in a north-south direction was significantly larger than in

an east-west direction. Average differences between directions within pruning treatments varied from 3 to 5 feet

after 5 years and from 5 to 8 feet after 10 years (table

2).

After 5 and 10 years, live crown ratios by pruning treatments ranged from 60 to 97 percent and from 58 to 89

percent in the order (l/2), (l/3), (17), and (C). If crown

shape is considered as a parabola, lateral crown surface

area of pruned trees compared to (C) were 0.40 to 0.61

as large at age 5 and 0.50 to 0.90 as large at age 10;

crown volumes were 0.27 to 0.49 as large at age 5 and

0.40 to 0.86 as large at age 10 (table 3).

AGE AGE AGE 5 AGE IO

I

I I I 1 1 1 1

5 6 7 6 6 IO I I 12 I3 14 15 I 6 17

DIAMETER (INCHES)

Figure 2. -Average diameter at given stem heights for pruning

treatments at 40- by 40-foot spacing and for lo- by

lo-foot spacing at ages 5 and 10.
Table 2. - Maximum crown widths in a north/south (N/S) and east/

west (El W) direction at ages 5 and 10 by spacings and

pruning treatments

Spacing Treatment

Age 5

N/S E/W

Age 10

N/S E/W

ft ------------------_-______ __________________________ ft

4 0 by 4 0 Control 27.1 22.2 32.6 24.9

One-third 21.6 18.0 29.0 21.3

One-half 18.0 15.1 25.2 20.0

17 feet 21.0 17.1 30.7 24.4

1 0 by 1 0 Control 1 1 9.1 7.3

‘Directions not separated, overall average 8.8 feet.

Table 3. - Crown surface areas and crown volumes at ages 5 and

10 by spacings and pruning treatments

Spacing Treatment

Age 5

CSA’ CW

Age 10

CSA CV

it fP fP fP flcr

4 0 by 4 0 Control 3,151 14,322 5,076 26,964

One-third 1,924 7,086 3,625 16,798

One-half 1,250 3,857 2,548 10,673

17 feet 1,839 6,524 4,557 23,313

1 0 by 1 0 Control 364 603 426 682

Crown surface area.

Zrown volume, where crown shape considered a parabola with base

an ellipse using maximum crown width measurements.

Height to live crown averaged 10 feet (range 6 to 19

feet) after 10 years for (C). No dead limbs were noted

above the 17-foot level pruned prior to the 5th year in

(17) or above final 8th year pruning of 26.8 feet in (l/3)

and 38.4 feet in (l/2).

Doyle board foot volume growth in the 10th year varied

by pruning treatment from 470 to 500 board feet per acre,

including ingrowth, for trees ~12.5 inches d.b.h. to a 1 Oinch top (table 4). Board foot mai through 10 years ranged

from 100 to 277 board feet per acre per year.

In contrast to trees in the 40- by 40-foot planting, cottonwood in the lo- by 1 O-foot planting from the 5th through

10th years increased in d.b.h. from 6.5 to 7.5 inches, in

height from 60 to 73 feet, in ratio of diameter at 17 feet

to d.b.h. from 0.76 to 0.83, in diameter at 17 feet from

4.9 to 6.2 inches, in height to live crown from 41 to 48

feet, and in live crown length from 19 to 24 feet. The

trees decreased in average maximum crown diameter

from 8.8 to 8.3 feet. Crown volume increased 13 percent,

although crowns were very sparse and only 0.03 to 0.06

as large as crowns of ?rees at 40 by 40 spacing, depending on pruning treatment.

The 40 by 40 spacing had four to six times more volume per tree by pruning treatment after 10 years than

the 10 by 10 spacing, but only 0.3 to 0.4 as much total

volume per acre. Relative cubic volumes were calculated

from mean values of trees where upper stem diameters

were measured, using (d.b.h.)z * height l form.

DISCUSSION

It should be noted that the clone used in this study was

classified as a select clone primarily because of fast early

growth when planted at close spacings (Mohn et al. 1970).

Other clones with different crown shapes and rates of

crown development may respond differently at wide

spacings.

In the last 3 years, d.b.h. growth of all pruning treatments has generally stabilized, rather than steadily decreased as during the earlier years. Thus, 17- to 1 g-inch

pruned trees are a possibility in 15 years and 20- to 23-

inch trees in 20 years. Although past experience indicates a slowing of d.b.h. growth with time rather than

stable growth, past experience has not included wide

spacings.

During periods of maximum d.b.h. growth (within the

first 5 years), treatments that affect crown length have a

greater effect on d.b.h. growth. While pruning tends to

influence d.b.h. growth during the year of treatment, the

effects have little influence on future growth.

Upper stem diameter growth in this study did not agree

with several generally found stem form concepts (Larson

1963) of maximum growth occurring lower on the boles

of open-grown trees than those of close-grown trees, and

of open-grown trees differing markedly in stem form from

close-grown trees. Results did agree with the concept of

maximum growth moving up the tree as the crown recedes.

Two spacing systems for growing cottonwood through

10 years have produced quite different results. Trees

under close spacing have fully utilized the site while individual trees under wider spacing have grown much

faster. Either pulpwood from close spacing or sawtimber

from wide spacing can be grown in 10 years.

Nearly equal lo-year returns of about $150 per acre

are obtained for both pulpwood and sawtimber. Pulpwood estimates are based on average d.b.h. and height

to give 9 cubic feet per tree at 10 by 10 spacing, 70

percent survival, and 90 cubic feet and $5 per cord stumpTable 4. - Doyle board foot volumes per acre in trees ~12.5 inches

d.b.h. to lo-inch top inside bark by pruning treatments for

8th through 10th years

8th year 9th year 10th year

Treatment N/A’ fbm/A N/A fbm/A N/A fbm/A

Control 2 6 1,795 2 6 2,261 2 7 2,770

One-third 2 5 942 2 6 1,428 2 7 1,899

One-half 4 9 8 1 4 502 2 2 1,004

17 feet 2 6 1,387 2 7 1,889 2 7 2,383

‘Trees/acre.

4
age. Sawtimber estimates are based on 1,900 board feet

from one-third pruning at 40 by 40 spacing and $80 per

M stumpage.

But, there is no option to go on from this point with the

10 by 10 spacing as the stand is essentially stagnant,

awaiting either cutting or mortality. Thinning now would

not benefit leave trees as they could not respond; earlier

thinning, which would have reduced volume, should

probably have been done by age 5 when the live crown

ratio was already reduced to 32 percent. With the wider

spacing, however, options exist either to cut at age 10,

or to let the trees grow.

True cost comparisons between the two spacings cannot be made because of inadequate economic information. Planting costs for the close spacing appear to be at

least four times greater and cost of cuttings 16 times

greater than for the wide spacing. However, near 100

percent survival is needed at wide spacing and the alternatives of long cuttings or multiple cuttings per planting spot would be more costly.

Pruning at wide spacing is necessary in order to produce quality lumber. But when and how much to prune?

In this study (l/3) looked best, but annual pruning is not

feasible. One possible method, which appears workable

and not too severe, would be three prunings to a two-log

height: prune by half-log increments, when tree heights

are about three times greater than pruning heights, to a

one-log height and then prune an additional log when a

55 to 60 percent live crown ratio can be maintained. By

this method in this study, half-logs would have been

pruned at the start of the 3rd and 5th years, and the

second log at the start of the 7th or 8th years.

A better sawtimber spacing would be one that provided

more trees but that would not greatly restrict diameter

growth over the first 10 years compared to 40 by 40

spacing. The crown spread of (l/3) after 10 years suggests that possibly 50 to 55 trees per acre would have

similar early growth. Nearly twice the trees would give

better site utilization and would provide some safety

against mortality.

LITERATURE CITED

Broadfoot, Walter M. Hardwood suitability for and properties of important Midsouth soils. Res. Pap. SO-1 27,

New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station; 1976.

84 p.

Kennedy, Harvey E., Jr. Planting season for cottonwood

can be extended. South. J. Appl. For. 3(2):54-55; 1979.

Krinard, R. M. Five years’ growth of pruned and unpruned cottonwood planted at 40- by 40-foot spacing.

Res. Note SO-252, New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest

Experiment Station; 1979. 5 p.

Krinard, R. M.; Johnson, R. L. Ten-year results in a cottonwood plantation spacing study. Res. Pap. SO-1 06,

New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station; 1975.

10 p.

Land, Samuel B., Jr. Forest tree improvement: Mississippi certifies Nation’s first “blue tag.” J. For. 72(6):353;

1974.

Larson, Philip R. Stem form development of forest trees.

For. Sci. Monogr. 5; 1963.

Mohn, C. A.; Randall, W. K.; McKnight, J. S. Fourteen

cottonwood clones selected for Midsouth timber production. Res. Pap. SO-62, New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station; 1970. 17 p.

5