’ a Research Note so-264

1961

Thinning and Pruning Influence Glaze Damage in a

Loblolly Pine Plantation

James D. Burton

SUMMARY

An old-field plantation was thinned and pruned at

age 11 and again at age 14 to 4 basal area levels

and 3 crown percent levels. A survey was made to

determine how damage by an ice storm at age 15

was influenced by treatment. Severe damage was

heaviest in the densest stands and in stands with the

shortest crowns, while the percent of stand destroyed

was least under the heaviest thinning and lowest

pruning. However, basal area and stems/acre in remaining growing stock were greatest under the lightest thinning and lowest pruning.

Additional keywords: Pinus taeda L., freezing rain,

understocking.

INTRODUCTION

Cultural practices such as heavy thinning and high

pruning can markedly shorten sawtimber rotations

and enhance the value of the product. But they can

also have an unfavorable effect on a stand’s resistance to ice damage.

The belt of greatest severity and frequency of

glaze storms extends from north-central Texas to

southern New England; ice 0.25 to 0.50 inches in

radial ‘thickness on needles, branches, and utility

wires may be expected once every 3 years somewhere in this belt. Nearly all the country east of the

Rocky Mountains (except the southern half of Louisiana, Mississippi, Alabama, Georgia, South Carolina,

and’all of Florida) can expect such thicknesses somewhere once in 6 years; and deposits of 1 radial inch

or more have been recorded in most of this area and

on the Louisiana coast (Bennett 1959).

Loblolly pine is less resistant than shortleaf and

more resistant than slash pine to injury by glaze and

snow (Wakeley 1954).

The most resistant stands are those with wide,

uniform spacing-stands frequently thinned so as

to maintain sturdy boles and compact, symmetrical

crowns; recently thinned stands tend to be susceptible to glaze injury for a few years after thinning; in

the long run, thinning results in sturdier stems and

stronger branches, but in the short run it robs residual

trees of mutual support (Brender and Romancier

1960).

A glaze storm heavily damaged thinned and unthinned stands in southern Arkansas in January 1974.

The storm provided an opportunity to study how

thinning and pruning intensity affect resistance of a

15year-old loblolly pine plantation to ice loading on

foliage and branches.

METHODS

The plantation is on an old cotton field near

Monticello, Arkansas’. It was thinned and pruned at

age 11 (12 years from seed germination) and again

at age 14-4 years before and 1 year before the

storm. Four levels of thinning and three of pruning

were applied factorially and replicated 3 times in

randomized complete blocks. Treatments were applied

to 0.6acre square gross plots; a 0.1 -acre, square

measurement plot was established in the center of

each gross plot.

Thinning at age 11 to four levels (very light, light,

heavy, and very heavy) left basal areas of 100, 80,

- -

‘Land and technical assistance were provided by the Crossett

Division, Georgia-Pacific Corporation.

Southern Forest Experiment Station/T-l0210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, Eastern Oklahoma, Tennessee, Eastern Texas.
60, or 40 ft*/acre in merchantable trees (> 4.6 inches

dbh containing 2 two 63-inch bolts to a gin top dib).

Trees smaller than this were considered submerchantable and were removed in the initial thinning,

Where possible, thinning was mainly from below.

But the need to achieve uniform spacing and to

remove forked and crooked trees, wolf trees, and

trees with fusiform rust stem galls forced cutting of

many dominant and codominant trees. So did the

reduction of basal area to 40 ft*/acre. Dominant and

codominant trees at age 11 averaged 37 ft in total

height and had a live crown length of 22 ft. After

thinning, live crowns were reduced by pruning to 52,

41, or 37 percent of total height (low, medium, and

high pruning levels, respectively).

Following inventory at age 14, thinning and pruning were repeated and intensified. Residual basal

area levels were 90, 70, 50, or 30 ft*/acre, and

crown lengths were pruned to 55, 40, or 25\percent

of total height.

After treatment at age 14, some stand attributes

representative of the four thinning levels were:

Thinning Quadratic Mean total height of

level Trees/acre mean dbh dominants and codominants

fP/acre number inches feet

90 273 7.8 4 3

7 0 199 8.1 4 4

5 0 123 8.8 4 5

3 0 8 8 9.0 4 4

The low level of pruning did not remove all the

lower branches already killed by shading at age 14

in the lightest thinning. It did at all other levels of

thinning. But even at the heaviest thinning, low

pruning was very mild. High pruning, however, was

severe (fig. 1).

A complete inventory of the plots was made 1

month after the storm. Glaze injuries were tallied

according to severity under three headings: loosening of roots in soil, stem bending, and loss of crown.

Trees with 2 50 percent of the live crown lost or with

the bole bent or butt deflected > 45’ from vertical

were considered severely damaged and were marked

for salvage. All other living trees were retained as

growing stock.

Treatment-associated differences in proportion of

stand destroyed and in density of remaining growing

stock were examined through analysis of variance

at the .05 level. Duncan’s Multiple Range Test was

used to evaluate differences in main effects of thinning and pruning when the T x P interaction was

not significant.

Crown loss was the principal kind of injury. Stem

bending and root springing account for < 5 percent

of the injured trees. Some crown loss resulted from

breaking branches, but a substantial majority was

caused by stem break, within or below the live crown

(fig. 2). Mean stem diameter at the break was 3.0

inches (range 1.5 to 3.7 inches by treatment).

Crown loss was heaviest in the high pruning treatments and was lightest in the two heaviest thinning

treatments (table 1). Forty-four percent of the basal

area in high-pruning plots, all thinning levels combined, was in severely damaged trees. In high-pruning

plots thinned to 70 ft* or 90 ft*, 50 to 60 percent of

the stand was lost. The mean for high pruning was

significantly different from the means for medium

and low pruning; the means for the 30-ft*, 50-ft*, and

90-ft2 thinning levels, all prunings combined, were

significantly different.

Severe damage, in numbers of trees/acre, was

centered in the 7-inch and 6-inch dbh classes in the

very light and light thinning treatments (table 2). In

the very heavy and heavy thinnings, severe damage

occurred more frequently in the g-inch than in the

7-inch class.

Table 1 .-Fraction of stand basal area in severely damaged trees

Pruning Thinning level Pruning

level very heavy heavy light .very light means

High 2 8

Medium 3

Low 8

3 9 5 0 8 0 44b’

. 2 4 2 0 3 6 21a

9 1 8 1 8 13a

Thinning

means 13a 24b 29bc 38c 2 6

‘Means followed by the same letter, within a row or a column, are

not significantly different.

Even though the very light thinning plots sustained

a greater percentage loss than the very heavy and

heavy thinning treatments combined, they contained

mare growing stock after the storm (table 3). The

heavy thinning plots contained a greater manageable

basal area than did the very heavy. The T x P interaction was not significant. Uniform levels of basal

area within thinning treatments no longer existed,

because pruning level affected storm damage. All

three pruning means were significantly different.

Burton is Soil Scientist, Southern Forest Experiment Station, Forest Service-USDA, Pineville, Louisiana 71360.
Figure 1 .-In many trees, high pruning left only 2 annual whorls

and the leader.

Figure 2.-Many trees suffered stem breakage below the crown.

Table 2.-Diameter distribution of trees severely damaged by the

glaze storm

Thinning

level

Pruning

Dbh (inches)

Ieve’ Total

5 6 7 8 9 1 0

---------.----treesjacre----------------

Very heavy High 0 0 3 7 1 3 0 23

Medium 0 0 0 3 0 0 3

Low 0 0 0 0 6 0 6

Heavy High 0 7 1 0 20 1 0 1 0 5 7

Medium 0 0 3 1 0 1 3 7 33

Low 0 0 3 3 7 7 1 3

Light High 0 1 3 7 3 1 0 1 7 4 117

Medium 0 7 1 3 20 7 0 4 7

Low 0 0 1 3 1 7 7 3 40

Very light High 0 1 3 4 7 60 33 7 160

Medium 7 1 7 40 26 20 3 113

Low 0 ‘ 7 1 3 2 7 7 0 54

Table 3.-Basal area remaining in undamaged and lightly damaged

trees after the glaze storm

Pruning

Thinning level

Pruning

level very heavy heavy light very light means

High

Medium

Low

- ------___________ft2/acre _________________

23 3 2 3 7 3 7

3 2 4 2 60 63

3 1 5 1 64 8 1

32a1

49b

57c

Thinning

means 29a 42b 54c 6Oc 46

‘Means followed by the same letter, within a row or a column, are

not significantly different.

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Table 4.-Diameter distribution of undamaged and lightly damaged

trees/acre after the glaze storm

Thinning Pruning Dbh (inches)

level level Total

6 , 6 9 1 0 1 1 1 2

Very heavy High

Medium

Low

Heavy High

Medium

Low

Light High

Medium

Low

Very light High

Medium

Low

.--.---..--treeslacre ..-_. .-.. --..

0 7 13 7 20 3 0 50

0 0 7 17 20 13 3 60

0 3 10 14 13 20 0 60

0 0 10 17 26 10 0 63

0 3 13 40 27 7 0 80

0, 6

0 33

7.. ;; 20 10 0 113

3 0 0 103

0 13 63 57 17 0 0 150

0 7 50 40 23 17 3 140

14 13 40 27 3 3 0 loo

10 27 87 43 13 7 0 167

10 67 100 33 13 3 0 226

Stand density in trees/acre remaining after the

storm followed a pattern similar to that of basal area

(table 4). All four thinning means were significantly

different when all pruning levels were combined.

All three pruning means were significantly different

when all thinning levels were combined. The very

light thinning plots contained twice as much basal

area (table 3) and nearly three times as many trees/

acre (table 4) as the very heavy thinning plots, all

pruning levels combined. In the very heavy thinning

plots, 92 percent of the trees/acre were in the 8-

through 1 l-inch dbh classes. In the heavy thinning

plots, the 8- through 1 l-inch classes contained 96

percent of the trees/acre. Seventy percent of the

treed‘in the light thinning plots were in the 7- and

a-inch classes. The 7- through lo-inch classes included 91 percent .of the trees in the high pruning,

91 percent in the medium, and 88 percent in the low

pruning treatments, all thinning levels combined,

Wahlenberg (1960) stated: “In general, natural

stands appear just as susceptible to injury from ice

as are planted stands properly spaced and thinned.

Ice damage is found in both open-grown and dense

stands, but the trees that show the effects of crowd-

” ing are somewhat more susceptible. Although dense

stands tend to suffer the highest percentage of loss,

they are likely to emerge with the most undamaged

trees. This is the situation where calm weather outlasts the ice overload”.

Results of this study indicate that plantations

pruned to a 25”percent live crown are more likely

to lose half or more of the remaining crown in a

glaze storm of this severity than are stands of similar

basal area, within the limits of this study, pruned to

a 4@ or S&percent live crown. This does not necessarily indicate that high pruning uces a’ stem’s

mechanical strength more than medium or low pruning (although it could be true). It does indicate that

mean stem diameter at breaking point is more likely

to be at or below the crown midpoint in a highly

pruned tree than in a similar tree given low or medium

pruning.

Trees ‘in this study appeared to be very severely

damaged, particularly in dense stands with high

pruning. This does not mean- that plantations so

damaged are immediate candidates for liquidation.

Loblolly pine, more than any other southern pine,

recovers rapidly from understocking and vigorously

exploits available space. Some of the intensivemanagement plots in the Sudden Sawlogs study

(Burton 1976) contained only 37 ft2/acre in 92

trees/acre at age 16; 3 years later their basal area

was 61 to 63 ft*/acre.

LtTiRATlJREOtTBD

Bennett, I.

1959. Glaze-its meteorology and climatology,

geographic distribution, and economic effects.

U.S. Army Quartermaster Research and Engineering Command, Natick, MA. Tech. Rep.

EP-105,27 p.

Brender, E. V. and R. M. Romancier.

1960. Glaze damage in loblolly pine plantations.

South. Lumberman 201(2513):168.

Burton, J. D.

1976. How a timberland owner can grow sawlogs

in a pulpwood rotation. For. Farmer 36( 1): 8-9.

Wahlenberg, W. G.

1960. Loblolly pine. Duke University School of

Forestry, 603 p.

Wakelay, P. C.

1954. Planting the southern pines. USDA For. Serv.,

Agric. Monog. 18, 233 p.

.-.- _ EEWQUM: 2nd col, , 1st par.

Sentence, “This does not.. .I’

should read,“This does not necessarily indicate that high

pruning reduces a stem’ 8.. .*I

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