United States

ie Department of

@& Agriculture

0

54s Forest Service Research Note * Southern Forest

Experiment Station

SO-31 2

May 1985

Prescribed Winter Burns Can Reduce the Growth of

Nine-Year-Old Loblolly Pines

Michael D. Cain

SUMMARY METHODS

Prescribed winter burning was done in a precommercially thinned, g-year-old, natural stand of

loblolly and shortleaf pines (Pinus taeda L. and

P. echinata Mill.). Growth and survival of 174 loblolly pines were monitored one growing season

after this burning. Mortality was highest for pines

with less than 2 inches groundline diameter or

pines that were less than 8 feet tall. Diameter

and height growth during the year after burning

showed statistically significant, negative correlations with increasing crown scorch.

Additional keywords: Crown scorch, fire intensity, Pinus taeda L.

In mid-January 1983 prescribed burns were conducted on six 0.4-acre plots of a precommercial

thinning study in a g-year-old natural stand of

loblolly-‘shortleaf pine. The precommercial thinning study is located on the Crossett Experimental Forest in south Arkansas. The six plots had

been mechanically thinned in October 1979 by

rotary mowing 12-foot-wide swaths that alternated

with l-foot-wide uncut strips. Prior to the prescribed burns, pine density averaged 2,000 stems

per acre, mean total height was about 10 feet,

and mean diameter at breast height (dbh) was

about 1 inch.

INTRODUCTION

Prescribed burning is a useful silvicultural treatment to reduce fuel hazard and to control unwanted

competition in even-aged pine management in the

South. Thus, knowledge about fire’s effects on

survival and growth of southern pines is important in the decision-making process of forest managers.

Published information is available on the effects

of prescribed fire in 17- to 20-year-old stands of

loblolly pine (Mann and Gunter 1980, McNab 1977,

Villarrubia and Chambers 1978, Chambers and Villarrubia 1980, Waldrop and Van Lear 1984), but

documentation of fire effects in younger stands

has been lacking (Muckenfuss 19?4). This study

addresses that information gap.

Burning was done between 10:00 am and 3:30

pm; air temperature ranged from 49’ to 60” F;

and relative humidity ranged between 31% and

52%. Wind speed was 5 mph from the northwest.

There had been no precipitation for 7 days prior

to burning. Surface fuels consisted of pine litter

and grasses on the l-foot-wide uncut strips, but

on the 12-foot swaths there were mostly grasses,

briars, and undecomposed wood fiber from the

mechanical thinning that occurred 3 years earlier.

Surface fuels were sampled down to mineral soil

and at the time of burning had a moisture content of 28% and a dry weight of 6 tons per acre.

Because of the patchiness of combustible surface

fuel, headfires were required to insure complete

area coverage. The rate of fire spread was 5.5

chains per hour. Nearly 200 ocular estimates of

flame length were made during the burns. Mean

flame lengih over all 6 plots was 2.8 feet with a

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

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Loulsiana, Misslssippl, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
standard deviation of 1.1 feet. Fireline intensity

was calculated from flame length estimates (Byram

1959) and averaged 60 Btu/ft-sec.

Two weeks after the prescribed fires, 174 loblolly pines were tagged to provide a full range of

sizes and scorch damage within the following categories:

Number Height class Crown Scorch

of pines (feet) (percent)

24 <5 550

25 5to10 150

25 >lO 550

35 c5 >50

33 5to10 >50

32 >lO >50

Total=1 74

For each pine, groundline diameter (gld) and

dbh were measured to the nearest 0.1 inch. Although gld and dbh were measured after the burns

and some bark-char occurred, the fires were not

hot enough to consume the bark. Total height,

height to live crown, height of crown scorch, and

height of stem-bark char were measured to the

nearest 0.1 foot on individual pines. Percent crown

scorch, the proportion of live crown delineated

by yellowing nor browning needles, was determined

2 weeks after burning by ocular estimation to the

nearest 5 percent. Mortality was assessed in

October 1983, one growing season after burning,

when height and diameter measurements were retaken for calculation of growth.

Statistical analyses included linear regression

and correlation coefficients. The arc sine transformation was used for percent values in statistical analyses. All analyses were carried out at

the 0.05 level of significance.

RESULTS AND DISCUSSION

Either total height or groundline diameter of 9-

year-old loblolly pines appear to be suitable predictors of pine survival following a winter prescribed fire. In general, pines survived the prescribed fire if, at the time of burning, they were

taller than 8 feet (fig. 1) and larger than 2 inches

in groundline diameter (fig. 2), regardless of the

degree of crown scorch. Regression analysis indicated that total height and groundline diameters

were highly correlated in this study (fig. 3).

During the year following the prescribed fire,

growth in total height (fig. 4) and growth in dbh

(fig. 5) were significantly reduced as crown scorch

increased. Regression analysis provided the following equations:

Height growth (Y)

Pines ~10 feet tall; Y = 3.6366 - 0.0253(X), r 2 = 0.58

Pines 5 to 10 feet tall; Y = 2.7572 -0.0216(X), r’J = 0.44

Pines ~5 feet tall; Y = 1.5632 - 0.0102(X), r 2 = 0.33

Dbh growth (Y)

Pines >lO feet tall; Y = 0.5632 - 0.0058(X), r 2 = 0.57

Pines 5 to 10 feet tall; Y = 0.2910 - 0.0025(X), r 2 = 0.40

where (X) was percent crown, scorch by arc sine

transformation.

All of the above growth correlations with crown

scorch were statistically significant, but the strongest were for pines 10 feet and taller for which the

variation in height growth and diameter growth

associated with crown scratch, explained by the

regression equation, was more than 50 percent.

Somewhat more pronounced decline appeared in

mean annual growth relative to increasing crown

scorch for surviving pines taller than 10 feet (figs.

4, 5). For pines up to 10 feet tall, there was

higher mortality with more crown scorch (fig. 1);

so, survivors in those smaller height classes may

have been less stressed from crown scorch for

subsequent growth response compared to survivors 10 feet tall and taller. Without mortality for

pines less than 10 feet tall, declining height

growth trends similar to that of taller pines may

LIVE

. :

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--DEAD

0

I 1 I I I I I I I

0 10 20 30 40 50 60 70 so so 100

CROWN SCORCH (PERCENT)

Figure l-Survival of lob/o//y pines, one growing season after

a prescribed winter burn, relative to total height

at the time of burning and crown scorch after

burning.

Michael D. Cain is Associate Silviculturist, Southern Forest Experiment Station, USDA Forest Service, Crossett, AR 71635.

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: :

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. 0 -DE&D 0 0 0 fJ

01 I t I I I I I I I

0 IO 20 30 40 SO 60 70 60 60 100

CROWN SCORCH (PERCENT)

Figure 2-Survival of lob/o//y pines, one growing season after

a prescribed winter burn, relative to groundline

diameter at the time of burning and crown scorch

after burning.

’ :

r-- ---- --- -A< 0. z-10’ 5’ T O5' TALL IO’TALLTALL

CROWN SCORCH (ARC SINE B PERCENT)

Figure 4-/mpact of crown scorch on height growth of Q -

year-old lob/o//y pines one growing season after a

prescribed winter burn.

have been expected relative to increasing crown

scorch.

Results of this investigation are not necessarily

applicable to all loblolly pine stands. Working in

the South Carolina Piedmont, Waldrop and Van

Lear (1984) found that diameter growth of 17-

year-old, pole-size loblolly pines was not significantly affected in the year following prescribed

burning, even when fire intensity was reported to

be over 235 Btu/ft-sec.

The use of measured variables often provides

more definitive results than subjective judgments.

Yet ocular estimates of crown scorch were more

useful in this investigation. For example, when re2 2 -

Y= 2.1963 + 3.0476X

2 r2= 0.93

t la4

0 I 2 3 4 5 6

GROUNDLINE DIAMETER (INCHES)

Figure 3-Relationship of total height to groundline diameter for Q-year-old lob/o//y pines.

8 I I

4: JO 6 0 I 1 WI I 941 L-P.i.- 701 ko IO 2 0 3 0 7 0 6 0 9 0

CROWN SCORCH (ARC SINE B PERCENT) - -

Figure 5-Impact of crown scorch on dbh growth of Q-yearold lob/o//y pines one growing season after a prescribed winter burn.

lated to length of crown, height of crown scorch

overestimated the degree of scorch because such

measurements assume uniform discoloration of

needles throughout the crown when, ‘in fact, only

one side of the crown may have been affected.

Ocular estimation compensated for nonuniformity

of crown scorch. Also, ocular estimates of crown

scorch were more easily made in the field than

measurements of scorch height of char height. In

post-burn situations, however, measurements of

scorch height or char height on loblolly pines

may provide forest managers the dependent variables needed for estimation of relative fireline

intensity (Cain 1984).

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MANAGEMENT IMPLICATIONS

Groundline diameters and total heights may be

useful in predicting mortality of young loblolly

pines prior to burning-that is, whether or not

prescribed burning can or should be done. High

mortality of loblolly pines can result from prescribed winter burns if total heights average less

than 8 feet, groundline diameters average less

than 2 inches, and predicted fireline intensity

approaches or exceeds 60 Btu/ft-sec.

Backfires are the recommended burning technique in young loblolly pine stands in order to

minimize crown scorch. When crown scorch does

occur, ocular estimates of scorch percentage may

be useful in assessing potential mortality of young

loblolly pines following a winter fire. As crown

scorch increases, both dbh growth and height

growth of young loblolly pines can be significantly

reduced during the growing season that immediately follows a prescribed winter burn.

LITERATURE CITED

Byram, G.M. Combustion of forest fuels. In: Davis,

K.P., ed. Forest fire: control and use. New York:

McGraw-Hill Book Co., Inc.; 1959: 61-89.

Cain, M.D. Hejght of stem-bark char underestimates flame length in prescribed burns. Fire

Manage. Notes 45( 1): 17-21; 1984.

Chambers, J.L.; Villarrubia, C.R. An assessment

of the effects of crown scorch on loblolly pine

growth and survival. La. State Univ. Forestry

Notes 131; 1980. 2 p.

Mann, W.F., Jr.; Gunter, E.R. Predicting the fate

of fire-damaged pines. Forests and People 10(l):

26-27,43; 1960.

McNab, W.H. An overcrowded loblolly pine stand

thinned with fire. South. J. Appl. For. l(1):

24-26; 1977.

Muckenfuss, G.E. Use of prescribed fire in young

loblolly pine stands. In: Proc. Symp. on Manage. of Young Pines; 1974 October 22-24; Alexandria, La. 1974 December 3-5; Charleston, S.C.:

U.S. Department of Agriculture, Forest Service,

Southeastern Area State and Private Forestry,

Southern and Southeastern Forest Experiment

Stations; 1974: 337-339.

Villarrubia, C.R.; Chambers, J.L. Fire: its effects

on growth and survival of loblolly pine, Pinus

taeda L. La. Acad. Sci. 51: 85-93; 1978.

Waldrop, T.A.; Van Lear, D.H. Effect of crown

scorch on survival and growth of young loblolly pine. South. J. Appl. For. 8(l): 35-40; 1984.