United States

Department of

Agriculture

Forest Service

Southern e s

Research Station

Research Note

SRS-09 July 2001

Abstract

An unreplicated demonstration was established in the Ouachita Mountains

in which shottleafpine (I’inus echinara Mill.) trees were harvested and

overstory hardwoods were retained. A new stand was established by

underplanting shortleaf pine seedlings. After the third growing season, five

0.5~acre plots were established, and one of five overstory hardwood

retention treatments 0, IO, 20, 30, or 40 square feet per acre of residual

basal area (RBA) was randomly assigned to each. Pine seedlings were

measured after the third, fifth, and seventh growing seasons. Over time,

pine seedling density changed very little by treatment, but seedling basal

area varied inversely with increasing overstory retention. After the lifth

and scvcnth growing seasons, the basal area of the average tree in the RBA

0 treatment was greater than in any other treatment, and differences in

height among treatments were also observed. Between the third and

seventh growing seasons, average annual growth of pine in both height and

basal area declined with increasing overstory hardwood basal area. The

decline in height growth occurred uniformly from the RBA 0 through the

RBA 40 treatments; but the decline in seedling basal area growth occurred

between the RBA 0 and RBA IO treatments. To date, seedlings have

survived and growth has increased across all treatments. Future monitoring

will determine if and when growth rates cease on any of the plots.

Keywords: Ouachitd, plantation, shortlcaf pine, survival, underplanting.

Introduction

In 1990, the USDA Forest Service initiated New

Perspectives-the first step in a philosophical reorientation

of land management within the agency. One element of this

program was to demonstrate partial cutting methods that

might be used as alternatives to clearcutting, especially in

visually sensitive areas.

One such demonstration was established in a mixed pinehardwood stand on the Fourche Ranger District of the

Ouachita National Forest (NF) in Arkansas. Rather than

clearcutting both pines and hardwoods in this stand, only the

shortleaf pine (Pinus echinata Miil.) was harvested.

Over-story hardwoods, primarily oaks (Que~u.s spp.) and

hickories (Caya spp.) were retained as forest cover for the

James M. Guldin, Project Leader, USDA Forest Service, Southern

Research Station, Hot Springs, AR 7 I902 and Gcrdid Heath, District

Silviculturist, USDA Forest Service, Ouachita National Forest, Danvillc,

AR 72833.

site. Shortleafpine seedlings were then under-planted

beneath the hardwoods. However, foresters questioned

whether the pines could develop successfully beneath the

hardwoods. Thus, an unreplicated experimental case study

was established in this stand to monitor the survival and

growth of planted shortleaf pines beneath the residual

hardwood overstory.

ethods

Study Area

The demonstration area is located on the Fourche Ranger

District of the Ouachita NF in Yell County, AR,

approximately 6 miles due south of the town of Ola. This

region of the Ouachita Mountains is characterized by long

east-west ridges, and the study area occupies a north-facing

aspect with less than 1 O-percent slope near the top of one of

these ridges. The stand also lies within the viewshed of Lake

Nimrod, a U.S. Army Corps of Engineers flood control

facility on the Fourche LaFave River. The lake is a popular

fishing spot among residents in the area.

All overstory shortleaf pines were harvested in November

1987. Over 40 square feet (ft?) per acre of residual overstory

and midstory hardwoods remained after the pines were cut.

Management plans on the ranger district originally

prescribed herbicide injection of these hardwoods, followed

by ripping and planting. However, because the area was

visible from the lake, managers decided to keep between

one-half and two-thirds of the hardwoods. This led to a

modification in the planting prescription--the site was not

ripped because ripping would disrupt the roots of the

hardwoods.

The site was underplanted in February 1989, using

genetically improved shortleaf pine on a 7- by 8-ft spacing.

‘The unwanted part of the hardwood component (smaller

trees of poor form and trees of inferior species) was injected

in March 1989 with 0.46pound (lb) active ingredient (ai)

per acre of Garlon 3A.
In July 199 1, five 0.5-ac plots measuring 2 by 2.5 chains

were established within a relatively uniform area of the

stand. Five residual basal area (RBA) treatment levels--O,

10, 20, 30, and 40 ft’ per acre-were randomly assigned,

one to each plot.

All overstory trees were measured in each treatment plot.

Trees were marked for removal from below, and an updated

tally was kept in the field. All trees marked as surplus above

the specified treatment target were injected in August 199 1

using Garlon 3A at a rate of 1 -lb ai per ac.

In March 1992, after the third growing season (GS3), a

central measurement area of 1 by 1.5 chains was established

in each treatment plot. In this measurement area, six O.Olacre fixed-radius subplots (radius = I 1.78 ft) were

systematically established on a 0.5-chain grid. All planted

trees with heights > 1.5 ft were numbered and tagged in each

subplot. On each sample tree, the root collar diameter (RCD)

was measured to the nearest 0.1 inch (in.) using a caliper,

and the total height of the seedling was measured using a

logger’s tape to the nearest 0.1 ft.

In December 1993, after the fifth growing season (GS5), and

again in February 1996, after the seventh growing season

(GS7), all tagged pines were remeasured. Root collar

diameter (RCD) was measured to the nearest 0.1 in. using a

caliper or diameter tape. Tree heights were measured to the

nearest 0.1 ft using either a range pole (graduated in feet and

tenths) or a clinometer.

Data Analysis

Data were analyzed in three ways. Preliminary analyses

quantified changes in stand density (trees per acre) and basal

area (ft? per acre) of planted pines from the third to the

seventh growing season. Because the treatments were

unreplicated and only six 0.0 l-acre subplots were used per

treatment, results should be interpreted as general trends.

The second analysis quantified changes in average tree

diameter and height across the five treatments in each

measurement period. Tree height (HT) was analyzed directly

using the height data recorded in the field. All statistical tests

for RCD were conducted using root collar basal area

(RCBA), because RCBA is more robust for statistical

analyses than RCD. To help interpret RCBA analyses,

quadratic root collar diameter (QRCD, the RCD of the tree

having mean RCBA) was calculated for mean RCBA results.

The statistical analysis of RCBA and HT was conducted

using a parametric general linear models procedure (SAS

Institute Inc. 1989). The hypothesis of no difference in mean

tree size among treatments was tested using analysis of

variance. The sampling error of the subplots per treatment

was used as a surrogate for experimental error. The

2

statistical analysis was complicated by differences in initial

tree RCBA and HT by treatment after the third growing

season, for which the common mathematical transformations

were unable to compensate. Thus, treatment differences in

RCBA and HT at GS5 and GS7 were tested using analysis of

covariance (Snedecor and Cochran 1967) where the initial

RCBA (or HT) at GS3 was used as the covariate for RCBA

(or HT) at GS5 and GS7.

The third analysis tested for differences in individual tree

growth by treatment. Data were calculated as average annual

growth in RCBA and HT for GS3 to GS5, and GS5 to GS7;

RCBA growth was converted back to QRCD to facilitate

interpretation. The hypothesis of no difference in mean

annual growth among treatments was tested using analysis of

variance (SAS Institute Inc. 1989). In all analyses, Sidak’s

test (SAS Institute Inc. 1989) was used for comparisons

among means.

Results

Plantation Density and Basal Area

Pine seedling density showed very minor changes over time

within any of the treatments (fig. 1). Three treatments

showed no change from GS3 to GS7, and two of the

treatments showed a slight decline. Seedling density among

treatments differed at GS3. These differences could be

attributed to variations in planting density, mortality of

planted seedlings between the time of planting and the third

growing season, or both. The former is more likely. Because

the planting was done as an educational project by

volunteers from the community, variation in planting density

was not unexpected.

Pine basal area increased over time (fig. 2), and the basal

area at GS7 in the RBA 0 treatment was clearly higher than

that in the remaining treatments. In both the GS5 and GS7

measurements, the difference in basal area between the RBA

0 and RBA 10 treatments was larger than the difference

between the RBA 10 and RBA 40 treatments. At both GS5

and GS7, the basal area of planted pines in the RBA 10

treatment was approximately one-half that in the RBA 0

treatment.

Mean Sapling Diameter and Height

At GS3, differences in the diameter and height of the

average tree were observed among treatments (table 1).

Generally, the RBA 0, RBA 10, and RBA 40 treatments had

the largest seedlings in GS3; and the RBA 30 treatment had

the smallest. These differences probably resulted from

variation in the residual overstory before the third growing

season. However, these differences are small for all practical

purposes. Variation in diameter (QRCD) was only about 114

in. across all treatments (fig. 3), and variation in height was

only 213 ft (fig. 4).
G&3 G?K

Growing season

G&7

i

I

A0

v

RBA 10

-eRBA 20

::3

Figure I Pine plantation density after the third, fifth, and seventh growing season (GS3, GSS, and GS7) by

treatment. RBA O-40 are treatment levels.

,’

’

,’ .rt

_,

i

G.93 GS5 Gh7

Growing season

RBA 0

v

RBA 10

0

RBA 20

A 30

RBA 40

Figure 2 Plantation basal area a&x the third, fifth, and seventh gowing seasons (GS3, GSS, and GS7) by

overstoly residual basal arca (RBA) tl-eatment. RRA 0 40 are treatment levels.

3
RBA 10

RBA 20

f=3

A 30

RBA 40

GS3 GS5

Growing season

GS7

Figure 3 Mean sapling quadratic root collar diameter (QRCD) aficr the third, filih, and seventh growing seasons (GS3, GS.5,

and GS7) by overstoly residual basal area (RBA) treatment. RBA 0 40 arc treatment levels

1 6

1 4

6

- w

RBA 0

w

RBA IO

RBA 20

Es

A30

~-

RBA 40

Growing season

Figure 4 Mean sapling height (HT) after the third, filih, and seventh growing seasons (GS3, GSS, and GS7) by overstory

residual basal arca (RBA) treatment. RBA 0 40 are treatment levels.

4
Table l-Mean quadratic root collar diameter (QRCD), root collar basal area (

T) after the third, fifth, and seventh growing seasons (GS3, (235, and GS7)

Growing

season

GS3

RBA 0”

RBA IO

RBA 20

RBA 30

RBA 40

MSE’

GS5

RBA 0

RBA IO

RBA 20

RBA 30

RBA 40

MSE

GS7

RBA 0

RBA 10

RBA 20

RBA 30

RBA 40

MSE

2.13

1.79

I .62

I .59

I.56

3.87

3.1 I

2.71

2.69

2.60

Ft’ Ft

0.0030

0.0024

0.0016

0.0014

0.0022

0.00 13

0.0248

0.01 75

0.0143

0.0138

0.0132

0.007 I

0.8189

0.0529

0.0401

0.0394

0.0370

0.0197

a

ab

bc

c

ah

a

b

b

bc

c

3.09

2.96

2.65

2.50

3.18

0.75

8.68

8.04

7.27

7.07

7.50

1.57

14.78

13.75

12.42

I I .95

12.02

2.40

a b

abc

bc

c

a

a

ab

bc

bc

c

a

ab

bc

C d

d

‘Within a column, means lbllowed by different letters indicate differences in six after accounting for size

differences at the start ofthe study.

’ RBA 0 40 are treatment levels in all growing seasons.

’ MSE is the root mean square error of the sample (not included for QRCD, because QRCD was calculated

directly from RCBA).

At GS5, differences in basal area and height among

treatments were observed based on analysis of covariance

(table 1). Average diameter (QRCD) was largest in the RBA

0 treatment and smallest in the RBA 40 treatment, and the

remaining treatments ranked in order between them (fig. 3).

The mean RCBA in the RBA 0 treatment was greater than

that in any other treatment. Average height was largest in the

RBA 0 treatment and smallest in the RBA 30 treatment (fig.

4). However, the RBA 40 treatment had the smallest mean

height after compensating for initial differences in height

using the covariance analysis.

At GS7, size differences in mean RCBA and height were

similar to those observed in GS5 (table 1). Average diameter

(QRCD) was largest in the RBA 0 treatment and smallest in

the RBA 40 treatment, and the remaining treatments were

ranked in order between them (fig. 3). The GS7 RCBA in

the RBA 0 treatment was greater than that in any other

treatment. Average height was largest in the RBA 0

treatment and smallest in the RBA 30 treatment (fig.4).

Again, the mean height for the RBA 40 treatment was

greater than the mean height in the RBA 30 treatment but

looked poorer in the analysis after compensating for

unequal mean height at GS3.

Annual Growth Rates Over Both

Measurement Periods

Between GS3 and GS5, the average annual growth rate of

pine differed among treatments (table 2). The RBA 0

treatment exceeded all others in diameter growth and

exceeded all but the RBA 10 treatment in height growth. For

the RBA 10 through the RBA 40 treatments, individual tree

growth in both basal area and height decreased slightly with

increasing RBA, but the differences were not great.

Between GS5 and GS7, the average annual growth rate of

pine also differed among treatments (table 2). Mean RCBA

growth in the RBA 0 treatment was greater than all other

treatments; mean RCBA growth also differed between the

RBA 10 and RBA 40 treatments. Differences in height

growth rates among treatments were also observed. Saplings

in the RBA 0 treatment grew faster than those in either the

RBA 30 or RBA 40 treatments, and saplings in the RBA 10

treatment grew faster than those in the RBA 40 treatment.

5
Table 2-Mean annual growth in quadratic root collar diameter (QRCD), root

collar basal area (RCBA), and height (HT) for two growth intervals--the third to fifth

growing seasons (GS%GS5), and the fifth to seventh growing seasons (CSS-CS7)

Growing

season QRCD RCBA Diff” H T Diff”

GS3-GS5

RBA 0”

RBA IO

RBA 20

RBA 30

RBA 40

MSE’

GS5-GS7

RBA 0

RBA 10

RBA 20

RBA 30

RBA 40

MSE

Ill.

0.69

0.56

0.54

0.55

0.46

0.87

0.66

0.55

0.55

0.52

Ft'

0.0109

0.0075

0.0063

0.0062

0.0055

0.003 I

0.0283

0.01 77

0.01 29

0.0128

0.01 I’)

0.0072

a

h

lx

bc

c

Ft

2.79

2.54

2.3 I

2.29

2.16

0.59

3.00

2.85

2.58

2.44

2.26

0.76

a

ah

b

b

b

“Within a column, means followed by different letters indicate differences in six after accounting

for si/e differences at the start ofthe study.

” RBA 0 40 arc treatment levels in all growing seasons.

’ MSE is the root mean square errorofthe sample (not included Ihr QRCD, because QRCD was

calculated directly from RCBA).

6

a

ah

abc

bc

c

Four-Year Growth Trends

Over the 4-year period, growth in both diameter and height

decreased with increasing RBA (fig. 5). The decrease in

height growth was roughly proportional to RBA; treatments

that differ by 10 ft’ of residual basal area show less than a 10

percent difference in relative height growth.

However, basal area growth was 36 percent less in the RBA

10 treatment than in the RR.4 0 treatment, and 24 percent

less in the RBA 20 treatment than in the RBA 10

treatment-whereas the difference between the RBA 20 and

RBA 40 treatment was less than 10 percent. This basal area

response also appears when mean RCBA growth is

compared between the GS3-GSS growing season and the

GS5-GS7 growing season (fig. 6). In the RBA 20 through

the RBA 40 treatments, RCBA growth in the GS5-GS7

period is double that in the GS3-GS5 period. However,

RCBA growth in the RBA 10 treatment is 2.3 times greater

in the GS5-GS7 interval than in the GS3-GS5 interval and

RCBA growth in the RBA 0 treatment is about 2.6 times as

great.

iscussion

These data indicate that pines planted in the RBA 0

treatment have developed more rapidly than pines planted in

the other treatments on the site during the 4-year period of

this study. This is not surprising; the absence of overstory

hardwoods maximized the availability of site resources to

the underplanted pines. However, with increasing residual

overstory, basal area growth declined disproportionally even

under the RBA 10 treatment. Despite this effect, pines

underplanted beneath the RBA 40 treatment survived and

continued to grow through seven growing seasons.

The trends observed in plantation basal area by treatment

(fig. 2) are attributable to two factors: tree density and

overstory shade. First, the RBA 0 treatment had 18 percent

more trees than the RBA 10 treatment at GS7, which means

it will have higher basal area. But the RBA 0 treatment also

had 44 and 89 percent more basal area at GSS and GS7,

respectively, than the RBA 10 treatment. This suggests the

second factor -that even the initial 10 ft’ of overstory shade

results in a disproportionate decline in basal area.

With respect to individual trees, the data suggest that

retaining overstory trees results in reductions in height more

or less proportional to the overstory basal area. If the annual

height growth for RBA 0 is set at 100 percent, annual height

growth for RBA 10, RBA 20, RRA 30, and RBA 40 is 92,

84, 8 1, and 76 percent, respectively. This trend is roughly a

linear decline in height with increasing overstory basal area.

Conversely, data suggest that for diameter or basal area,

even a slight retention of overstory hardwoods

disproportionally reduces seedling diameter. If the annual

basal area growth for RBA 0 is set at 100 percent, annual
1

QRCD !

2

1.8

1.6

1.4

1.2 g-j

-5

1 .-c

s

0.8 g

0.6 5

%

0.4

20 i 30 ’ 40

RBA treatment

Figure 5 Mean annual sapling quadratic root collar diameter (QRCD) growth and mean annual sapling height (HT)

growth by overstory residual basal area (RBA) treatment OYW the 4-year study.

3-G%

GS5GS7

RBA treatment

Figure 6 Mean annual root collar basal area growth by overstory residual basal area (RBA) treatment ftx the

first 2-year period (GS3 GS5) and the second 2-year period (CSS-GS7) ofthc study.
basal area growth for RBA 10, RBA 20, RBA 30, and RBA

40 is 64,49,48, and 44 percent, respectively. The first 10 ft’

of basal area account for a greater reduction in seedling

diameter than the additional 30 ft’ of overstory basal area.

If no catastrophic event intercedes, these trends in height and

diameter growth should continue until the planted seedlings

reach crown closure, if not longer. If fast early growth is the

most important goal for plantation establishment on this type

of site, no overstory hardwoods should be retained.

However, at this point in the development of the stand,

experimental evidence does not suggest that seedlings

underplanted beneath 40 ft? per acre of residual overstory

will become suppressed to the point of mortality. Because

volume is a function of basal area and height, this study

suggests that early volume development would be much

greater in the RBA 0 treatment than in any of the partial

overstory retention treatments.

None of the underplanted plots can be considered a failure 4

years after treatment and seven growing seasons after

underplanting beneath the residual overstory. In all plots,

individual tree height growth exceeds 2 ft per year, and both

height and basal area growth were greater in the GSS-GS7

period than in the GS3-GS5 period. The basal area trends

suggest that pines in the RBA 0 and RBA 10 treatments are

in a better position to remain competitive with hardwoods in

the developing stand than those in the RBA 40 treatment.

Future monitoring of this study would provide more

information about whether the underplanted pines in the

higher RBA plots can maintain adequate growth through the

first decade of development.

Literature Cited

SAS institute Inc. 1989. LAS/STAT user’s guide, Version 6. 4”’ ed. Gary,

NC: SAS Institute Inc. Vol. 2. 846 p,

Snedecor, G.W.; Cochraa W.G. 1967. Statistical methods, 6”‘cd. Ames,

IA: The Iowa State University Press. 593 p.

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advent

sheville,

gri~ulture, Forest Service,

28802