Department of Effects of Seed Production, Seedbed 

Agriculture Condition, and Over~toty Basal Area 

Forest Service on the Establishment of Shortleaf 

Southern Forest 

Experiment Station Pine Seedlings in the Ouachita Mountains 

New Orleans, 

Louisiana 

Michael G. Shelton 

Research Paper 

SO-293 

September 1995 
SUMMARY 

First-year seedling establishment was determined across an array of seedbed conditions and overstory basal areas in stands of shortleaf pine (Pinus 

echinata Mill.) and hardwoods following the initial harvest implementing 

uneven-aged silviculture. Seedbed treatments were mineral soil, partial litter, 

undisturbed litter, and enhanced litter, and overstory treatments consisted of 

three hardwood basal areas (0,15, and 30 ft2/acre) retained with 60 ft2/acre of 

pine. The seedbed treatments were established over a 3-year period, which 

permitted evaluation of the seedlings resulting from a seed-crop failure, a below average seed crop, and an above-average seed crop. Seedling density was 

positively related to the level of seed production and negatively related to litter depth and overstory basal area. However, some seedlings became established even in the deepest litter, indicating that the barrier presented by litter 

is not complete. Results indicate the importance of regulating overstory basal 

area in the application of uneven-aged silviculture in stands featuring shortleaf pine. When total overstory basal area was within the guidelines for 

uneven-aged stands (45 to 75 ft2/acre), seedbeds of mineral soil and partial 

and undisturbed litter resulted in ample regeneration even with seed crops 

that were slightly below regional averages. Composition of the overstory basal 

area apparently did not strongly affect initial seedling establishment. However, results of this study apply only to initial seedling establishment in areas 

having sparse ground vegetation and should not be extended to longer time 

periods or other conditions. 
Effects of Seed Production, Seedbed Condition, 

and Overstory Basal Area on the Establishment of 

Shortleaf Pine Seedlings in the Ouachita Mountains 

Michael G. Shelton 

INTRODUCTION 

Natural regeneration is a viable alternative for establishing shortleaf pine (Pinus echinata Mill.) stands 

in the Ouachita Mountains (Baker 1992, Lawson 

19861, and it may be the best technique for landowners who have limited capital (Willett and Baker 1991) 

and who desire the favorable visual properties of partial cutting techniques (Stignani 1986). Successful 

natural regeneration of shortleaf pine depends on obtaining acceptable levels of seeds and limited resources-such as light, water, and nutrient-and 

having suitable seedbed conditions. Each of these factors is under a different degree of silvicultural control. For example, seedbed conditions can be readily 

modified by site preparation, such as controlled burning and mechanical treatments. To some degree, seed 

production can be enhanced by retaining additional 

seed trees, but production principally varies in response to uncontrollable fluctuations in weather and 

pest populations. The availability of light and water 

is controlled by regulating the stocking of the merchantable stand through harvesting and by controlling understory composition through vegetation 

management, but uncontrollable variation in moisture 

can also strongly affect seedling survival during critical stages of development (Larson and Smith 1969, 

Trousdell and Wenger 1963). In addition, all of the 

determinants of natural regeneration interact, and 

deficits in one may be compensated for by surpluses 

in another (Becton 1936, Grano 1949, Trousdell1950). 

However, unacceptable conditions in a single determinant, such as a poor seed crop or a severe drought, 

may result in a regeneration failure regardless of the 

suitability of the other factors. 

Investigations of the seedbed requirements of shortleaf pine exist (Ferguson 1958, Haney 1962, Wood 

1939,Yocom and Lawson 19771, but most of these studies have contrasted only disturbed and undisturbed 

seedbeds or have evaluated only a single seed crop. In 

this study, three of the determinants of natural regeneration of shortleaf pine-seed production, seedbed condition, and overstory competition-were evaluated. This research utilized an ongoing study designed 

to test the limits for hardwood retention within an 

uneven-aged silvicultural system that features shortleaf pine as the dominant species. 

METHODS 

Study Site 

The study was located in the Winona Ranger District of the Ouachita National Forest in Perry County, 

AR. Plots were oriented along an east-west ridge, 

which is typical of the physiography of the Ouachita 

Mountains. Elevations ranged from 640 to 790 R above 

sea level, a 150-ft difference in relief. Blocks were located on the following slope positions: lower, middle, 

and upper north slope and the upper south slope. 

Slopes ranged from 8 to 21 percent, and aspects ranged 

from north to northwest on the north-slope positions 

and from southeast to southwest on the south-slope 

position. 

Soils of the study area are mapped as the Carnasaw 

and Pirum series, both Typic Hapludults. These are 

well-drained, moderately deep soils that developed in 

colluvium and residuum weathered from sandstone 

and shale. Natural fertility and organic matter are 

low, and the soils are strongly acidic. 

Site index for shortleaf pine averaged 57 ft at 50 

years and ranged from 53 to 64 ft, which is typical of 

upland sites in the Ouachita Mountains (Graney 

1992). The lower north slope was slightly higher in 

site index than the other three slope positions (61 versus 56 ft). Site index averaged 53 ft at 50 years for 

white oak (Qwrcus alba L.) and 54 R for black oak (Q. 

velutina Lam.). 

Vegetation in the study area was typical of much of 

the forested landscape in the Ouachita Mountains, 

where upland forests are dominated by shortleaf pine 

and mixed oaks (Guldin and others 1994). Before treatment implementation, overstory basal area (trees 

Michael G. Shelton is a research foreater at the Forestry Sciences Laboratory, U.S. Department ofAgriculture, Forest Senice, Southern 

Forest Experiment Station, Monticello, AR 71656, in cooperation with the School of Forest Resources and the Arkansas Agricultural 

Experiment Station, University ofArkaneas at Monticello, Monticello, AR 71656. 
23.6 inches in d.b.h.) in this mature, second-growth 

shortleaf pine-oak stand averaged 90 ft2/acre for shortleaf pine and 32 ft2/acre for hardwoods. Oaks accounted for 84 percent of the total hardwood basal 

area. White oak was the most prevalent hardwood, 

with lesser amounts of post oak (Q. stellata Wang.), 

black oak, blackjack oak (Q. rnarilandica Muench.), 

and southern red oak (Q. falcata Michx.). Overstory 

pines and oaks in the initial stand ranged in age from 

30 to over 110 years (Shelton and Murphy 1991). However, most of the pines were 50 to 80 years old, and 

the oaks were 40 to 70 years old. The scarcity of young 

trees in the overstory indicated that regeneration and 

its subsequent development in both the pines and oaks 

had been limited for 30 to 40 years of stand development before study implementation. 

The precipitation and temperature that occurred 

during the study were fairly typical of the normal conditions for this locale. Annual precipitation and temperature average 52 inches and 61.6 OF, respectively, 

at the nearby Alum Fork Weather Station (U.S. Department of Commerce, NOAA 1992). Precipitation occurring from Odober 1989 through September 1990 totaled 

59.3 inches, and temperatures averaged 63.6 OF. 

Comparable values from October 1990 through September 1991 were 60.5 inches for precipitation and 

61.6 OF for temperature. Precipitation that occurred 

during the period of germination was above normal 

for the 1989 seed crop and below normal for the 1991 

seed crop (fig. 1). Monthly precipitation was about onethird below normal during June for the 1989 seed crop 

and during August for the 1991 seed crop. 

Study Design and Treatments 

Overstory !lkeatments.-This seedbed study was 

superimposed within an ongoing study testing the limits for hardwood retention within an uneven-aged silvicultural system featuring shortleaf pine as the 

dominant species (Shelton and Baker 1992, Shelton 

and Murphy 1991). Twelve square, 1.6-acre plots were 

installed in 1988, and each consisted of an interior 

0.5-acre plot that was surrounded by a 1.1-acre isolation strip (58.2 ft wide). The interior plot and isolation strip were treated in an identical manner. The 

basal area of overstory pines (23.6 inches in d.b.h.1 

was reduced to 60 ft2/acre in all plots. Hardwood treatments were basal areas of O,15, and 30 ft2/acre. Overstory treatments were assigned in a randomized, 

complete block design with four replications for each 

treatment. 

The pine harvest was implemented using the basal 

area-maximum diameter-quotient method of singletree selection (Farrar 1984). Targets were 60 ft2/acre 

for basal area, 18 inches for maximum diameter, and 

a quotient of 1.2 for 1-inch diameter classes. Targets 

10 

I" 

0 6 z 

4 

2 

O OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT 

PRECIPITATION 

TEMPERATURE 

DISPERSAL STRlTlFlWTlON GEAMlN4llON ESTAWUSUMEM 

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT 

MONTH 

ESTABUSHMEM 

Figure 1.- Precipitation and temperature occurring during the dispersal, stratification, germination, and establishment 

of the 1989 and 1991 seed crops compared to normal 

values. Data are for the nearby Alum Fork Weather Station (U.S. Department of Commerce, NOAA 1989-921. 

DlSPERSLL GWUlWlTON 

for maximum diameter and quotient were followed as 

closely as feasible because the stand lacked a balanced 

reverse-J structure. Hardwood retention favored the 

higher quality white and red oaks; these were typically the largest hardwoods in the study area. 

Plots were harvested from December 1988 through 

early March 1989 using mules to skid logs to landings. Because no local markets existed for hardwoods, 

all hardwoods 21 inch in d.b.h. that were not specifically designated for retention were killed with 

stem-injected triclopyr during April 1989. Herbicide 

treatments were applied by contract crews following 

STR*TIFICATIMI 

label directions; research crews did some followup injection work. 

Seedbed !lkeatments.-Along the interior boundary of the isolation strip of each 1.6-acre plot, three 

0.01-acre areas were selected where the existing forest floor had not been disturbed by 1ogging.Areas were 

randomly assigned for establishment of seedbed treatments in 1989,1990, and 1991. 

During the early summer of the year designated for 

establishment, the ground vegetation was controlled 

within the area by a foliar application of glyphosate 

(2 percent in an aqueous solution) to the point of satu- 
ration. This treatment was done to ensure that initial 

levels of ground vegetation were equal in all overstory 

hardwood treatments and in all years. During the late 

summer, four contiguous, square subplots (10 ft on a 

side) were established and randomly assigned a seedbed treatment. The location of each subplot was permanently marked for identification. The following 

seedbed conditions were then created using hand tools: 

(1) mineral soil-All organic materials were removed, which exposed the mineral soil surface. 

This treatment simulated areas where the forest floor had been scraped away by logging traffic and/or mechanical site preparation. 

(2) partial litter--The surface litter, consisting 

principally of foliage deposited during the previous year, was removed, which exposed the 

older and more highly decomposed material. 

This treatment represented areas where litter 

had been disturbed but not completely removed 

during logging and/or site preparation. 

(3) undisturbed litter-The existing forest floor 

was not disturbed. 

(4) enhanced litter-The existing forest floor was 

enriched by the litter removed in seedbed treatments (1) and (2). This condition represented the 

piling of litter along the edges of skid trails, 

where the forest floor had been scraped from the 

center and displaced along the edges. 

Welve of these seedbed-treatment areas were established each year over a 3-year period, resulting in 

a total of 36 seedbed-treatment areas or 144 individual 

seedbed subplots. Subplots installed in 1989 and 1990 

were prepared for the 1991 seed crop by treatment 

with a second application of glyphosate during the 

early summer of 1991 to control the ground vegetation that had developed since installation. The subplots were inspected later that summer to ensure that 

all existing pine seedlings had been killed. 

Measurements 

The forest floor material present before establishing seedbed treatments was determined by collecting 

three 0.9-ft2 samples from just outside each seedbedtreatment area (a total of 36 samples per year). The 

relatively fresh litter that was principally deposited 

during the previous year (the litter layer) was collected 

separately from the older and more highly decomposed 

litter existing below (the fermentation layer). Very 

little humus material (dark, amorphous organic matter) was observed in these forest floors because of the 

rapid incorporation of organic material within the 

mineral soil; this condition was also noted by Shelton 

and Lawson (1994) elsewhere in the Ouachita Mountains. The observed boundaries between forest floor 

layers within these small sampling frames were also 

complied with when implementing the seedbed treatments. Samples were weighed after drying to a constant weight at 167 OF. Litter depth was also measured 

on each side of the sampling frames. 

In 1989,1990, and 1991, pine seed production was 

monitored from October through February of the next 

year in four 0.9-ft2 seed traps (Cain and Shelton 1993) 

in each 0.5-acre plot. Seed traps were located about 

30 ft from the plot center in a square pattern and were 

about 100 ft from the outer boundary of the 1.6-acre 

plot. Collections were made during the middle and the 

end of the October-to-February period. Viability was 

determined by splitting seeds and inspecting the contents (Bonner 1974). Seeds with 111, firm, undamaged, 

and healthy tissue were judged to be potentially viable and were tallied as sound. 

During the fall of the year following establishment, 

pine seedlings were counted on a milacre plot (3.72 ft 

in radius) centered in each of the seedbed subplots. 

Heights of the two tallest shortleaf pine seedlings, if 

present, were measured. Coverage of understory vegetation was ocularly estimated on milacre plots in 1990 

and 1992 for the following groups: grasses, herbs, 

vines, shrubs, and hardwoods and for total coverage. 

Coverage was not evaluated in 1991. Pine seedlings 

were counted and measured for all installations in 

1992. 

The basal areas of pines and hardwoods for each of 

the 36 seedbed-treatment areas were determined using a 10-basal area factor prism. Litter depth was 

again evaluated in 1992 by taking 20 measurements 

across each of the 144 milacre plots where seedling 

counts were made. The seedbed treatments were 1 to 

3 years old at this time. 

Data Analysis 

Analysis of variance for a split-plot, randomized 

complete block design was used to compare the effects 

of the overstory hardwood treatments and the seedbed treatments on the number of l-year-old seedlings 

resulting from each seed crop. Overstory hardwood 

treatments constituted the main effects, and seedbed 

treatments were the subeffects. Differences among 

treatment means were isolated by the Ryan-EinotGabriel-Welsch Multiple Range Test (P=0.05). This test 

is one of the most powerful step-down, multiple-range 

tests available, and it controls the experiment-wise 

error rate (SAS Institute 1989). 

A modeling approach was employed to combine the 

effects of all independent variables. This approach 

allowed greater flexibility in accounting for yearly 

variation in seed production and local variation in forest floor conditions and overstory basal areas. The 

concept of a potential and modifier function, which 

has widespread application in growth modeling, was 
employed to predict the number of 1-year-old seedlings resulting from a seed crop as follows: 

Regeneration =Potential x Modifier (1) 

Seed production represents the potential for obtaining regeneration in equation (1) that is modified by 

existing environmental conditions. The logistic function, which is constrained between the interval 0 to 1, 

was used as a modifier function. Several notential 

functions were examined, but none was found to be 

better than a fixed proportion of the sound-seed production. This conclusion may reflect the fact that 

evaluated seed crops had a fairly narrow range. For 

example, the best seed crop evaluated averaged 

180,000 sound seeds per acre, whereas over a million 

seeds may be produced during bumper years (Shelton 

and Wittwer, in press). The resulting function was: 

R = boS/{l + exp (bl + b2 D + b, P + b, H)) (2) 

where R is the number of 1-year-old seedlings per acre, 

S is the number of sound seeds per acre the preceding 

year, D is the depth of the litter in inches just before 

seed dispersal, P is the pine basal area in square feet 

per acre, H is the hardwood basal area in square feet 

per acre, and b,'s are coefficients to be estimated. 

Data for fitting equation (2) were the seedling counts 

made each year following the annual installation of 

the seedbed treatments. Data also included the 1992 

seedling counts of seedbed treatments installed in 

1989 and 1990, when seedbed conditions had existed 

for 1 and 2 years, respectively, before dispersal of the 

1991 seed crop. Litter depths for these treatments were 

evaluated in 1992 (the year following the 1991 seed 

crop) but were adjusted for a 1-year change in depth 

using equations that express the change in depth 

through time (equations (7) and (8) in the results and 

discussion section). Seed ~roduction was the mean of 

the four seedtraps located in each 0.5-acre plot, and 

pine and hardwood basal areas were those determined 

by prism for the specific seedbed-treatment area. Coefficients were calculated by nonlinear least squares 

regression using the SAS procedure MODEL (SAS 

Institute 1988). Variables were eliminated from the 

full model if their coefficient did not significantly differ from zero at a probability level of 50.05. Transforming the independent variables improved the fit 

index of equation (2). However, determining specific 

values for exponents was compromised by convergence 

problems in the regression procedure. Thus, square 

and square root transformations of independent variables were tested. If the fit index was improved, the 

transformation was retained; otherwise, it was 

dropped. 

The following function was used for fitting the relationship between other response variables, such as 

seedling height and coverage of competing vegetation, 

and litter depth and overstory basal areas: 

where Y is the specific response variable, and the other 

symbols and fitting procedures are as previously 

described. 

RESULTS AND DISCUSSION 

Independent Variables 

Annual seed production averaged 180,000, 3,000, 

and 74,000 sound seeds per acre for the 1989, 1990, 

and 1991 seed crops, respectively (table 1). The overall average was 86,000 sound seeds per acre. No significant differences were observed among the overstory 

hardwood treatments. Shelton and Wittwer (in press) 

reported that the long-term annual seed production 

for shortleaf pine within the Ouachita and Ozark 

Mountains averaged about 100,000 sound seeds per 

acre. Thus, the 1989 seed crop was above average, and 

the 1991 seed crop was slightly below average. The 

seed-crop failure in 1990 was a fairly common occurrence in shortleaf pine stands; about one-half of the 

seed crops within the region over a 9-year period were 

classified as failures (Shelton and Wittwer, in press). 

The undisturbed forest floor of this stand weighed 

15,000 lWacre and had a mean depth of 0.9 inch. These 

values are typical of pine-hardwood stands within the 

Ouachita Mountains, for which Shelton and Lawson 

(1994) reported a regional mean weight of 17,000 lb/ 

acre and a depth of 1.2 inches. The slightly higher 

regional means may reflect the difference in sampling 

times of the two studies; Shelton and Lawson sampled 

after the fall litter nulse. while forest floors were 

sampled before the pulse in this study. About one-third 

of the forest floor weight was in the litter layer or the 

relatively fresh material in the upper portion of the 

forest floor. A weak but significant negative relationship occurred between the undisturbed forest floor 

weight and the hardwood basal area (correlation coefficient=4.37; P>F=0.02). A similar relationship reported by Shelton and Lawson (1994) was attributed 

to the higher decomposition rates of hardwood foliage 

when compared to pine foliage. 

Establishing the seedbed treatments redistributed 

the original forest floor from one treatment to another, 

but mean values were not affected because all of the 

material was used. Litter weight for seedbed treatments ranged from 0 to nearly 60,000 lblacre, and 

depth ranged from 0 to 3.7 inches (table 2). Establishing the seedbed treatments greatly increased the variability of litter weights and depths, and coefficients of 
Table 1.- Annual production of shortleafpine seeds by overstory hardwood treatment after 

implementing uneuen-aged silviculture in shortleafpine-hardwood stands 

Seed Hardwood basal area* Mean 

crop - square 

year 0 ft2/acre 15 ft2/acre 30 ft2/acre ermrt P>F 

~p 

------ Thousands of sound seeds per acre ------ 

1989 197 144 200 6.7839 0.58 

1990 3.1 3.1 3.1 3.4837 1.00 

1991 75.1 75.1 70.9 7.7838 0.97 - 

*No significant differences occurred among row means according to the Ryan-Einot-GabrielWelsch Multiple Range Test (P=0.05). 

+In this table and elsewhere in this paper, values for mean square error are expressed in exponential notation; for example, 1.2334 = 1.23 x lo4 = 12,300. 

variation were increased by four times when compared 

to the forest floors before disturbance. The mean 

weights of the seedbed treatments aRer establishment 

were 0,9,700, 15,300, and 36,300 lblacre for the mineral soil, partial litter, undisturbed litter, and enhanced litter treatments, respectively. Means for depth 

of the four seedbed treatments were 0, 0.6, 0.9, and 

2.1 inches, respectively. 

Overstory basal areas in the vicinity of the 

seedbed-treatment areas averaged 62 and 25 @/acre 

for pines and hardwoods, respectively (table 2). The 

mean basal area for pines was very close to the postharvest target of 60 ft2/acre in the residual stand, but 

the mean basal area for hardwoods was higher than 

the treatment mean of 15 ft2/acre (that is, the mean of 

0,15, and 30 ft2/acre). The reason for this discrepancy 

is not known. It may have been due to locating the 

seedbed-treatment areas in the undisturbed portion 

of the stand; such areas may have had a higher hardwood component, which might have resulted in less 

logging disturbance. The wide range in basal areas of 

both pines and hardwoods indicates the high spatial 

variability that occurs in stands managed under guidelines for uneven-aged stands; canopy gaps of 0.1 to 

0.2 acre in size can occur when such guidelines are 

implemented. There was no significant correlation 

between the basal areas of pines and hardwoods. 

Seedling Density of Treatments 

The density of seedlings resulting from a seed crop 

was affected by the size of the seed crop and the seedbed conditions when seeds were dispersed (fig. 2). The 

above-average 1989 seed crop resulted in an average 

of 3,040 seedlings per acre across all seedbed treatments, and the below-average 1991 seed crop resulted 

in an average of 790 seedlings per acre for the O-yearold seedbed treatments. These values provided a mean 

seedling-to-seed ratio of 1.7 percent for the 1989 seed 

crop and 1.1 percent for the 1991 seed crop. The seedcrop failure that occurred in 1990 resulted in no seedlings. This result was expected because the carry over 

of pine seeds from one year to the next has been observed to be virtually nil (Barnett and McGilvray 1991, 

Little and Somes 1959). The mineral-soil seedbed 

yielded the greatest number of seedlings, generally 

followed by the partial-litter seedbed. Differences between the undisturbed and enhanced litter treatments 

Table 2.- Seedbed conditions and ouerstory basal areas existing after treatment implementation - 

Range 

Mean Standard 

value deviation Minimum Maximum 

- ----.------.-----------...... Seedbed conditions ....-.------------------....-- 

Litter weight (1Wacre) 15,320 14,130 0 59,990 

Litter depth (inches) 0.87 0.82 0 3.70 

-------.......----.-------.. Overstory basal area% ----.--.....-----.--.-.-...... 

Pine (ft2/acre) 62 24 20 120 

Hardwood (R2/acre) 25 23 0 80 

*Determined by prism at each seedbed-treatment area. 
1989 SEED CROP / 0-YEAR-OLD SEEDBED 1991 SEED CROP / 0-YEAR-OLD SEEDBED 

10 

1 

g 2 

< 

. 0.5

u, 

0.2 

3 0.1 

5 0.05 

0 1991 SEED CROP / I-YEAR-OLD SEEDBED 1991 SEED CROP / PYEAR-OLD SEEDBED 

10 - 5 

u) 

P>F = 0.0002 

C3 2 

1 4 

4 0.5 $ 0.2 

0.1 

0.05 

MINERAL PARTIAL UNDIST. ENHANCED MINERAL PARTIAL UNDIST. ENHANCED 

SOIL LITTER LlllER LlllER SOIL UllER LITER LIllER 

SEEDBED CONDITION 

Figure 2.- Density of 1-year-old shortleafpine seedlings by seedbed treatment, seed crop, and seedbed age (time between 

treatment establishment and dispersal of the seed crop). Note that a logarithmic scale is used for seedling 

density. Bars with different letters are significantly different according to the Ryan-Einot-Gabriel-Welseh 

Multiple Range lkst P=0.05). 

were not significant for any seed crop or seedbed age. 

The favorable effects of a mineral-soil seedbed diminished through time; thus, the 2-year-old mineral-soil 

seedbed did not significantly differ from the other seedbed treatments for the 1991 seed crop. This occurs because the overstory trees had continued t~ produce litter, 

which eventually covered the exposed mineral soil. 

The effects of overstory hardwood treatments are 

shown in table 3. Means for overstory treatments can 

be compared across all seedbed treatments because 

no significant interactions were observed between 

seedbed treatments and overstory hardwood treatments. For each seed crop and seedbed age, the seedling density was consistently greatest in the overstory 

treatment with no hardwoods, but differences were 

significant for only the 1991 seed crop and the l-yearold seedbed. The lack of consistent significant differences reflects, to some degree, local variation in the 

basal area of overstory trees in the vicinity of the 

seedbed-treatment areas. This observation suggests 

that a modeling approach using the basal areas determined at each seedbed-treatment area would be 

more sensitive to this local variation. 

Modeling Seedling Density 

The following equation combines the effects of sound 

seed production (S in number per acre), litter depth 

(D in inches), and basal area of pines and hardwoods 

(P and H, respectively, in square feet per acre) on the 

density of 1-year-old seedlings (R in number per acre): 

R = 0.1182s 1 {1+ exp (-3.151 + 1.0740°.5 + 

0.5870p.5 + 0.1364@.5)) (4) 

where number of obsenrations was 240, root mean 

square error was 1,183, fit index was 0.41, and mean 

R was 1,000 seedlings per acre. The coefficients of all 

independent variables were significant at the 0.05 
probability level, and the model explained 41 percent for a bumper seed crop of over a million sound seeds per 

of the variation in the resulting regeneration. acre, when the seedling-bseed ratio was 20 percent. 

Seedling densities calculated from equation (4) are 

plotted in figure 3 for a seed crop of 100,000 sound 

seeds per acre and a reasonable range in values for 

litter depth and pine and hardwood basal areas. A 

single level of seed production is adequate for demonstrating trends in equation (4) because seedling density varies directly with the number of sound seeds; 

that is, doubling seed production doubles the number 

of seedlings and halving seed production halves the 

number of seedlings. This observation may have been 

an artifact of the relatively narrow range of seed proSeedling density decreased with increasing litter 

depth (fig. 3). Grano (1949) reported a similar decline 

in seedling density with litter depth in uneven-aged 

loblolly-shortleaf pine stands in southern Arkansas, 

and Shelton (1995) described a similar relationship 

between the density of loblolly pine seedlings and litter weight. In addition, the decline in seedling density with increasing litter has commonly been observed 

for many species with small, wind-disseminated seeds. 

Seedling density decreases because: (1) litter modifies environmental conditions while seeds are dormant 

duction sampled over the 3-year monitoring period. and germinating and as emerging seedlings become 

However, some support for this finding exists in data established, (2) litter acts as a physical barrier that 

reported by Cain (1991) for five pine seed crops and interferes with development of the radicle andlor 

the resulting seedlings in uneven-aged pine stands in shoot, (3) litter teems with life, some of which may be 

southern Arkansas. Calculated values for the unfavorable to dormant and germinating seeds, and 

seedling-to-seed ratios of a mineral-soil seedbed, (4) litter has distinctive chemical properties, some of 

coupled with hardwood control, were fairly narrow which may be unfavorable to germination and seed- 

(ranging from 5 to 7 percent) for all seed crops except ling establishment (Shelton 1995). 

0 FT 2/ ACRE HARDWOODS 

( FT~/ ACRE ) 

@) ----- 

.......... I 

... . . - 

- --. .... - - -----__ ....... ... ... "......... ---------______. .................... 

....... ......... I.I.I.I.I.I 

15 FT 2/ ACRE HARDWOODS 

L 0 

0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 

LITER DEPTH ( INCHES ) LITER DEPTH ( INCHES ) 

u E 

O 0.0 0.5 1.0 1.5 2.0 2.5 

LITER DEPTH ( INCHES ) 

Figure 3.-Effects of litter depth and overstory basal areas of pines and hardwoods on the number of I-year-old 

shortleafpine seedlings when the seed crop of the preceding year was 100,000 sound seedsper acre. Values 

were calculated from equation (4) in text. 
Table 3.- Effects of overstory hardwood treatments on the number of I-year-old shortleafpine 

seedlings by seed crop and seedbed age 

Seed 

mop 

year 

Seedbed 

-- 

age+ 

----- years ---- 0 

0 

1 

2 

Hardwood basal area* 

0 ftz/acre 15 ft2/acre 30 fta/acre 

. . -- - - - - Number of seedlings per acre -------- 5,560 1,000 2,560 

1,810 190 375 

1,940a 190b 125b 

1,120 125 0 

Mean 

square 

error P>F 

*Row means followed by different letters are significantly different according to the RyanEinot-Gabriel-Welseh Multiple Range Test (P=0.05). 

+Time from treatment establishment until seed dispersal. 

The decline in seedling density with increasing overstory basal areas reflects a response of seedlings to 

levels of overstory competition for light and moisture. 

Guidelines for uneven-aged pine stands specify the 

upper and lower limits for basal area to keep overstory competition within acceptable bounds. Guidelines specify: (1) retention of a minimum of 45 ft2/acre 

of pine basal area to provide acceptable rates of merchantable growth and (2) never letting basal area exceed a maximum of 75 ft2/acre during a cutting cycle 

because regeneration will be adversely affected (Baker 

and others, in press). Although results of this study 

are too short term to confirm these guidelines, they 

do indicate that moderate litter depth (up to 1 inch) 

will not be a serious detriment to securing an adequate 

density of 1-year-old seedlings iE (1) seed production 

is at least near the regional average, (2) the total overstory basal area is reduced to specified guidelines and 

structural targets are imposed, and (3) understory 

hardwoods are controlled and ground vegetation is 

sparse. Because of the short-term nature of these results, they should not be extended to other situations 

or extrapolated to longer time periods. 

Results seem to indicate that the composition of the 

overstory is not so critical in determining seedling 

density after the first growing season. This finding 

has also been observed in other studies in which the 

basal areas of overstory pines and hardwoods were 

reduced to near specified guidelines and understory 

hardwoods were controlled (Shelton and Murphy 1993, 

in press). Pine seedlings are apparently able to become established and subsist under fairly dense canopies for several years before dying (Becton 1936, 

Wahlenberg 1960). This observation suggests that 

newly established pine seedlings are moderately shade 

tolerant but become shade intolerant as the seedlings 

age. Bormann (1956) reported, for example, that the 

photosynthetic efficiency of loblolly pine seedlings at 

low light intensities declined substantially as secondary characteristics developed. 

The subsequent development and survival of seedlings is not expected to continue where a substantial 

hardwood component exists. Hardwoods have been 

observed to produce about twice the amount of shade 

per unit of basal area as do pines because of their large 

crowns, broad leaves, and generally shorter heights 

(Tappe and others 1993). Based on this generality, an 

overstory composed of 60 ft2/acre of pines and 7.5 ft2/ 

acre of hardwoods would produce the equivalent shade 

of a pure pine overstory at the upper limit specified 

for uneven-aged pine stands (that is, 75 ft2/acre). Thus, 

the opportunity for hardwood retention within unevenaged shortleaf pine stands under single-tree selection 

appears to be low (Shelton and Murphy 1993). 

The seedling-to-seed ratios for the values presented 

in figure 3 range from nearly 0 to 4 percent. (The nurnber of seedlings in thousands is numerically equal to 

the seedling-to-seed ratio in figure 3 because it is for 

100,000 sound seeds per acre.) These values are fairly 

typical of the seedling-to-seed ratios reported elsewhere for natural regeneration of shortleaf pine. Yocom 

and Lawson (1977), for example, reported a seedlingto-seed ratio of 0.4 percent for undisturbed areas 

within a seed tree stand in the Ouachita Mountains 

and 1.0 percent for disturbed areas. Haney (1962) reported a seedling-to-seed ratio of 0.3 percent for undisturbed areas and 2.0 percent for scarified areas in 

a sawtimber stand. In poorly stocked shortleaf pine 

stands, the seedling-to-seed ratio was 0.4 percent for 

untreated areas, 1.3 percent for burned areas, and 2.9 

percent for areas where a brushcutter had been used 

(Maple 1965). In seed tree stands, Dale (1958) reported 

a mean seedling-to-seed ratio of 6.4 percent when the 

seedbed was bulldozed, 4.2 percent when disked, and 

1.4 percent for the untreated control. 

Seedling Height 

The height of 1-year-old seedlings (HT in feet) was 

negatively correlated with the basal area of pines and 
hardwoods (P and H, respectively, in square feet per 

acre), as shown in the following equation: 

HT = exp (-0.7937 - 0.008401P - 0.01439H) (5) 

where number of observations was 71, root mean 

square error was 0.131, fit index was 0.29, and mean 

HT was 0.23 ft. The effect of litter depth was not significant. Calculated values for equation (5) are plotted in figure 4. The greatest heights occurred at the 

lowest basal areas, and then heights declined as the 

basal areas of both pine and hardwood increased. 

Heights ranged from 0.16 to 0.33 R, which are typical 

of 1-year-old shortleaf pine seedlings on these poor 

sites (Shelton and Murphy, in press). This relationship simply reflects the increased competition from 

the overstory as basal area increases. Overstory competition is perhaps most easily demonstrated by the 

reduction in light intensity that results from increasing basal area. Tappe and others (1993), for example, 

reported a strong negative relationship between light 

intensity in the understory and the basal area of overstory pines and hardwoods. However, this relationship 

also undoubtedly involves competition for other resources such as soil moisture. Retained overstory trees 

in uneven-aged stands clearly suppress the growth of 

regeneration, but traditional guidelines for unevenaged pine stands are designed to provide an acceptable environment for the survival and development of 

sufficient regeneration to sustain future harvests 

(Baker and others, in press). 

Understory Vegetation 

Coverage of herbs, vines, shrubs, and hardwoods 

averaged 3.7, 1.6, 0.3, and 1.1 percent, respectively, 

and did not vary significantly with litter depth or overPINE BASAL 

AREA ( FT2/ ACRE) 

80 - - - - - - . 

..._ -. .... -. -_ ', ... -. ... .... -. -_ - - -. - - --__ -..., *-. 

... - - ..., 

-. ..., ,-., 

0.151, , , , , , , , , ,I 

0 5 10 15 20 25 30 

HARDWOOD BASAL AREA ( FT ACRE) 

Figure 4.- Effects ofpine and hardwood basal areas on the height 

of I-year-old shortleafpine seedlings. Values were calculated from equation (5) in text. 

story basal area. By contrast, coverage of grasses (G 

in percent) was significantly related to litter depth (D 

in inches) and basal areas of pines and hardwoods (P 

and H, respectively, in square feet per acre) as shown 

in the following equation: 

G = exp (3.166 - 1.2690 - 0.0102lP - 0.04273H) (6) 

where number of observations was 192, root mean 

square error was 5.52, fit index was 0.25, and mean G 

was 2.8 percent. The effects of litter depth and overstory basal areas were negative, and the equation explained 25 percent of the variation in the coverage of 

grasses. Calculated values for equation (6) are plotted in figure 5. Coverage of grasses was about 15 percent for a mineral-soil seedbed with 45 ft2/acre of pine 

basal area and no hardwoods. Values declined to near 

zero at the greater litter depths and the higher basal 

areas. Grasses observed in these seedbeds developed 

mostly from seeds because the herbicide application 

killed existing grasses. Seeds were stored in the soil 

and litter and were dispersed into the treated areas 

from outside the seedbed subplots. 

The similar response of shortleaf pine seedlings and 

grasses indicates that these competitors have similar 

environmental requirements for germination and establishment. ~hese results suggest that the favorable 

effects of a mineral-soil seedbed on the establishment 

of shortleaf pine seedlings need to be considered within 

the context of the entire understory community, especially where ground vegetation is not controlled with 

herbicides as was done in this study. Shelton (1995), 

for example, reported that loblolly pine seedlings developing on a mineral-soil seedbed were smaller than 

those developing on a seedbed where litter was present 

and attributed this result to the suppression of competing vegetation by litter. Thus, when the pine seed 

crop is good, litter seedbeds may actually be favorable 

to natural regeneration because of a reduction in herbaceous vegetation. 

Changes in Seedbed Conditions Through Time 

Litter production continues in stands that have been 

partially harvested; thus, the exposure of mineral soil 

through stand disturbance is short lived. In such areas, litter accumulates until a steady state occurs between rates of litter production and subsequent 

decomposition of the litter (Olson 1963). By contrast, 

areas with enhanced litter will decompose at higher 

rates than are supported by litter production, and 

depth of the enhanced litter will decrease through time 

until a steady state is attained. 

Data collected in this study allowed calculating the 

recovery rates of the seedbed treatments. In the developed equations, D is the current depth expressed 
as a percentage of the depth before disturbance, T is 

the number of years since disturbance occurred, and 

His the hardwood basal area in square feet per acre. 

The equation for the mineral-soil seedbed is as follows: 

D = exp (3.22 + 0.206T + 0.00629H) (7) 

where number of observations was 36, root mean 

square error was 12.9, fit index was 0.43, and mean D was 45.3 percent. The equation for the enhanced-litter 

seedbed is: 

D = exp (5.20 - 0.0983T + 0.0027- (8) 

where number of observations was 36, root mean 

square error was 29.4, fit index was 0.27, and mean D 

PINE : HARDWOOD 

BASAL AREA ( FT '1 ACRE 1 

-45:O 

------ 75 : 0 

. . , . . . . . . . . 45 : 30 

--- 75 : 30 

0.0 0.5 1 .O 1.5 2.0 2.1 

LITER DEPTH ( INCHES ) 

was 16' percent' The above for the Figure 5.-Effects of litter depth and overstory basal areas on the 

partial-litter seedbed was not significant. For both the coverage ofgrasses I year after disturbance. Values were 

mineral-soil and enhanced-litter seedbeds, the hard- calculated from equation (6) in text. 

wood basal area simificantlv - influenced the recoverv 

rate, but the pine basal area was not significant. 

Values calculated from equations (7) and (8) are plot- ship were positive, and thus, an increase in hardwood 

ted in figure 6. For the mineral-soil seedbed, an in- basal area retarded the recovery to undisturbed 

crease in hardwood basal area and time since depths. This finding is consistent with the concept of 

higher litter production being associated with the disturbance increased the recovery toward the depths greater hardwood basal areas. existing before disturbance. After 3 years of recovery, 

litter depths in areas with exposed mineral soil were 

within 45 to 60 percent of the original depths before 

disturbance. The positive effects of hardwood basal CONCLUSIONS AND MANAGEMENT 

area probably reflect the increased litter production IMPLICATIONS 

associated with the higher basal areas. ~bdwoods in 

the Ouachita Mountains have been observed to pro- Initial establishment of naturally regenerated shortduce more litter per unit of basal area than shortleaf leaf pine aRer implementing uneven-aged silviculture 

pine (Shelton and Lawson 1994). in pine-hardwood stands is affected by seed producDepth of the enhanced-litter seedbed decreased tion, seedbed conditions, and overstory basal areas. 

through time (fig. 6). After 3 years, depths were 135 The greatest uncertainty in the determinants examto 150 percent of those occurring before disturbance. ined in this study is seed production, which varies 

The effects of hardwood basal area on this relation- widely due to uncontrollable factors. However, seed 

MINERAL SOIL ENHANCED LITER 

180 

1 70 

160 

150 

140 

YEARS AFTER DISTURBANCE 

Figure 6.- Recovery of mineml-soil and enhanced-litter seedbeds following disturbance. Values were calculated from 

equations (7) and (8) in text. 
production was adequate for obtaining regeneration 

in 2 of the 3 years of monitoring. The presence of litter inhibits the germination and early establishment 

of shortleaf pine, a finding that has strong support 

from the existing literature for many species with 

small, wind-disseminated seeds. Litter affects the complicated processes of germination and establishment 

through a number of environmental, physical, biological, and chemical mechanisms. However, the inhibiting effects of litter are far from complete; some 

seedlings in this study became established on the deepest litter seedbeds tested. Apparently, some suitable 

microsites for germination and establishment occur 

even within a generally unfavorable seedbed. 

Although litter reduces seedling establishment, this 

relationship must be considered within the goal of securing a sufficient number of seedlings to successfully 

regenerate the stand. Results of this study indicate 

that seedbed conditions after implementing 

uneven-aged silviculture in shortleaf pine stands will 

not seriously limit adequate regeneration if: (1) seed 

production is at least near the regional average; (2) 

overstory basal areas are within guidelines for 

uneven-aged pine stands, and structural targets are 

imposed; (3) competition from understory hardwoods 

and ground vegetation is low; and (4) other environmental factors, such as moisture and temperature, are 

within acceptable limits. Seedbed conditions after 

harvesting of uneven-aged stands will be very heterogeneous and will range from exposed mineral soil to 

deep accumulations of litter and logging debris 

(Shelton and Wittwer 1992). Thus, all the seedbed 

conditions tested in this study will exist after harvesting; exposed mineral soil will occur where traffic 

scrapes litter away, and heavy litter accumulations 

will occur where this litter is deposited. The areal extent of each seedbed condition and the spatial distribution are the critical features affecting regeneration. 

In most cases, the seedbed conditions after harvesting of uneven-aged stands will be suitable for shortleaf regeneration, and additional disturbance does not 

seem justified for seedbed preparation, although treatments may be needed to control competing understory 

vegetation. In addition, conventional seedbed-preparation treatments, such as prescribed burning and 

disking, are difficult to apply in uneven-aged stands 

because of the presence of residual trees and advanced 

regeneration. However, pre-harvest prescribed burning might be beneficial to control ground vegetation 

and reduce litter depths if few seedlings are present 

in the size classes susceptible to fire damage. 

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Some of the factors affecting the establishment of shortleaf pine regeneration after the initial treatments implementing uneven-aged silviculture in shortleaf pine-hardwood stands are quantitatively described 

in this publication. 

Keywords: Forest floor, natural regeneration, pine-hardwood stands, 

Pinus echinata Mill., site preparation, uneven-aged silviculture.