United States

Department of

Agriculture

Forest Service

Southern 

Research Station

Research Paper

SRS—50

Site Establishment Practices 

Influence Loblolly Pine Mortality 

Throughout the Stand Rotation

Felipe G. Sanchez and Robert J. Eaton
The Authors:

Felipe G. Sanchez, Research Chemist, and Robert J. Eaton,

Biological Support Scientist, U.S. Department of Agriculture, 

Forest Service, Southern Research Station, Research Triangle 

Park, NC 27709. 

Cover photo: A long term soil productivity study installation on the

Croatan National Forest, New Bern, NC. 

July 2010

Southern Research Station

200 W.T. Weaver Blvd.

Asheville, NC 28804
Site Establishment Practices Influence 

Loblolly Pine Mortality Throughout the 

Stand Rotation

Felipe G. Sanchez and Robert J. Eaton

Abstract

During a rotation, land managers need to estimate yields, update inventories, 

and evaluate stand dynamics. All of these factors in land management 

are heavily influenced by tree mortality. Tree mortality can, in turn, be 

influenced by land management practices from the inception of the stand and 

throughout the rotation. We describe the impact of organic matter removal 

and soil compaction during stand establishment on loblolly pine (Pinus taeda 

L.) mortality over the course of 15 years. We also describe the impact of 

understory control throughout the study on tree mortality. In this study, soil 

compaction did not impact tree mortality at any time. Removal of surface 

organic matter impacted tree mortality, with the least intensive practices (bole 

only removal) resulting in the greatest tree mortality. This effect, observed 

very early in the rotation, was probably due to immobilization of essential 

nutrients. Control of the understory became a significant factor to tree 

mortality late in the rotation as intraspecific competition for light became an 

important consideration. Site amelioration, via regional Best Management 

Practices, negated the impact of competition control on mortality. 

Keywords: Compaction, loblolly pine, mortality, organic matter.

Introduction

The long-term soil productivity (LTSP) experiment was 

designed to investigate the impact of soil compaction and 

surface organic matter removal during a harvest on net 

ecosystem productivity (NPP). The core experiment was 

installed on more than 140 sites across the United States and 

Canada, making it the world’s largest coordinated effort to 

examine the relationship between soil processes and NPP. 

Since the establishment of the first LTSP installations in 

1991 and over the course of the experiment, cooperators 

from the Forest Service, U.S. Department of Agriculture, 

National Forest System, Forest Service Research and 

Development, British Columbia Ministry of Forests, Canadian 

Forest Service, and various university and private industry 

cooperators have published more than 300 manuscripts that 

describe various aspects of the experiment, including soil 

processes, soil macro- and micro-fauna, and above- and 

belowground biomass. 

The information gained from the LTSP experiment has been 

invaluable in advancing understanding of the soil processes 

that impact stand productivity. However, a critical piece of 

information that has not received as much attention is the 

treatment impacts on tree survival. Understanding treatment 

impacts on tree survival is a complicated matter because of 

multiple factors that can directly or, in combination with 

other factors, indirectly impact survival. For example, trees 

weakened by one stressor, such as insects or disease, may 

be susceptible to additional stressors, such as drought and 

storm damage, resulting in tree mortality. Neither stressor in 

isolation directly led to tree mortality but the combination of 

stressors was instrumental in the death of the tree. Researchers 

have linked management practices, such as soil compaction 

(Lockaby and Vitrine 1984) and herbicide application 

(Edwards 1994, Lauer and others 1993, Wittwer and others 

1986), to tree mortality. But linking specific treatments 

to mortality over the duration of a study is not always 

straightforward. Nevertheless, land managers need to predict 

tree survival throughout a rotation in order to estimate yields, 

update inventories, and evaluate stand dynamics (Amateis and 

others 1997). 

The LTSP installations in North Carolina utilize loblolly pine 

(Pinus taeda L.) plantations. Loblolly pine is an important 

tree species in the Southeastern United States, covering, 

together with shortleaf pine (Pinus echinata Mill.), more than 

20 million ha, approximately a quarter of the South’s total 

timberland area (Conner and Hartsell 2002). Research from 

the North Carolina LTSP installations has yielded information 

on loblolly pine tree growth (Sanchez and others 2006a, 

Scott and others 2004), root development (Ludovici 2008), 

soil nutrient contents and cycling (Butnor and others 2006, 

Li and others 2003, Sanchez and others 2006a, 2006b, Scott 

and others 2004), soil fauna (Eaton 2006, Eaton and others 

2004), and fusiform rust incidence (Eaton and others 2006). 

The objectives of this manuscript are to describe the general 

treatment impacts on loblolly pine survival throughout 
2

the life of the Croatan LTSP installations and to identify 

potential contributing factors. This information will help 

land managers understand the interrelationship of factors that 

affect their yields.

Materials and Methods

Study Site

The LTSP installations are located in the Croatan National 

Forest in Craven County, NC, in the Atlantic Coastal Plain. 

The study site has a mean annual temperature of 16 oC and 

receives 1360 mm of precipitation. The installations were 

established in 1991 as a component of the LTSP cooperative. 

Prior to study installation, the site was occupied by a 60-yearold pine-hardwood forest.

Treatments

Prior to harvesting the existing stand, three replicated blocks 

were established. Block 1 was located in the Goldsboro soil 

series (fine-loamy, siliceous, thermic Aquic Paleudults), while 

blocks 2 and 3 were in the Lynchburg series (fine-loamy, 

siliceous, thermic aeric Paleudults). Within each block, nine 

experimental plots (0.4 ha) were established representing 

a 3 x 3 factorial of organic matter removal and compaction 

treatments. Three levels of organic matter removal were 

applied: merchantable bole removal (OM0), whole-tree 

removal (OM1), and whole-tree plus forest floor removal 

(OM2). Additionally, three levels of soil compaction were 

applied: no compaction (C0), moderate compaction (C1), 

and severe compaction (C2). The C2 treatment was intended 

to approximate 80 percent of growth limiting bulk density 

in the upper 0 to10 cm of the mineral soil (Daddrow and 

Warrington 1983). The C1 treatment was intended to be 

mid-range between the C0 and C2 treatments. To achieve the 

desired compaction levels, plots were either not compacted 

(C0 treatment); compacted with one pass of a smooth drum 

vibrator roller without vibration (C1 treatment); or compacted 

with two passes of a smooth drum vibrator roller with full 

vibration (C2 treatment). The plots were then planted with 

loblolly pine bare-root seedlings on a 3 x 3 m spacing, and 

half of each plot was maintained with native understory 

(U+) while the other half had complete understory control 

(U-) where the understory was controlled by mechanical and 

chemical means.

One of the hypotheses of the LTSP experiment is that 

any negative effects due to the main treatments could be 

ameliorated by the Best Management Practices (BMPs) of 

the region (Powers and others 1990). To test this hypothesis, 

an additional plot, referred herein as the ameliorated plot, 

was installed in each block. The ameliorated plot replicated 

the OM1C1 experimental treatment followed by bedding 

and a one-time fertilization application of 224 kg ha-1 of 

triple superphosphate, which supplied 45 kg P. The splitplot subtreatment of understory control was applied as in 

the core LTSP experimental design. Over the course of the 

study, inventories of tree mortality have been documented 

for each plot. 

Statistical Analysis 

Analysis of variance (ANOVA) using the GLM procedure for 

split-plot design (SAS 9.2 2002) was used to test for treatment 

effects on tree mortality. Soil compaction and organic matter 

removal treatments were the main effects while the understory 

control treatments were the split plot effects. Differences 

between treatments were determined significant at α ≤ 0.05 

level using the Tukey’s Paired Comparison Procedure. 

Student’s t-test comparison was used to detect differences in 

the ameliorated plots and the OM1C1 experimental plots for 

each understory control split plot subtreatment. 

Results

Soil compaction did not impact tree survival in any of the 

years measured (fig. 1); however, significant impacts were 

detected for the organic matter removal and understory control 

treatments. Since the significance levels for the compaction 

treatments (or any compaction interaction) were not 

significant (no value was lower than p = 0.53) for any year, the 

compaction values were pooled to increase the power of the 

statistical tests for the organic matter removal and understory Percent survival 

No compaction

Moderate compaction

Severe compaction

100

95

90

85

80

75

0 2 4 6 8 10 12 14 16

Age (year)

Figure 1—Percent survival of loblolly pine trees over 15 years for soils 

with three levels of compaction.
3

control treatments. Using the pooled data, the OM0 plots had 

significantly lower survival than the OM1 and OM2 treatment 

plots early in the rotation but percent survival between the 

treatments was not significantly different after 5 years (fig. 2) 

(table 1). The presence of an understory (U+ plots) resulted 

in lower survival than plots without an understory (U- plots); 

however, this effect was significant only in two years of the 

study (fig. 3) (table 1). The interaction between organic matter 

removal and understory control treatments was nearly, but not 

quite, significant at the p ≤ 0.05 level after 6 years (table 1).

For the comparison between the ameliorated and experimental 

(OM1C1) treatment plots, tree survival was profoundly 

impacted by the understory. When the understory was present, 

the ameliorated plots had significantly (p ≤ 0.05) greater 

survival than the experimental plots for the majority of the 

time (fig. 4a). However, when the understory was absent, 

there was no significant difference in survival between the 

ameliorated and experimental plots for any of the years 

measured (fig. 4b).

Discussion

Soil Compaction

The lack of significant compaction effects on tree mortality 

(fig. 1) (table 1) is not surprising since compaction did not 

impact tree growth (Sanchez and others 2006a), insect and 

disease incidence (Eaton and others 2006), and soil nutrients 

(Powers and others 2005, Sanchez and others 2006a, 2006b). 

The reason for the lack of a compaction effect is because soil 

bulk density on these plots recovered rapidly (Page-Dumroese 

and others 2006). The only exception to this general trend was 

the observation that compaction decreased root biomass and 

increased the ratio of aboveground to belowground biomass 

(Ludovici 2008). While the decreased root biomass may lead 

to future productivity declines, it did not impact tree mortality 

to date. Since soil bulk density recovered rapidly and survival 

of loblolly pine, after crown closure, is determined primarily 

by intraspecific competition (Amateis and others 1997), it is 

unlikely that soil compaction (or its interaction) during stand 

establishment will become a factor later in the rotation.

Organic Matter Removal

 

With a lack of any significant or nearly significant compaction 

effect (table 1), we decided to pool the compaction plots in the 

Percent survival 

100

95

90

85

80

75

0 2 4 6 8 10 12 14 16

Age (year)

Bole only removal

Whole tree removal

Whole tree plus forest 

floor removal

Percent survival 

100

95

90

85

80

75

0 2 4 6 8 10 12 14 16

Age (year)

U+

U_

Figure 2—Percent survival of loblolly pine trees over 15 years for soils 

with three levels of surface organic matter levels.

Year OM U OM x U

1 <0.0001 0.67 0.89

2 0.00017 0.061 0.56

3 0.00085 0.033 0.34

4 0.008 0.47 0.25

5 0.17 0.96 0.14

6 0.33 0.29 0.075

7 0.43 0.27 0.054

8 0.35 0.14 0.05

10 0.27 0.12 0.066

15 0.36 0.011 0.15

Table 1—Significance levels (p values) for the organic 

matter (OM) removal and understory control (U) 

treatments and their interaction (OM x U) for each year

Figure 3—Percent survival of loblolly pine trees over 15 years for plots 

with no or complete understory control, U+ and U-, respectively.
4

examination of the effects of organic matter removal and 

understory control on tree mortality. Analysis of the pooled 

data showed that early in the rotation, the OM0 plots had the 

greatest mortality with no difference in tree mortality between 

the OM1 and OM2 plots; however, there was no significant 

difference (p > 0.05) between the organic matter removal 

treatments after year 4 (fig. 2). Assuming minimal betweenplot variation in planting stock or planting efficiency, early 

rotation mortality is primarily determined by competition for 

site resources, e.g., nutrients and water (Amateis and others 

1997, Haywood and others 1997). The OM0 treatment plot 

is the least invasive organic matter removal treatment and 

should result in a condition favoring soil moisture retention. 

Additionally, removal of the bole results in relatively little 

nutrient loss from the forest floor as compared with the more 

intensive organic matter removal treatments (Powers and 

others 2003). With little soil moisture or nutrients loss in 

the OM0 treatment, then why was early rotation mortality 

greatest on these treatment plots? The answer may lie in 

the availability of nutrients. Although most of the nutrients 

are contained in the limbs and foliage (Powers and others 

1990), these nutrients are sequestered in the forest floor and 

are essentially unavailable until the forest floor decomposes 

(Gholz and Fisher 1982). Nutrient immobilization would be 

the greatest on the least disruptive plots (OM0 treatments). 

The more intensive practice of removing parts or the entire 

forest floor (OM1 and OM2 treatments, respectively) would 

create conditions (increased soil temperature and mineral 

soil disruption) that would result in increased carbon and 

nutrient mineralization (Jenkinson 1990). Thus the OM1 and 

OM2 treatments provided the highest level of soil nutrients 

to young seedlings despite the fact that they contained the 

lowest absolute amount of nutrients (Sanchez and others 

2006a, 2006b). By the time the forest floor decomposed and 

the nutrients contained in it were made available, the trees had 

developed enough that nutrient availability had a small impact 

on mortality.

Understory

The presence or absence of an understory did not significantly 

impact tree survival for the majority of the study and was only 

significant in the 3rd and 15th year (fig. 3) (table 1). In the third 

year of the study, the trees were damaged by the herbicide 

application that was exasperated by below-average growing 

season precipitation (Goodwin 1989). This damage was also 

exhibited in stunted tree growth on the U- plots during that 

year. In the 15th year, tree mortality was higher in the U+ plots. 

After crown closure, tree mortality is primarily influenced 

by intraspecific competition (Amateis and others 1997). 

Consequently, the understory is now becoming an important 

factor for tree mortality at these installations.

Ameliorated Plots

Soils on the Atlantic Coastal Plain are historically nutrient 

deficient and often times poorly drained. Thus, the silvicultural 

practices for this region generally call for bedding and 

phosphorus fertilization as a means of achieving high loblolly 

pine productivity. Bedding increases soil aeration and soil 

macropore space, and lowers soil moisture, thus creating an 

environment for favorable root development for the newly 

planted seedlings (Will and others 2002). Additionally, the soil 

disruption of the bedding results in increased mineralization of 

soil organic matter and consequently nutrient release (Edwards 

1994). This coupled with inorganic fertilization should provide 

Percent survival 

100

95

90

85

80

75

70

0 2 4 6 8 10 12 14 16

Age (year)

Experimental

Ameliorated

Percent survival 

0 2 4 6 8 10 12 14 16

Age (year)

100

95

90

85

80

75

70

Experimental

Ameliorated

Figure 4—Percent survival of loblolly pine trees over 15 years for 

ameliorated and experimental plots when (A) the understory was 

present and (B) the understory was absent.

(A)

(B)
5

optimal conditions for seedling establishment. The availability 

of essential nutrients limits growth of pine forests in the 

Southern United States (Allen 1987, Gurlevik and others 

2003) and maintaining adequate levels of soil nutrients is the 

dominant factor for maintaining high productivity within the 

natural range of loblolly pine (Jokela and Long 2004). 

For the first 3 years of the study, amelioration of the 

experimental plots (OM1C1) by bedding and fertilization did 

not impact tree survival, suggesting that the conditions on the 

experimental plots were sufficient for seedling establishment. 

However, after the third year, amelioration did result in lower 

tree mortality but the effect was dependent on the presence 

(fig. 4a) or absence (fig. 4b) of an understory. Competition for 

site resources is the primary factor for tree mortality before 

crown closure (Amateis and others 1997) and this effect is 

manifested early (fig. 4a) in this study. As the tree developed 

larger and deeper root systems, this intraspecific competition 

became less of a factor and is demonstrated by the parallel 

path of tree mortality for the ameliorated and experimental 

plots (fig. 4a). The abrupt increase in tree mortality in the 

experimental plots compared with the ameliorated plots may 

be a response to the drought that occurred at these sites in 

year 3. Bedding and fertilization increase root development 

(Haywood and others 1997, Will and others 2002) making the 

trees on the ameliorated plots better equipped to survive the 

drought.

Conclusion

The site preparation methods employed at the Croatan 

National Forest LTSP installations impacted loblolly pine 

mortality throughout the life of the study. Soil compaction 

was the only treatment that did not impact tree mortality. This 

is consistent with the observed lack of a significant treatment 

effect on the majority of the metrics, i.e., tree growth and soil 

nutrient status, at the installations. Although the forest floor 

regulates soil moisture and temperature levels and contains 

considerable amounts of essential nutrients, its retention 

actually increased tree mortality during the first few years of 

the study. This can be attributed to nutrient immobilization 

within the forest floor prior to its decomposition. Finally, the 

presence of an understory also impacted tree mortality because 

of competition for site resources such as water and nutrients 

(early in the study) or light (after crown closure). Regional 

BMPs negated the negative effect of understory on tree 

mortality. Consequently, land managers should be cognizant 

that the silvicultural practices during stand establishment can 

have long-term consequences on yield estimates, annual tree 

inventories, and stand dynamics.

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Sanchez, Felipe G.; Eaton, Robert J. 2010. Site establishment practices influence loblolly pine 

mortality throughout the stand rotation. Res. Pap. SRS–50. Asheville, NC: U.S. Department of 

Agriculture Forest Service, Southern Research Station. 6 p.

During a rotation, land managers need to estimate yields, update inventories, and evaluate stand 

dynamics. All of these factors in land management are heavily influenced by tree mortality. Tree 

mortality can, in turn, be influenced by land management practices from the inception of the stand 

and throughout the rotation. We describe the impact of organic matter removal and soil compaction 

during stand establishment on loblolly pine (Pinus taeda L.) mortality over the course of 15 years. 

We also describe the impact of understory control throughout the study on tree mortality. In this 

study, soil compaction did not impact tree mortality at any time. Removal of surface organic 

matter impacted tree mortality, with the least intensive practices (bole only removal) resulting 

in the greatest tree mortality. This effect, observed very early in the rotation, was probably due 

to immobilization of essential nutrients. Control of the understory became a significant factor 

to tree mortality late in the rotation as intraspecific competition for light became an important 

consideration. Site amelioration, via regional Best Management Practices, negated the impact of 

competition control on mortality. 

Keywords: Compaction, loblolly pine, mortality, organic matter.
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