United States

Department of

Agriculture

Forest Service

Southern

Research Station

General Technical

Report SRS-58

Local Soils Information Needed to

Define the Root Zone in Process

Models on the Gulf Coastal Plain

Mary A. Sword and Allan E. Tiarks
The Authors

October 2002

Southern Research Station

P.O. Box 2680

Asheville, NC 28802

Mary A. Sword and Allan E. Tiarks, Research Plant Pysiologist and Soil Scientist,

respectively, USDA Forest Service, Southern Research Station, Alexandria Forestry

Center, 2500 Shreveport Highway, Pineville, LA 71360.
1

Abstract

We combined published information and our own experimental results

from the Gulf Coastal Plain to evaluate how soil aeration and strength

interact with loblolly pine root growth. Our results demonstrate that soil

aeration and strength differ by soil series and year and are subject to

vertical and horizontal spatial variation. Comparison of loblolly pine root

phenology and seasonal redox potential indicates that oxygen is frequently

limiting to new root growth. The strength of these soils may also restrict

loblolly pine root growth. Physiological process models that predict

southern pine productivity should apply accurate calculations of plantavailable soil in simulations of potential root zone water storage and plantavailable water. We propose a conceptual root zone submodel that predicts

the volume of plant-available soil with soil aeration, strength, and water

retention by horizon. Model parameters are components of the Natural

Resources Conservation Service (NRCS) Soil Interpretation Record (SIR)

database collected across the United States.

Keywords: Soil aeration, soil strength, spatial variation, redox potential,

root phenology, root zone sub-model.

Introduction

While the demand for pine timber in the United States is

increasing, some reports show a decline in net annual pine

productivity in the South (Bechtold and others 1991, Ruark

and others 1991, U.S. Department of Agriculture, Forest

Service 1989). Land use changes have contributed to this

decrease with hardwood encroachment on nonindustrial

private land and conversion of timberland to crop, pasture,

and urban uses (U.S. Department of Agriculture, Forest

Service 1989). To meet present and future timber demands,

land managers must maximize southern pine productivity

while sustaining site quality.

Process models that predict forest productivity and

sustainability in response to climate and site variables are

valuable forest management tools. Existing models express

forest productivity relative to the horizontal crown position,

age, and retention of foliage (McMurtrie and others 1990,

McNulty and others 1996, Wang and Jarvis 1990). Models

with parameters that predict root zone size may also assist

forest managers.

Process Model Applications

Physiological process models combine important relationships among climate, physiology, and resource availability

Local Soils Information Needed to Define

the Root Zone in Process Models on the

Gulf Coastal Plain

Mary A. Sword and Allan E. Tiarks

to predict carbon fixation and subsequent forest

productivity. New tools have led to predictions of tree and

stand productivity in response to environmental change. For

example, TREGROW evaluates the effects of environmental

stress on potted red spruce (Picea rubens Sarg.) saplings

(Weinstein and Beloin 1990). Simulations provide

information on ozone-induced imbalances between the

carbon, water, and nutrient dynamics of woody plants

(Weinstein and Beloin 1990, Weinstein and others 1991).

Other process models serve as intensive forest management

tools. The process model BIOMASS predicts the

productivity of managed Monterey pine (Pinus radiata D.

Don.) plantations based on simulated water and carbon

balances (McMurtrie and Landsberg 1992, McMurtrie and

others 1990). Using SPM (slash pine model), Cropper and

Gholz (1993) successfully simulated the stem growth of 22-

year-old slash pine (P. elliottii Engelm. var. elliottii) on a

Florida flatwoods site. Both BIOMASS and SPM provide

information on the carbon dynamics of managed pine in

response to fertilization (Cropper and Gholz 1993,

McMurtrie and Landsberg 1992).

Process models can predict forest productivity at the

regional level with fewer parameters and can represent more

generalized ecophysiological relationships than tree- and

stand-level process models (Landsberg and Gower 1997).

For example, when Running and Coughlan (1988) used the

process model FOREST-BGC to predict forest productivity

at seven sites with different climates, the ranking of

simulated hydrologic and carbon balances corresponded to

measurements in mature forests near each site. After

validation at the ecosystem level, PnET-II predicted regional

water and carbon responses to elevated temperature and

carbon dioxide (Aber and Federer 1992, Aber and others

1995). Such process models have potential to predict forest

responses to climate change.

Successful validations suggest that process models can

answer contemporary forest management questions. At the

same time, forest managers should address examples of

failed or questionable validations before using process

models to guide decisions. For example, McMurtrie and

others (1994) found good correlation between the observed

and predicted growth of Monterey pine saplings in Australia

and New Zealand with BIOMASS. In the same analysis,

however, they noted discrepancies between measured and
2

predicted aboveground net primary productivities of 2-yearold Monterey pine in New Zealand, 28-year-old red pine (P.

resinosa Ait.) in Wisconsin and 22-year-old slash pine in

Florida. They suggested that BIOMASS was a poor

predictor of slowly growing stands. With PnET-II during

periods of water deficit in northern red oak (Quercus rubra

L.) -red maple (Acer rubrum L.) and in red pine stands, Aber

and others (1995) found that rates of carbon exchange did

not correspond with predicted rates. One explanation for the

failed validation was the assumption that available water

was constant across the region.

Role of the Root Zone in Process Models

Field capacity defines the potential amount of water stored

in the root zone, and both field capacity and permanent

wilting point define the amount of plant-available water in

the root zone. Generally, field capacity is -0.004 to 0.03

MPa, and permanent wilting point is -1.5 MPa (Marx 1988,

McMurtrie and others 1990, McNulty and others 1996, Soil

Survey Staff 1996). With this information, plant-available

water simulated by a water balance submodel is applied to

process model predictions of carbon fixation and allocation

(Aber and Federer 1992, Aber and others 1995, McMurtrie

and others 1990, McNulty and others 1996, Running and

Coughlan 1988). BIOMASS, for example, calculates net

photosynthesis and stomatal conductance as a function of

plant-available water (McMurtrie and Landsberg 1992,

McMurtrie and others 1990). FOREST-BGC quantifies leaf

water potential as a function of actual soil water content

divided by soil water content at field capacity. These values

help determine canopy conductance and net photosynthesis

(Running and Coughlan 1988). PnET calculates net

photosynthesis as a function of water stress (Aber and

Federer 1992; Aber and others 1995; McNulty and others

1996, 1997). Specifically, PnET and PnET-II use plantavailable water to quantify actual transpiration; they express

water stress as the product of actual and potential

transpiration (Aber and Federer 1992, Aber and others

1995). In PnET-IIS, water stress is a function of average

growing season soil water divided by soil water content at

field capacity (McNulty and others 1996, 1997).

Conceptual Root Zone Submodel

Forest productivity is dependent on the amount of plantavailable mineral nutrients and water in the soil and on their

acquisition by the absorbing roots of trees. Phosphorus and

nitrogen deficiencies and severe water deficits commonly

occur on the Gulf Coastal Plain (Allen and others 1990,

Dougherty 1996). Root system proliferation plays a critical

role in soil resource uptake throughout this physiographic

region.

Soil physical properties influence the size and shape of tree

root systems and, therefore, the volume of soil available for

water and mineral nutrient uptake (Bennie 1996, Rogers and

Head 1969). Seasonal precipitation and soils with low

porosities and water-holding capacities, aquitards, and high

bulk densities all constrain root proliferation. Specifically

during periods of elevated precipitation, water table

dynamics may result in limited oxygen availability for root

metabolism. Alternatively with reduced precipitation, soil

strength may increase to levels that mechanically impede

root elongation.

With experimental results and published information, we

propose a conceptual submodel that uses limiting soil

physical properties to define the root zone. Once developed,

this model will broaden the range of sites where

physiological process models are valid and can increase the

accuracy of physiological process model predictions. To do

so, we must define the root zone, potential root zone water

storage, and plant-available water based on seasonal and

spatial changes in soil physical properties. Although we base

our root zone submodel on observations from the Gulf

Coastal Plain, it can serve as a template to characterize the

root zone in other physiographic regions.

Materials and Methods

At 10 installations of the North American Long-Term Soil

Productivity (LTSP) Program and at 1 site on the Bienville

National Forest in central Mississippi, investigators have

intensively monitored the soil environment. At another

location on the Gulf Coastal Plain, we are investigating the

ecophysiology and growth of plantation loblolly pine (P.

taeda L.).

Soil environment studies—Of the LTSP Gulf Coastal Plain

installations, three each are located on the Davy Crockett

National Forest in eastern Texas and the DeSoto National

Forest in eastern Mississippi, and four are located on the

Kisatchie National Forest in central Louisiana (Tiarks and

others 1993, 1997). Installations initially supported mature

loblolly pine forests, ages 39 to 55, that were uniform in

stand structure, productivity potential, aspect, topography,

and soil series. We retained a nonharvested portion of each

stand adjacent to all installations. The Texas and Mississippi

installations were on Kurth fine sandy loam and Freest fine

sandy loam soil series, respectively (table 1). The Louisiana

installations were on Glenmora silt loam, Malbis fine sandy
3

loam, Mayhew silty clay loam, or Metcalf silt loam soil

series. On the Bienville National Forest in central

Mississippi, the soil environment was an Ichusa silty clay

loam. These soil series, which commonly support loblolly

pine, are characterized by a flat to gently sloping

topography, a relatively low saturated hydraulic

conductivity, and an aquitard that restricts percolation.

At all LTSP installations, we applied three levels each of

organic matter removal and soil compaction to nine

treatment plots (0.4 ha) in a 3- by 3-factorial design (Tiarks

and others 1993). In Mississippi and Texas, installations on

the same soil series represent three blocks of a randomized

complete block design. To eliminate organic matter, we

removed stems, whole trees, or all aboveground biomass

plus the forest floor. On plots that were stem-only harvested,

we uniformly distributed logging slash. Levels of soil

compaction were severe, moderate, and no compaction

(Tiarks and others 1993). We planted installations at 2.5 by

2.5 m with 10 genotypes of container-grown loblolly pine.

To monitor soil redox potential (Eh), we equipped an

adjacent nonharvested area at one LTSP installation each in

Texas and Mississippi and at all installations in Louisiana

between 1990 and 1997 (Tiarks and others 1995). We also

monitored Eh at the Bienville National Forest location

between 1995 and 1999.

At three microsites 2 m apart at each LTSP location, we

installed a platinum electrode at two depths (50 and 100 cm)

to measure Eh. At the Bienville National Forest, we placed

six electrodes randomly at a 25-cm depth within a 25-m2

area. We calibrated readings to reflect the millivolt

difference between platinum and standard hydrogen

electrode readings by adding 244 mV to electrical current

readings (Tiarks and others 1995). Because soil pH at the

depth of sensor placement was approximately 5.0, we did

not adjust the data to account for the effect of soil pH on Eh

(Tiarks and others 1995). We used data recorded by a data

acquisition unit at 5-minute intervals to calculate hourly

means.

We determined bulk densities of the Malbis soil at one

Louisiana LTSP installation by coring to a depth of 180 cm

(Veihmeyer 1929), with six samples taken at 30-cm

increments in depth. We cored at five randomly selected

locations in each plot and in an adjacent nonharvested area.

At the three LTSP installations in Mississippi, we measured

soil strength using a recording penetrometer equipped with a

30o

 conical tip, 10 mm2 in area. We recorded resistance at

10-cm increments to a depth of 60 cm and averaged 35

measurements from random locations in each plot by depth

increment.

Ecophysiology study—At the ecophysiology study site on

the Kisatchie National Forest in central Louisiana (Sword

and others 1998a), the soil is a Beauregard silt loam

(table 1). In 1981, we planted container-grown loblolly pine

from a genetically unimproved source at a 1.8- by 1.8-m

spacing. In 1988, we established twelve 0.06-ha plots, 13 by

13 trees each. In early 1989, we applied 2 levels each of

thinning (not thinned: 2,990 trees per ha; thinned: 749 trees

per ha) and broadcast fertilizer application with

diammonium phosphate (none; 135 kg N and 150 kg P per

ha) in a factorial design to 3 replications. In early 1995, we

rethinned the 13-year-old thinned plots (18.1 m2

 per ha)

from below (not thinned: 40.4 m2 per ha; thinned: 15.4 m2

per ha) and refertilized the fertilized plots by broadcast

application of urea, monocalcium phosphate, and potash

(200 kg N, 50 kg P, and 50 kg K per ha).

Based on the influence of topography on soil drainage, we

chose two replications as blocks for measurement of root

system growth, soil environment, and crown physiology. At

2- to 4-week intervals in 1993 through 1998, we quantified

root elongation in three (1993) to five (1994–98) vertical

Plexiglas rhizotrons (0.3 by 35.4 by 76 cm) per plot (Sword

and others 1998a, 1998b). We expressed net lateral root

elongation as mm per dm2

 per day. In 1994 through 1998,

we measured the volumetric water content of the soil at 2-

week intervals by time domain reflectometry (TDR) with

one sensor placed at a depth of 15 cm through a port in each

of three randomly chosen rhizotrons per plot. We calibrated

the TDR measurements to gravimetric soil-water content

values. Radio towers and wooden walkways provided access

to the canopy. In 1998, we identified three south-facing

branches in the upper crown of three dominant or codominant trees per plot of one replication and measured net

photosynthesis at 1100, 1300, and 1500 hours between June

and November (Tang and others 1999).

Results and Discussion

Soil Aeration

In normal years, Gulf Coastal Plain soils are characterized

by aquic conditions (Soil Survey Staff 1998). In aquic soils,

during periods of high precipitation and low evapotranspiration, low saturated hydraulic conductivity results in

episaturation and the formation of a perched water table

within 200 cm of the soil surface (Soil Survey Staff 1998).

In Gulf Coastal Plain soil series descriptions, the high or
4

Table 1—Descriptive characteristics relative to water table dynamics of eight Gulf Coastal Plain soil series

High water Depth to

Soil series Location Family Texture class table depth Drainage class gray mottlesa

 - - - - cm - - - - - - - - cm - - - -

Beauregard Kisatchie National Fine-silty, siliceous, thermic Silt loam 46 – 91 Moderately well 30 – 76

Forest, Louisiana Plinthaquic Paleudults drained

Freest DeSoto National Fine-loamy, siliceous, Fine sandy loam 46 – 76 Moderately well 46 – 76

Forest, Mississippi active, thermic Aquic drained

Paleudalfs

Ichusa Bienville National Fine, smectitic, thermic Silty clay loam 46 – 91 Somewhat poorly 10 – 28

Forest, Mississippi Aquic Dystuderts drained

Glenmora Kisatchie National Fine-silty, siliceous, thermic Silt loam 61 – 91 Moderately well 25 – 76

Forest, Louisiana Glossaquic Paleudalfs drained

Kurth Davy Crockett Fine-loamy, siliceous, Fine sandy loam 76 –102 Moderately well 46 – 76

National Forest, semiactive, thermic drained

Texas Oxyaquic Glossudalfs

Malbis Kisatchie National Fine-loamy, siliceous, Fine sandy loam 76 –127 Well drained or 76 –180

Forest, Louisiana subactive, thermic Plinthic moderately well

Paleudults drained

Mayhew Kisatchie National Fine, smectitic, thermic Silty clay loam 0 – 30 Poorly drained 0 – 10

Forest, Louisiana Chromic Dystraquerts

Metcalf Kisatchie National Fine-silty, siliceous, Silt loam 46 – 76 Somewhat poorly 41 – 58

Forest, Louisiana semiactive, thermic Aquic drained

Glossudalfs

a Redox depletions with chromas less than those of the matrix (Soil Survey Staff 1998).
5

sites with different high water tables. Less productivity

occurred on the Caddo soil that had a high water table depth

of 4 to 14 cm than on the Beauregard soil and the AcadiaBeauregard-Kolin soil complex that had high water table

depths of 19 to 49 cm and 23 to 38 cm, respectively. In an

earlier study, Tiarks and Shoulders (1982) surveyed the

productivity of 20-year-old loblolly and slash pine growing

on six soil series with different high water table depths in

Mississippi. They found a strong positive relationship

between tree height and high water table depth. Moreover,

this relationship was stronger on soil series with high water

table depths <60 cm.

Aerobic respiration requires oxygen as an electron acceptor.

As oxygen becomes less available, the concentration of

electrons, and, therefore, the reducing strength of the soil,

increases. Soil redox potential is a measure of soil reducing

strength (Lindsay 1979, Wolt 1994). Depending on soil pH,

the Eh of oxidized soil ranges from 400 to 1000 mV

(Lindsay 1979, Wolt 1994). In the Gulf Coastal Plain with

an average soil pH of 5.0, complete depletion of oxygen

occurs at Eh values of approximately 300 mV (Faulkner and

Patrick 1992, Patrick and Jugsujinda 1992).

During 1993 through 1997 in 12- to 16-year-old loblolly

pine on a Beauregard silt loam soil in the Kisatchie National

Forest, maximum net new root elongation occurred during

late April through late July (fig. 1). Also in the Kisatchie on

Glenmora soil, which is similar to Beauregard soil, values of

Eh averaged from late April through late July suggested that

winter water table represents the depth to the perched water

table. In the present study, for example, the Mayhew soil has

a high water table within 30 cm of the soil surface; whereas

that of the Malbis soil occurs between 76 and 127 cm (Soil

Survey Staff 1997c, 1997d) (table 1). High water table

depths of the Beauregard, Freest, Glenmora, Ichusa, Kurth,

and Metcalf soils are intermediate between those of the

Mayhew and Malbis soils (Soil Survey Staff 1997a, 1997b,

1999a, 1999b, 1999c, 1999d). Waddell (1997) also reported

distinct differences among the high water table depths of

Malbis, Glenmora, and Mayhew soils.

Loblolly pine is tolerant of low concentrations of soil

oxygen associated with flooding in winter and spring

(Schultz 1997). For a limited period, adaptations to low soil

oxygen, such as accelerated anaerobic respiration (DeBell

and others 1984) and development of hypertrophied

lenticels and aerenchyma (Hook 1984, Topa and McLeod

1986), allow maintenance respiration and survival of

loblolly pine root systems.

Lorio and others (1972) reported the effects of prolonged

anaerobic conditions on loblolly pine root growth; they

observed root growth parallel to the soil surface where the

high water table was shallow. A greater number of roots

were vertically oriented in soil that was less saturated. On

sites where mineral nutrient deficiencies are common,

constrained root growth may limit stand productivity. For

example, Haywood and others (1990) evaluated the

productivity of 10-year-old loblolly and slash pine on three

Figure 1—Net lateral root elongation averaged among thinning and fertilization treatments of 12- to 16-yearold plantation loblolly pine in the 0-to 30-cm depth of rhizotrons on a Beauregard soil in the Kisatchie

National Forest, Louisiana. Maximum root elongation generally occurred in midspring (April) through

midsummer (July) (black symbols).
6

oxygen deficiency limited aerobic metabolism at the 50-cm

depth in 1993 and 1994 and at the 100-cm depth in 1993 to

1995 (table 2). On the drier Malbis soil, average Eh values

at the 50- and 100-cm depths were within the range

associated with oxygenated soil. However, minimum Eh

values of the Malbis soil indicate that at least one of three

sensors at the 50-cm depth in 1993 and at the 100-cm depth

in all three years was exposed to anaerobic conditions.

Waddell (1997) reported similar results for a Metcalf soil in

May through July where Eh at 50 cm ranged between 230

and 330 mV in 1995 and 410 and 610 mV in 1996. We

conclude that the depth of oxygenated soil and the volume

of soil available for root proliferation during midspring

through midsummer vary by soil series and year.

Spatial variation also affects the oxygenation of Gulf

Coastal Plain soil series. Between the winters of 1995 and

1999, we measured Eh at the 25-cm depth with six sensors

in a newly established loblolly pine stand on an Ichusa silty

clay loam in Mississippi. In 1995 in midspring through

midsummer, average Eh between 370 and 488 (fig. 2).

Values >300 mV suggested that soil oxygen was not

limiting and, therefore, did not hinder root survival and

growth. However during this period, approximately 25

percent of the Eh sensors indicated anaerobic conditions.

Similarly in 1997 and 1998, approximately 25 percent of the

Ichusa soil area at the 25-cm depth was not oxygenated in

midspring through midsummer when the rate of new root

growth may have been greatest.

Soil Strength

Ultisols predominate on the Gulf Coastal Plain (Barrett

1995), and a fragipan commonly occurs on poorly drained

Ultisols (Buol and others 1980). A fragipan is a subsurface

horizon with a lower macroporosity and a higher bulk

density than proximal horizons; when dry, it limits root

penetration (Buol and others 1980, Soil Survey Staff 1998).

As soils dry on the Gulf Coastal Plain during the growing

season and fragipan characteristics develop, soil strength

may limit root elongation (Wraith and Wright 1998).

Average penetrometer resistance measurements

demonstrated the strength of a Freest fine sandy loam soil in

Mississippi (table 3). Although measured during winter and

spring when water availability was high, average values at

the 10- to 20-cm depth approached or exceeded 2 MPa,

which can impede root growth (Whalley and others 1995).

Because mechanical impedance to root elongation increases

as soil water potential decreases (Bennie 1996, Eavis and

Payne 1969), soil strengths >2 MPa may have occurred

throughout the soil profile as the growing season

progressed.

In addition to strength, bulk density measurements of the

soil indicate low macroporosity and the likelihood of

reduced root elongation. Bulk densities exceeding 1.55 g per

cm3

 potentially inhibit the elongation of tree roots in fine

texture soils (Pritchett 1979). However, because soil texture

and macropore distribution affect root elongation, growthlimiting bulk density differs by soil horizon (Daddow and

Warrington 1983). For the Malbis fine sandy loam soil in

Louisiana, we predicted the growth-limiting bulk densities

at six depth increments (Daddow and Warrington 1983)

(fig. 3). Our data indicated that bulk density limited root

elongation at approximately 35 cm.

On the Freest fine sandy loam soil in Mississippi, we

observed maximum soil strength at the 10- to 20- and 50- to

60-cm depths both before and 6 years after the application

of soil compaction and organic matter removal treatments

(table 4). Coefficients of variation associated with Freest

soil strengths indicated that a larger amount of variation

generally occurred in the 0- to 30-cm depth than in the 30-

to 60-cm depth (table 3). On the Malbis soil in Louisiana,

the variation associated with bulk density also differed by

Table 2—Mean and minimum hourly soil redox potential (Eh, mV) at the

50- and 100-cm depths of Glenmora and Malbis soils in the Kisatchie

National Forest, Louisiana, during late April through late July of 1993,

1994, and 1995

50-cm depth 100-cm depth

Soil series Statistic 1993 1994 1995 1993 1994 1995

Glenmora Mean 265 265 355 226 271 209

Minimum -120 -132 -142 -144 15 -75

Malbis Mean 566 673 727 309 449 504

Minimum 294 355 560 -108 -52 299
7

Figure 2—Redox potential at the 25-cm depth of an Ichusa soil in the Bienville National Forest, Mississippi, between the winters of

1995 and 1999. We recorded data from six sensors placed randomly in a 25-m2 area at 5-minute intervals to calculate daily means

(center black line). Upper and lower black lines present the minimum and maximum range of daily average Eh (n = 6). Shaded and

open bands around the mean distinguish 25th, 50th, 75th, and 100th percentiles. The soil is considered anaerobic at Eh 300 mV.

depth with more variation at the 20- to 75-cm depth than at

the 0- to 20- and 75- to 180-cm depths (fig. 3).

Although average values of soil strength and bulk density

may inhibit root elongation at a particular depth, the spatial

variation associated with these variables suggests that a

portion of the soil volume is continuously available for root

system expansion. For example, on the Malbis fine sandy

loam soil in Louisiana, we compared the range of 50 bulk

density measurements and the predicted growth-limiting

bulk density at each of six depth increments (Daddow and

Warrington 1983) (fig. 3). At the 0- to 30-cm depth, 15

percent of the bulk density measurements exceeded the

growth-limiting bulk density. Below the 30-cm depth, more

than 50 percent of the bulk density measurements were

greater than the growth-limiting bulk density. If root system

access to the soil is based on bulk density alone, <40

percent of the Malbis soil volume below 30 cm is available

for root system advancement.

The majority of southern pine roots occur in the upper

portion of the soil profile where mineral nutrient availability

is highest (Farrish 1991, Mou and others 1995, Parker and

Van Lear 1996, Torreano and Morris 1998, Van Rees and

Comerford 1986). However on sites that experience

drought, vertical roots must extend deep in the soil profile

for water uptake. Day and others (1998) found significant

horizontal and vertical water flow through interconnecting

prism faces of a sandy loam fragipan. The occurrence of

redoximorphic features on prism faces in the fractured

fragipan suggested that prism interfaces were channels for

vertical water and root movement.

In soil horizons with a high proportion of block- or prismlike peds, roots elongate vertically into interped spaces

(Parker and Van Lear 1996). Old root channels left by

previous vegetation also serve as conduits for root

elongation (Nambiar and Sands 1992, Van Rees and

Comerford 1986). On a sandy soil with a compacted layer

that impaired root growth, plants benefited from deep soil

access (Nambiar and Sands 1992). Vertical elongation of

Monterey pine seedling roots in perforations that were

uniformly distributed over 0.2 percent of the compacted

layer eliminated the negative effect on growth of surface soil

water deficit.

( (
8

Table 3—Means and probabilities of a greater F-value associated with the soil

strength of a Freest soil in response to two levels each of soil compaction and

organic matter removal

 No compaction— Severe compaction—

level of OM removed level of OM removed

Depth None All aboveground None All aboveground

cm

 Preharvest soil strength (MPa)

0–10 1.06 (71)a 0.96 (74) 0.94 (66) 1.13 (65)

10–20 1.82 (51) 1.74 (57) 1.53 (58) 1.95 (52)

20–30 1.32 (47) 1.23 (46) 1.25 (53) 1.34 (57)

30–40 1.08 (32) 1.19 (40) 1.17 (41) 1.31 (40)

40–50 1.26 (35) 1.41 (32) 1.34 (41) 1.45 (28)

50–60 1.51 (46) 1.72 (43) 1.50 (43) 1.72 (34)

 Age 6 soil strength (MPa)

0–10 0.89 (64) 0.83 (48) 1.17 (62) 1.71 (61)

10–20 1.22 (66) 1.12 (70) 1.94 (53) 2.45 (49)

20–30 .88 (50) .88 (68) 1.16 (54) 1.25 (59)

30–40 .94 (35) .98 (38) 1.17 (39) 1.24 (31)

40–50 1.14 (41) 1.16 (33) 1.43 (43) 1.44 (28)

50–60 1.52 (48) 1.48 (52) 1.78 (43) 1.80 (41)

 - - - - - - Probability > F-value - - - - - -

Effect (df) Preharvest Age 6

Block (2) 0.0001 0.0001

COMP (2) .5333 .0001

OM (2) .0059 .0550

DEPTH (5) .0001 .0001

COMPxOM (1) .0589 .0158

COMPxDEPTH (5) .9461 .0001

OMxDEPTH (5) .6891 .5180

COMPxOMxDEPTH (5) .4037 .1711

COMP = soil compaction treatment with levels of no compaction and severe compaction. Severe

compaction is defined as 80 percent of the difference between the growth-limiting bulk density and the

bulk density of noncompacted soil (Tiarks and others 1993);OM = organic matter removal treatment with

levels of no removal and removal of all vegetation and forest floor material.

a

 Coefficients of variation in parentheses reflect the standard deviation of 315 measurements.
9

0

30

60

90

120

150

180

15

60

78

89

96

98

85

40

22

11

4

2

Depth (cm)

Unavailable

%

Available

%

Growthlimiting

BD

Soil bulk density (g cm-3)

0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25

Range

Mean

The range of soil strength measurements on the Freest fine

sandy loam soil in Mississippi reflects the influence on soil

variation of soil structure, old root channels, and anomalies

such as old animal burrows and stump holes. For example,

on one plot that received no soil compaction or organic

matter removal, soil strength ranged between 0.01 and

4.6 MPa.

On a Beauregard silt loam soil after reduced precipitation

during the first half of 1998, soil water content in June and

July at the 15-cm depth was 40 percent less than the average

measured in the previous 4 years (fig. 4). In June and July

1998, loblolly pine root elongation in the 0- to 30-cm depth

was 76 percent lower than the average of the previous 5

years (fig. 5). In mid-July, we observed the appearance of

hydraulic lift of water from deep to shallow soil with a surge

of root elongation in 9 of 24 rhizotrons. July root growth

was not related to thinning, fertilization, or recent

precipitation. Of the trees measured for fascicle physiology,

we simultaneously evaluated six for root growth in

rhizotrons, and we observed a significant relationship

between net photosynthesis in the upper crown and root

elongation in rhizotrons (fig. 6). We speculate that fractures

and old root channels in dry subsurface soil facilitated deep

Figure 3—Mean bulk density (BD) and predicted growth-limiting bulk density (Daddow and Warrington 1983) 10 years after planting

on a Malbis soil in the Kisatchie National Forest, Louisiana. Because the LTSP soil compaction and organic matter removal treatments

were not significant 10 years after planting, we composited data for each of six 30-cm-depth increments (n = 50). We determined the

percentage of soil available for root growth by the range of measured bulk densities and the growth-limiting bulk density at each depth

interval.

Table 4—Mean soil strength (MPa) by depth of a

Mississippi LTSP installation on a Freest soil

before harvest and 6 years after planting loblolly

pine seedlings

Depth Preharvest Age 6

cm

0–10 1.02 e 1.15 b

10–20 1.76 a 1.68 a

20–30 1.29 c 1.05 c

30–40 1.19 d 1.08 c

40–50 1.37 c 1.29 b

50–60 1.61 b 1.65

Means in each column associated with different letters are

significantly different by the LSD test at P < 0.05.
10

Figure 4—Average monthly precipitation and soil water content at a soil depth of 15 cm in 1994 through 1997

and in 1998.

root growth, water acquisition, and physiological activity of

some loblolly pine trees at the site.

Development of the Root Zone Submodel

Where soils contain multiple layers with unique properties

that affect plant-available water and root proliferation, a root

zone submodel should predict the function of each distinct

layer. In their water balance submodel, Whitehead and

Kelliher (1991) assumed that plants extract water uniformly

from one root zone soil layer. They validated their submodel

in a New Zealand Monterey pine plantation with a soil

characterized by three texturally distinct layers. The surface

soil to a 0.5-m depth was sandy loam to loamy sand

overlying a sand and gravel layer, and the subsoil to a

rooting depth of 3.5 m was sandy loam to silt loam.

Although matric potential functions for the 0- to 1-, 1- to

1.4- and 1.4- to 3.5-m depths were considerably different,

they applied the matric potential function of the 0- to 1-m

depth to the entire 3.5-m root zone for calculation of

transpiration. They observed a large discrepancy between

predicted drainage and measured streamflow and attributed

the poor validation to errors in rainfall and streamflow

measurement. Application of one matric potential function

to the entire root zone may have also introduced error in

predictions of plant-available water, transpiration, and

drainage.

Previous modeling efforts have evaluated soil productivity

as a composite of environmental conditions in soil layers

(DeJong and Shaykewich 1981, Gale and others 1991,

Kiniry and others 1983, Phillips and others 1989). Kiniry

and others (1983) summed the rooting indices (RI) of

10-cm-depth increments to determine the sufficiency of soil

for the root growth of agricultural crops. They determined

the RI of each depth increment by potential root zone water

storage, aeration, bulk density, pH, and electrical

conductivity. Based on the model of Kiniry and others

(1983), Gale and others (1991) developed a Productivity

Index (PI) model to express forest productivity based on soil

properties, topography, and climate. Further development of

the PI model led to the characterization of pedogenic

horizons rather than depth increments (Gale and others

1991, Pierce and others 1984). Models that describe soil

layers by horizon are compatible with the U.S. Natural

Resources Conservation Service (NRCS) Soil Interpretation

Record (SIR) database, which is readily available and

includes extensive information from actual measurements.

Pertinent soil physical characteristics affect root

proliferation and function on the Gulf Coastal Plain. These

include the seasonal dynamics of soil aeration and strength

that impede root growth, soil structural attributes that

facilitate vertical root elongation, and plant-available water

defined as soil water between field capacity and wilting

point. Da Silva and others (1994) selected similar soil
11

physical properties to model the least limiting water range

(LLWR) or amount of soil available to agricultural crops.

They predicted the LLWR over a range of bulk densities as a

function of air-filled porosity, soil strength, and plantavailable water.

The NRCS soil series descriptions contain information on

the color of the soil matrix, as well as the frequency, size,

and color of redox depletions and vertical streaks by horizon

(Soil Survey Staff 1998). In 17 plots each on 3

topographically different Gulf Coastal Plain sites, Sprecker

and others (1999) placed redox sensors in each horizon to a

50-cm depth. They found high correlations between Munsell

color, water table depth, and redox potential. Furthermore,

with information from aquic soils in Louisiana, Daigle and

others (1996) expressed the association between the volume

of soil with redoximorphic features and Eh. We hypothesize

that the relationship between Eh and NRCS color can be

used to predict soil aeration limitations to root processes.

Once relationships between NRCS soil color and root

Figure 5—Average monthly net lateral root elongation by treatment in the 0- to 30-cm depth, during 1993

through 1997 (A), and in 1998 (B). The arrow in (A) represents the measurement period in (B).
12

survival and growth are established, the color of the soil

matrix and the spatial distribution of redox depletions in

each horizon of a soil series will reflect the volume of soil

that is and is not favorable for root proliferation.

Water content and bulk density are the primary factors that

control soil strength (Smith and others 1997). The NRCS

primary characterization data and SIR database convey

water retention and bulk density by horizon for all soil

series. In the root zone submodel, it may be possible to

predict seasonal soil strength as a function of bulk density,

plant-available water, and seasonal soil water content.

Before including soil strength in a root zone submodel,

however, we should determine the range of soil strength that

inhibits loblolly pine root elongation. Furthermore, we must

consider that soil strengths that mechanically impede root

elongation may not completely limit vertical and horizontal

expansion of the root system if well-developed interped

spaces, soil fractures, or old root channels are common. The

NRCS soil survey descriptions of soil structure provide

information on the occurrence of interped spaces.

Management history and stand age may also help determine

the prevalence of soil fractures and old root channels.

Conclusions

Our conceptual submodel of the root zone calculates the

volume of plant-available soil in each horizon as a function

of soil aeration, soil strength, and plant-available water.

Initial calculations by horizon assume that physical

restrictions are absent. As soil aeration and strength reach

levels that reduce root survival or elongation, or plantavailable water becomes limiting to root function, the

unrestricted soil volume is decreased accordingly. We sum

horizon values to give the amount of soil available for water

and mineral nutrient uptake. Once we calculate the volume

of plant-available soil, we can determine values of potential

root zone water storage and plant-available water that reflect

root zone limitations and apply them in physiological

process models.

This approach to modeling plant-available water and the

potential for root proliferation on the Gulf Coastal Plain is

promising. Preliminary research is needed, however, to

define limiting values of soil aeration and strength

associated with in situ loblolly pine root survival and

elongation. For soil physical limitations to root survival and

elongation, a root zone submodel should rely on data from

the NRCS National Soil Survey program, which is the only

source of extensive information on forest soils collected by

standardized methods. The NRCS SIR database is available

by soil association in the readily accessible State Soil

Geographic database (Soil Survey Staff 1995). This resource

allows application of our conceptual root zone submodel in

process model predictions of forest productivity at the

regional scale.

Figure 6—Relationship between net photosynthesis and net root elongation of six 17-year-old loblolly pine

trees in July 1998 on a Beauregard soil in the Kisatchie National Forest, Louisiana. We present the mean and

standard deviation of measurements taken at 1100, 1300, and 1500 hours.
13

Acknowledgments

The authors gratefully acknowledge Zhenmin Tang (School

of Renewable Natural Resources, Louisiana State

University) for providing the 1998 crown physiology data,

and Rick Stagg and Dan Andries for their long-term

dedication to site maintenance and data collection at the

LTSP and ecophysiology study installations, respectively.

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Station. 15 p.

We combined published information and our own experimental results from the Gulf

Coastal Plain to evaluate how soil aeration and strength interact with loblolly pine root

growth. Our results demonstrate that soil aeration and strength differ by soil series and year

and are subject to vertical and horizontal spatial variation. Comparison of loblolly pine root

phenology and seasonal redox potential indicates that oxygen is frequently limiting to new

root growth. The strength of these soils may also restrict loblolly pine root growth.

Physiological process models that predict southern pine productivity should apply accurate

calculations of plant-available soil in simulations of potential root zone water storage and

plant-available water. We propose a conceptual root zone submodel that predicts the

volume of plant-available soil with soil aeration, strength, and water retention by horizon.

Model parameters are components of the Natural Resources Conservation Service (NRCS)

Soil Interpretation Record (SIR) database collected across the United States.

Keywords: Soil aeration, soil strength, spatial variation, redox potential, root phenology,

root zone sub-model.
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