Effects of Vegetation Control and 

Organic Matter Removal on Soil 

Water Content in a Young DouglasFir Plantation

Warren D. Devine, Constance A. Harrington

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-568

August 2006
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Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by appropriate state or federal 

agencies, or both, before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

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pesticides and pesticide containers.

Authors

Warren D. Devine and Constance A. Harrington are research foresters, U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Research Station, 

Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512-9193.

Cover Photos (clockwise from top left): Total-tree harvest plus removal of woody 

material with vegetation control; bole-only harvest with vegetation control; boleonly harvest without vegetation control; PR1 Profile Probe® partially inserted into a 

soil access tube. 
Abstract

Devine, Warren D.; Harrington, Constance A. 2006. Effects of vegetation 

control and organic matter removal on soil water content in a young Douglas-fir 

plantation. Res. Pap. PNW-RP-568. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 28 p.

We evaluated the effects of vegetation control and organic matter (OM) removal on 

soil water content (SWC) in a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) 

plantation from age 3 through age 5. Treatments were presence versus absence of 

vegetation control through year 5 and bole-only harvest of the previous stand versus 

total-tree harvest of the previous stand including removal of all coarse woody 

residues. In the presence of vegetation control, SWC was approximately 0.02 to 

0.04 m3•m-3 greater between depths of 10 and 60 cm; the effect was greatest from 

July to September. Soil water content was negatively correlated with percentage 

cover of competing vegetation and positively correlated with tree diameter growth 

across all treatments. Soil water content at depths between 10 and 100 cm did not 

differ between OM removal treatments. Accurate measurement of SWC required a 

soil-specific instrument calibration. On this highly productive site with high annual 

precipitation, SWC was greater through plantation age 5 when competing vegetation was controlled.

Keywords: Soil water, vegetation control, organic matter, harvest residue, 

Douglas-fir, Pseudotsuga menziesii, Andisol, soil water measurement, instrument 

calibration.
Summary

Soil water content (SWC) in a forest stand is influenced by silvicultural treatments 

that affect the rates of plant transpiration and evaporation from the soil surface. 

We evaluated the effects of vegetation control and organic matter (OM) removal 

on SWC in a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation from 

age 3 through 5. The study area was a highly productive site in the Coast Range of 

southwestern Washington, USA, with mean annual precipitation of 2260 mm. Soil 

was a Typic Fluvudand, formed in basalt residuum with andic influence in surface 

horizons. Treatments were presence versus absence of vegetation control through 

year 5 and bole-only harvest of the previous stand versus total-tree harvest of the 

previous stand including removal of all coarse woody residues. Growing-season 

volumetric SWC was measured on 24 plots at 3- to 4-week intervals at soil depths 

of 10, 20, 30, 40, 60, and 100 cm with a PR1 Profile Probe® soil water probe (DeltaT Devices, Ltd.). Soil properties necessitated a soil-specific calibration, which 

differed greatly from the instrument’s standard calibration.

Soil water content increased with soil depth in all treatments (mean July SWC 

at 10 and 100 cm was 0.31 and 0.47 m3•m-3, respectively); within-treatment variability decreased with depth. The OM removal treatment did not influence SWC 

at depths between 10 and 100 cm. Soil water content was approximately 0.02 to 

0.04 m3•m-3 greater between depths of 10 and 60 cm when competing vegetation 

was controlled. This effect was greatest from July to September during growing 

seasons 3 and 4. The effect of vegetation control on SWC was less in year 5 when 

water utilization by trees appeared to have increased relative to earlier years. 

Estimated soil water potential in August of years 3 and 4 was significantly lower 

without vegetation control (-348 to -262 kPa) than with vegetation control (-126 to 

-60 kPa) at depths of 30 and 40 cm. Soil water content was negatively correlated 

with percentage cover of competing vegetation and positively correlated with tree 

diameter growth across all treatments. On this highly productive site with high 

annual precipitation, SWC was greater through plantation age 5 when competing 

vegetation was controlled. Management of OM on similar sites should be based on 

factors other than its effects on soil water.
Contents

1 Introduction

2 Materials and Methods

2 Study Site

5 Design and Treatments

6 Data Collection

6 Data Analysis

7 Results

7 Soil Water Content by Depth

7 Treatment Effects

9 Relationships Between SWC, Vegetation, and Tree Growth

14 Discussion

17 Conclusions

18 Acknowledgments

18 English Equivalents

19 Literature Cited

	23	 Appendix A: Soil-Specific Calibration for the PR1 Profile Probe

	25	 Appendix B: Procedure for Checking Consistency Among the Six Sensors of the

	 	 PR1 Profile Probe

26 Appendix C: Soil Water Content at Six Depths in Years 3, 4, and 5

Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

Introduction

Water is transferred from the soil profile to the atmosphere via plant transpiration 

and evaporation from the soil surface. Under a forest canopy, plant transpiration is 

the dominant of these two processes (Blanken 1997, Tsujimura and Tanaka 1999, 

Wilson et al. 2001), but prior to canopy closure, both processes may be important. 

Silvicultural practices that affect evapotranspiration rates subsequently influence 

the amount of plant-available soil water. Site preparation practices such as chemical or mechanical removal of competing vegetation reduce soil water loss, thus 

increasing the amount of water available to plantation trees (Flint and Childs 1987, 

Nambiar and Sands 1993, Powers and Reynolds 1999). Management of organic 

matter (OM) in the form of logging residues or legacy logs prior to regeneration 

can also increase soil water availability. When residue is distributed over the soil 

surface, it creates a vapor barrier between the soil and the atmosphere. Although 

this barrier may intercept some precipitation, it also may reduce soil temperature 

and evaporative water loss (Powers 2002, Troncoso 1997). Reducing soil water loss 

may alleviate water stress among young trees, and thus, prevent associated reductions in photosynthesis and growth (Brix 1979, Cienciala et al. 1994, Pabst et al. 

1990, Petersen et al. 1988).

In young forest stands where sunlight is abundant and soil water is growthlimiting (Drever and Lertzman 2001), control of competing vegetation may lead 

to greater soil water availability for trees, increasing stomatal conductance and 

transpiration rates (Fleming et al. 1996). Control of competing vegetation in 3- to 

10-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands resulted in 

reduced water stress and increased growth of trees (Pabst et al. 1990, Petersen et 

al. 1988). Similarly, summer water stress has been reduced by vegetation control in 

other species including loblolly pine (Pinus taeda L.) (Morris et al. 1993), ponderosa pine (Pinus ponderosa Dougl. ex Laws.) (Petersen and Maxwell 1987), and 

radiata pine (Pinus radiata D. Don) (Sands and Nambiar 1984).

Although many options exist for managing logging residue after harvest, leaving the residue dispersed on site may improve nutrient retention (Aber et al. 1978), 

and, in some cases, result in higher soil water content (SWC). Few data on the latter 

subject are available from the Pacific Northwest, but retention of logging slash 

significantly increased the summer SWC of a sandy soil in California (Troncoso 

1997). In Australian Eucalyptus globulus (Labill.) plantations on sandy and red 

earth soils, retention of harvest residue onsite resulted in greater soil water for 5 

years relative to removal of residue (O’Connell et al. 2004). Other studies, however, 

showed no differences in SWC after conventional and whole-tree harvest treatments 

(Hendrickson et al. 1985, Piatek and Allen 1999).
Research Paper PNW-RP-568



We examined the influences of vegetation control and OM removal on SWC in 

a young Douglas-fir plantation on a highly productive site in southwestern Washington, USA. A previous study on the same site (Roberts et al. 2005) examined 

plot-level treatment effects on SWC, soil temperature, nitrogen (N) availability, and 

tree growth through age 3. Because trees were relatively small, the study measured 

SWC at a depth of 0 to 20 cm. Roberts et al. (2005) found that both vegetation 

control and retention of OM positively influenced SWC during the second and 

third growing seasons after stand establishment. In the present study, our primary 

objective was to evaluate the effects of vegetation control and OM removal on SWC 

at six depths from 10 to 100 cm during growing seasons 3 through 5. Secondary 

objectives were to assess relationships between SWC, cover of competing vegetation, and annual growth of young Douglas-fir on a microsite basis.

Materials and Methods

Study Site

The study is located in Pacific County, in the Coast Range of southwestern 

Washington (46° 43’ N; 123° 24’ W). The site is gently sloping (10 percent) with 

a westerly aspect; elevation is 300 m above mean sea level. Mean annual temperature is 9.2 °C; mean minimum temperature in January is -0.1 °C, and mean 

maximum temperature in August is 23.1 °C (USDA, NWCC, and OSU 2006). 

Mean annual precipitation is 2260 mm, but an average of only 241 mm occurs from 

1 June through 30 September (USDA, NWCC, and OSU 2006). Total precipitation measured on site from 1 June to 30 September was 112, 120, and 322 mm in 

2002, 2003, and 2004, respectively (fig. 1). The soil, formed in basalt, is a medial 

over clayey, ferrihydritic over parasesquic, mesic Typic Fulvudand of the Boistfort 

series, with a silt loam A horizon (0 to 16 cm) underlain by a silt loam AB horizon 

(16 to 45 cm), both with andic influence. Beneath these layers are silty clay loam 

Bw1 (45 to 88 cm) and Bw2 (88 to 140+ cm) horizons. The soil is well drained and 

low in bulk density (0.59, 0.72, 0.99, and 1.14 g•cm-3 in the A, AB, Bw1, and Bw2 

horizons, respectively). At four locations across the study site, 68-cm3 soil samples 

were collected from each soil horizon to develop a soil water retention curve. Soil 

water content at saturation and at tensions of 6, 10, 80, 200, and 1500 kPa was 

determined for each sample at the Oregon State University Soil Science Physical 

Characterization Laboratory, Corvallis (fig. 2).

The study site was formerly occupied by an old-growth conifer stand that was 

clearcut in 1952–53. After a broadcast burn in 1953, the site was planted with 

Douglas-fir seedlings. The stand, consisting of planted Douglas-fir and naturally 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

Year 3

0

5

10

15

20

25

30

35

40

45

Year 4

0

5

10

15

20

25

30

35

40

Year 5

0

5

10

15

20

25

30

35

40

September

Month

April May June July August

Daily precipitation (mm)

Figure 1—Daily precipitation (mm) for 1 April through 30 September at the Fall River Long-Term 

Site Productivity study, Washington, in years 3 through 5 (2002, 2003, and 2004).
Research Paper PNW-RP-568

Soil water potential (kPa)

0 -300 -600 -900 -1200 -1500

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

A

AB

Bw1

Bw2

Soil water content (m3  m-3)

Figure 2—Soil water retention curves for four soil horizons at the Fall River Long-Term Site Productivity 

study, Washington.

regenerated western hemlock (Tsuga heterophylla (Raf.) Sarg.), was thinned in 

1971. At the time of harvest in April-July 1999, stand basal area was 69 m2•ha-1

(Terry et al. 2001). Douglas-fir site index at age 50 is 41 to 43 m (King 1966).

The study area is within the western hemlock vegetation zone (Franklin and 

Dyrness 1973). Prior to the study, the plant association was identified as Tsuga 

heterophylla (Raf.) Sarg./Polystichum munitum (Kaulfuss) K. Presl-Oxalis oregana

Nutt. (Franklin and Dyrness 1973). In 2004, the most prevalent vascular plant species in the young plantation were common velvetgrass (Holcus lanatus L.), fireweed 

(Epilobium angustifolium L.), hairy cat’s-ear (Hypochaeris radicata L.), western 

pearly everlasting (Anaphalis margaritacea (L.) Benth.), and California blackberry 

(Rubus ursinus Cham. & Schlecht.). 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

Design and Treatments

This study is part of the Fall River Long-Term Site Productivity study (LTSP), 

which examines the effects of silvicultural treatments on soil resources and processes and on planted Douglas-fir growth. The LTSP study was installed in 1999 

following harvest of the previous stand. The study follows a randomized block 

design including 12 treatment combinations of OM removal, vegetation control, 

fertilization, soil compaction, and soil tillage. Some of the treatments are more 

intensive than operational treatments because the study was designed to isolate 

treatment effects to determine their influences on site productivity. The present 

study uses 3 of the 12 treatment combinations:

1. Bole-only harvest without vegetation control (BO - VC). Merchantable 

stems to a diameter of approximately 10 cm were removed, and logging 

slash was scattered on site. No herbicide was applied.

2. Bole-only harvest with vegetation control (BO + VC). Merchantable stems 

to a diameter of approximately 10 cm were removed, and logging slash 

was scattered on site. Competing vegetation was treated with herbicides 

during each of the first 5 years of the study (2000–2004). Two weeks before planting, Oust® (DuPont Crop Protection)1

 and Accord Concentrate®

(Dow AgroSciences LLC) were broadcast by using backpack sprayers. 

In years 2 and 3, Atrazine® (Syngenta Crop Protection) and Oust® were 

broadcast in March and followed in April and May by spot application of 

Accord Concentrate® or Transline® (Dow AgroSciences LLC). In years 4 

and 5, a directed band of Velpar® (DuPont Crop Protection) was applied 

between rows and followed by spot application of Transline® and Accord 

Concentrate® during April to June.

3. Total-tree harvest plus removal of legacy coarse woody material, with vegetation control (TTP + VC). Entire trees, including needles and limbs plus 

all remaining dead limbs > 0.6 cm diameter, old-growth logs, and intact 

decaying wood were removed from the site. Competing vegetation was 

treated with herbicides as in treatment 2.

Each of the three treatments was applied to two plots (each 30 by 85 m) within 

each of four blocks (see Terry et al. [2001] for additional treatment information). 

In March 2000, all plots were planted with 1 + 1 Douglas-fir seedlings at 2.5- by 

2.5-m spacing, and the entire study area was fenced to eliminate potential deer and 

elk damage to seedlings.

1 The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.
Research Paper PNW-RP-568

Data Collection

We measured SWC at 3- to 4-week intervals between May and October in years 3, 

4, and 5 following study establishment (2002 to 2004) by using a 100-cm Profile 

Probe® type PR1 soil water probe with an HH2 moisture meter (Delta-T Devices, 

Ltd.). The probe has six capacitance sensors, mounted at 10-, 20-, 30-, 40-, 60-, and 

100-cm positions, that simultaneously take readings at these soil depths. Data collected with the Profile Probe were recorded in volts and subsequently converted to 

SWC by using a soil-specific calibration (app. A). Several times during the course 

of the study, we tested the six probe sensors on reference soil samples of known 

SWC to verify that all six sensors were functioning similarly (app. B).

Prior to using the Profile Probe, 100-cm fiberglass access tubes were installed 

vertically in the soil profile to allow insertion of the probe. In April of year 3, we 

installed an access tube near the center of each study plot (3 treatments x 2 replications x 4 blocks = 24 plots), equidistant from four nearest trees. In December of 

year 4, we installed a second tube in each plot, approximately 5 to 6 meters from 

the first but also equidistant from four nearest trees. At each measurement, we 

made three readings, rotating the probe 120° between each reading. Readings were 

not taken less than 48 hr after rain events greater than 10 mm. 

We surveyed vegetative cover in the vicinity of each access tube on 10 July 

2002, 21 August 2003, and 28 July 2004. We visually estimated percentage cover 

of herbaceous and woody vegetation on circular plots (radius = 1.0 m) centered on 

each access tube. Cover estimates between 10 and 90 percent were in 5-percent 

categories, and estimates above and below this range were in 1-percent categories. 

After each growing season from year 3 to year 5, we measured diameter (at a height 

of 0.15 m after years 2 and 3; at breast height [d.b.h.; 1.30 m] after years 3 through 

5) of the four trees closest to each access tube (n = 96 in years 2 and 3; n = 192 in 

years 4 and 5). We used these data to relate annual tree growth increment to SWC 

on a microsite basis.

Data Analysis

For each growing season, we normalized SWC based on the SWC value at each 

sensor location at the beginning of the growing season. The resulting value (SWC 

at a given date subtracted from the first value of the season) was the relative SWC 

(RSWC; m3•m-3), or the change since the beginning of that growing season. A preliminary analysis of true SWC revealed the same significant results as the RSWC 

analysis; however, trends in RSWC data are better defined owing to removal of initial variation among sensor locations. Treatment effects on RSWC may be slightly 

underestimated because they are normalized on initial readings from early May of 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

each year, and thus, any treatment effect on SWC already present in early May is 

excluded. Analyses of year 5 data were performed both with and without data from 

the additional 24 access tubes installed prior to that year; treatment means were 

similar for both analyses, and data presented here are from all access tubes.

Within each year, RSWC was analyzed by using repeated measures analysis of 

variance (ANOVA) (Proc Mixed; SAS Institute, Inc. 2005). The effects of vegetation control (BO - VC vs. BO + VC and TTP + VC) and OM removal (BO + VC 

vs. TTP + VC) were tested by using orthogonal contrasts. We also used ANOVA 

to analyze treatment effects on estimated late-growing-season soil water potential. 

We estimated soil water potential at each sensor depth by using the laboratory soil 

water retention data for the soil horizon at that depth, with linear interpolation 

between data points on the soil water retention curve. We made protected, postANOVA mean separations by using Fisher’s Least Significant Difference test.

We used correlation analysis to examine relationships between SWC and 

percentage cover of vegetation around each access tube (Proc Corr; SAS Institute, 

Inc. 2005). To assess relationships between SWC and annual tree diameter growth 

increment, we correlated SWC at specified dates at individual tube locations with 

the current year’s average growth increment of the four nearest trees. An alpha 

value of 0.05 was used to determine significance in all analyses.

Results

Soil Water Content by Depth

Soil water content increased consistently with increasing soil depth. Average SWC 

across all treatments and years in mid- to late-July was 0.31, 0.35, 0.38, 0.39, 0.43, 

and 0.47 m3•m-3 for soil depths of 10, 20, 30, 40, 60, and 100 cm, respectively. 

Variation in SWC within treatment decreased with increasing soil depth. In April of 

year 5 (prior to treatment effects associated with summer water depletion) withintreatment coefficients of variation (CV) for soil depths from 10 to 30 cm averaged 

35 percent. At 40, 60, and 100 cm, average CVs were 26, 14, and 4 percent, respectively. Within each sample depth, the amount of variation in SWC among sample 

locations was similar for the three treatments and the four experimental blocks.

Treatment Effects

In year 3, vegetation control resulted in significantly greater RSWC at all soil 

depths, although the magnitude of the effect was much less at 100 cm than at 

other depths (fig. 3). This treatment effect was greatest from July through the final 

measurement of the year on 17 October. In year 4, vegetation control resulted in 
Research Paper PNW-RP-568

10 cm

-0.06

-0.04

-0.02

0.00

0.02

0.04

-0.06

-0.04

-0.02

0

0.02

-0.06

-0.04

-0.02

0.00

0.02

20 cm

30 cm

40 cm

60 cm

100 cm

VC P = 0.002

OM n.s.

VC P < 0.001 VC P < 0.001

VC P < 0.001 VC P = 0.036

OM n.s. OM n.s.

VC P < 0.001

OM n.s.

OM n.s. OM n.s.

Month (year 3)

Relative soil water content

(m3  m-3)

Oc

M

tober

ay

June

July

August

September

Oc

M

tober

ay

June

July

August

September

BO - VC BO + VC TTP + VC

Figure 3—Relative soil water content under vegetation control (VC) and organic matter removal (OM) treatments, measured 

at six soil depths, during year 3 (2002). Significance of contrasts for vegetation control (bole only without vegetation control 

[BO – VC] vs. bole only with vegetation control [BO + VC] and total tree “plus” with vegetation control [TTP + VC]) and for 

organic matter removal (BO + VC vs. TTP + VC) are shown.
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

significantly greater RSWC at soil depths from 10 to 60 cm (fig. 4); effects were 

greatest from July through early September. In year 5, RSWC at soil depths of 40 

and 60 cm was significantly greater under vegetation control (fig. 5). The effect of 

OM removal (BO + VC vs. TTP + VC) on RSWC was not significant except at the 

100 cm depth in year 4. True SWC values (m3•m-3) appear in appendix 3.

Over the course of the study, treatments with vegetation control showed a progressively greater decline in late summer RSWC each year. This effectively reduced 

the magnitude of the vegetation control treatment effect in year 4 and again, to a 

greater extent, in year 5. In year 3, RSWC in the BO + VC and TTP + VC treatments remained relatively constant throughout the growing season; however, in 

year 5 the same treatments showed obvious depletions in RSWC in midsummer.

Estimated soil water potential differed significantly among treatments in late 

summer at depths of 30 and 40 cm, with the BO - VC treatment more negative than 

the treatments with vegetation control (fig. 6). At 10- and 20-cm soil depths, there 

was a nonsignificant trend in which the BO - VC treatment had more negative water 

potentials than the other treatments.

Relationships Between SWC, Vegetation, and Tree Growth

Vegetation control resulted in a dramatic reduction in vegetative cover. Treatments 

that included vegetation control averaged less than 10 percent vegetative cover, 

whereas those without vegetation control averaged greater than 80 percent vegetative cover (table 1). Late summer SWC was significantly and negatively correlated 

with vegetative cover in all 3 years of the study (table 2). The strengths of the correlations were similar for herbaceous or herbaceous plus woody vegetation. In years 

3 and 4, correlations between vegetative cover and SWC were significant at soil 

depths of 10 through 60 cm. In year 5, significant correlations occurred primarily at 

soil depths of 40 and 60 cm.

Annual tree diameter-growth increment (mean of the four trees closest to each 

access tube) was significantly correlated with SWC at 40- and 60-cm depths in 

summer of year 3 (mean r = 0.46 for SWC measurements in August and September) 

and at 30-, 40-, and 60-cm depths in year 4 (mean r = 0.60 for SWC measurements in August and September). For example, year 4 d.b.h. growth increment was 

significantly correlated (r = 0.66; P < 0.001) with SWC on 21 August 2003 at a soil 

depth of 30 cm (fig. 7).
Research Paper PNW-RP-568

10

10 cm

-0.06

-0.04

-0.02

0.00

0.02

0.04

-0.06

-0.04

-0.02

0

0.02

-0.06

-0.04

-0.02

0.00

0.02

20 cm

30 cm

40 cm

60 cm

100 cm

VC P = 0.008

OM n.s.

VC P < 0.001

OM n.s.

VC P < 0.001

OM n.s.

VC P < 0.001

OM n.s.

VC P < 0.001

OM n.s.

VC n.s.

OM P = 0.025

Month (year 4)

BO - VC BO + VC TTP + VC

Relative soil water content

(m3  m-3)

Oc

M

tober

ay

June

July

August

September

Oc

M

tober

ay

June

July

August

September

Figure 4—Relative soil water content under vegetation control (VC) and organic matter removal (OM) treatments, measured 

at six soil depths, during year 4 (2003). Significance of contrasts for vegetation control (bole only without vegetation control 

[BO – VC] vs. bole only with vegetation control [BO + VC] and total tree “plus” with vegetation control [TTP + VC]) and for 

organic matter removal (BO + VC vs. TTP + VC) are shown.
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

11

10 cm

-0.06

-0.04

-0.02

0.00

0.02

0.04

-0.06

-0.04

-0.02

0

0.02

-0.06

-0.04

-0.02

0.00

0.02

20 cm

30 cm

40 cm

60 cm

100 cm

October

VC n.s.

OM n.s.

VC n.s.

OM n.s.

VC n.s.

OM n.s.

VC n.s.

OM n.s.

VC P = 0.021

OM n.s.

VC P = 0.038

OM n.s.

Month (year 5)

Relative soil water content

(m3  m-3)

May

June

July

August

September

Oc

M

tober

ay

June

July

August

September

BO - VC BO + VC TTP + VC

Figure 5—Relative soil water content under vegetation control (VC) and organic matter removal (OM) treatments, measured 

at six soil depths, during year 5 (2004). Significance of contrasts for vegetation control (bole only without vegetation control 

[BO – VC] vs. bole only with vegetation control [BO + VC] and total tree “plus” with vegetation control [TTP + VC]) and for 

organic matter removal (BO + VC vs. TTP + VC) are shown.
Research Paper PNW-RP-568

12

-500

-400

-300

-200

-100

0

Estimated soil water potential (kPa)

-500

-400

-300

-200

-100

Soil depth (cm)

10 20 30 40 60 100

-500

-400

-300

-200

-100

Year 4 (21 August)

Year 3 (22 August)

Year 5 (19 July)

A

B

B

A

B

B

n.s.

n.s.

n.s.

A

B

A

B

n.s. n.s.

n.s.

n.s.

AB

A

B

n.s.

n.s. n.s.

n.s.

n.s.

B B

n.s.

BO - VC BO + VC TTP + VC

Figure 6—Estimated soil water potential (kPa) in late summer (22 August 2002, 21 August 2003, and 

19 July 2004, years 3, 4, and 5) under vegetation control and organic matter removal treatments. The 

earlier date for year 5 is because of early August rain that year. Same letter denotes no significant difference (P ≥ 0.05) within year and depth.
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

13

Table 1—Mean coverage (and standard deviation) of herbaceous and woody 

vegetation present on circular plots (radius = 1.0 m) centered on each access 

tube in three treatments (n = 8 in years 3 and 4; n = 16 in year 5)

Treatment

	 Year	 Cover type	 BO - VC	 BO + VC	 TTP + VC

Percent

	 3	 Herbaceous	 83.8	 (10.5)	 1.4	 (1.8)	 15.6	 (21.8)

	 	 Herbaceous and

	 	 woody	 90.3	 (21.9)	 1.6	 (2.1)	 16.1	 (21.7)

	 4	 Herbaceous	 85.8	 (27.9)	 4.3	 (8.4)	 2.4	 (2.9)

	 	 Herbaceous and

	 	 woody	 95.8	 (38.8)	 4.3	 (8.4)	 3.0	 (3.3)

	 5	 Herbaceous	 95.5	 (5.1)	 4.6	 (4.9)	 5.0	 (6.9)

	 	 Herbaceous and

	 	 woody	 107.1	 (16.7)	 4.6	 (4.9)	 5.4	 (7.1)

BO – VC = Bole-only harvest without vegetation control.

BO + VC = Bole-only harvest with vegetation control.

TTP + VC = Total-tree harvest with vegetation control.

Table 2—Significant (P < 0.05) correlation coefficients for the relationship 

between late summer (year 3 = 22 August 2002, year 4 = 21 August 2003, 

year 5 = 19 July 2004) soil water content at six soil depths and herbaceous 

and woody vegetation coverage on circular plots (radius = 1.0 m) centered 

on each access tube (n = 24 in years 3 and 4; n = 48 in year 5)

Soil depth (cm)

	 Year	 Cover type	 10	 20	 30	 40	 60	 100

	 3	 Herbaceous	 -0.44	 -0.38	 -0.68	 -0.39	 -0.64	 --a

	 	 Herbaceous and

	 	 woody	 -.48	 -.38	 -.60	 -.63	 -.59	 --

	 4	 Herbaceous	 -.40	 -.39	 -.59	 -.67	 -.58	 --

	 	 Herbaceous and

	 	 woody	 -.42	 -.39	 -.52	 -.60	 -.53	 --

	 5	 Herbaceous	 --	 --	 --	 -.38	 -.47	 --

	 	 Herbaceous and

	 	 woody	 -.30	 --	 --	 -.35	 -.40	 --

a Cells with no value had correlation coefficients that were not statistically significant.
Research Paper PNW-RP-568

14

Discussion

Vegetation control resulted in greater SWC during the third, fourth, and fifth 

growing seasons, although the magnitude of the treatment effect was less pronounced in year 5. This decreased treatment effect in year 5 may have been a result 

of increased utilization of water by trees relative to competing vegetation as trees 

became increasingly dominant on the site. In year 3, the summer decrease in RSWC 

without vegetation control (BO - VC) was likely due primarily to water utilization 

of competing vegetation, as RSWC remained relatively constant in treatments 

where trees were the primary form of vegetation (BO + VC and TTP + VC). In year 

5, RSWC in treatments with vegetation control declined sharply in summer, closely 

paralleling the trend of the BO - VC treatment. Thus, the substantial increase in tree 

size from years 3 to 5 (mean height increased from 150 to 340 cm) was apparently 

accompanied by an increased influence of trees on soil water. The gross decline 

in RSWC in the BO - VC treatment in year 5 was somewhat greater than that of 

the treatments with vegetation control, but transpiration by trees in the BO - VC 

SWC (m3  m-3)

0 0.1 0.2 0.3 0.4 0.5

Year 4

d.b.h. gro

wth (m

m)

0

5

10

15

20

25

30

r = 0.66

P < 0.001

Figure 7—Relationship between year 4 diameter at breast height (d.b.h.) growth increment and soil 

water content (SWC) on 21 August at a soil depth of 30 cm. Each point represents SWC at one access 

tube and mean year-4 d.b.h. growth of four nearest trees. 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

15

treatment was probably less than that of trees in the other treatments because they 

were, on average, smaller, and transpiration by competing vegetation was likely 

a substantial component of total transpiration in this treatment. In addition to the 

treatment effects, RSWC in year 5 was influenced by greater total precipitation 

from June through September (188 and 168 percent greater than in years 3 and 4, 

respectively). The increased year-5 precipitation is evident in all treatments in the 

early June RSWC spike and in higher September RSWC values relative to previous 

years.

Although the effect of vegetation control on estimated soil water potential was 

not statistically significant at 10- and 20-cm depths, treatment effects on RSWC 

and negative correlations between vegetative cover and SWC at these shallow soil 

depths indicate, as was shown by Roberts et al. (2005), an influence of vegetation on SWC near the soil surface. Using an intensive sampling protocol (324 to 

432 sample points) on the same study site, Roberts et al. (2005) found that SWC 

was slightly greater with vegetation control in the 0- to 20 cm soil depth interval 

during years 2 and 3. The increased variability in SWC that we observed near the 

soil surface compared to greater depths was probably due, in part, to an increased 

number of decaying tree roots as well as other woody materials, such as tree limbs 

and fragments of decaying logs, that were embedded in the soil profile across all 

treatments. During instrument calibration in late summer, we observed that where 

these woody materials were present in the soil profile close to access tubes, SWC 

values were lower than where woody debris was not present.

The influence of the mostly herbaceous competing vegetation on SWC extended 

to a soil depth of at least 60 cm. In a young Douglas-fir plantation in western Oregon, shrub competition reduced SWC to a depth of 100 cm (Petersen et al. 1988), 

and in a 2-year-old loblolly pine plantation, competing vegetation reduced soil water 

to an 80-cm depth (Morris et al. 1993). Although the effect on SWC that we observed at 60 cm may indicate that roots of competing vegetation are present at this 

depth, it is also possible that shallow-rooted herbaceous plants may have intercepted 

percolating soil water during light growing-season rain events, thus influencing 

SWC below their rooting zone. Furthermore, a portion of this precipitation was 

intercepted aboveground by leaves and stems of competing vegetation. Particularly 

in the treatment without vegetation control, trees probably utilized an increasing 

amount of water from lower in the soil profile over the course of the growing season 

after trees and other vegetation had depleted shallow soil water. This transition of 

water uptake from shallow to deeper soils during the growing season was quantified in a 20-year-old Douglas-fir stand (Nnyamah and Black 1977). In a 50-year-old 

Douglas-fir stand adjacent to our study site, cumulative growing-season depletion 
Research Paper PNW-RP-568

16

of soil water at depths from 10 though 100 cm was similar in magnitude to that 

reported in this study (Devine et al. 2005).

In contrast to the results of previous research conducted at the same study site 

(Roberts et al. 2005), we detected no significant effect of OM removal on SWC. 

During years 2 and 3 after plantation establishment, SWC from 0 to 20 cm was 

approximately 0.015 m3•m-3 greater during the mid-to-late growing season in the 

BO + VC treatment relative to the TTP + VC treatment (Roberts et al. 2005). The 

authors concluded that this was caused by shading or mulching effects that reduced 

evaporation at the soil surface. A possible reason why we did not detect this treatment effect when sampling during the same year as the earlier study (year 3) is that 

the sensor used in the earlier study measured soil water in the entire 0- to 20-cm 

interval, including the soil near the surface, which we did not sample and which 

may have had the greatest evaporative loss. In contrast, our shallowest measurements were taken at a 10-cm soil depth. 

Although our soil-specific instrument calibration and the PR1 Profile Probe’s 

generalized mineral soil calibration did not result in different conclusions regarding 

the significance of treatment effects on SWC, the soil-specific calibration produced 

a much smaller overall range in SWC values than the generalized calibration. The 

Profile Probe User Manual states that the accuracy of the generalized calibration 

is poorest in heavy clay soils, organic soils, or soils that are extreme in any respect 

(Delta-T Devices, Ltd. 2001). The large difference between our soil-specific calibration and the generalized calibration indicates that soil-specific calibration is vital 

when using this instrument in soils such as those of our study site (i.e., low bulk 

density, andic properties). Similar soils are common in the coastal Pacific Northwest. The improved accuracy of soil-specific calibration is necessary for comparing 

SWC among sites and to values reported elsewhere.

Estimated soil water potentials generally remained above -400 kPa; however, 

significantly lower late-summer values in the BO - VC treatment (-348 to -262 kPa) 

versus treatments with vegetation control (-126 to -60 kPa) at 30- and 40-cm depths 

may have adversely affected tree growth rates. Haase and Rose (1993) reported significantly greater terminal growth of Douglas-fir seedlings at a soil water potential 

of -10 kPa than at -100 kPa. When SWC was progressively lowered, transpiration 

rates of 4-year-old Douglas fir began to decline when soil water potential reached 

-100 to -200 kPa (Lopushinsky and Klock 1974). Meinzer et al. (2004) found a 

sharp increase in soil water utilization by 24-year-old Douglas-fir when the soil 

water potential increased from -400 kPa to near zero at the soil depth interval of 20 

to 60 cm. Therefore, it is possible that relatively small differences among treatments 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

17

in volumetric SWC in our study influenced soil water potential sufficiently to affect 

water utilization by trees.

The negative correlation between competing vegetation cover and SWC 

suggests that water was depleted by this vegetation, and the positive correlation 

between tree d.b.h. growth and SWC suggests that tree growth may have been 

limited by water availability. Many studies have previously shown that growth of 

young Douglas-fir is increased when soil water availability is increased by control 

of woody and herbaceous vegetation (Cole and Newton 1986, Flint and Childs 

1987, Pabst et al. 1990, Petersen et al. 1988, Rose and Ketchum 2002), although the 

influence of competing vegetation on SWC and tree growth diminishes over time 

as trees become larger and more deeply rooted (Nilsson and Örlander 1999, Sands 

and Nambiar 1984). In addition to soil water, competition for other resources may 

have influenced tree growth in this study. For example, foliar N concentration for 

Douglas-fir at the end of year 3 was significantly lower in the absence of vegetation 

control (Roberts et al. 2005). However, Roberts et al. (2005) concluded that because 

year-3 foliar N concentration in that treatment (BO - VC) was still within the range 

of sufficiency at 1.8 percent, reduced tree growth in the same treatment was probably not caused by N deficit, but more likely by competition for soil water. Foliar 

N concentration remained significantly lower in the BO - VC treatment at the end 

of year 4, but by the end of year 5, differences in foliar N between treatments were 

no longer significant.2 Further study is necessary to separate the relative effects of 

water and N availability in this system.

Conclusions

Microsite variability in SWC within treatment, particularly in the top 40 cm of the 

soil profile, was greater than anticipated given the uniform appearance of the study 

site. By measuring SWC at permanent sample points, we were able to detect treatment differences despite the spatial variability. However, soil-specific calibration 

was necessary for accurate measurement of SWC with the PR1 Profile Probe.

Removal of OM at the time of harvest did not have a detectable influence on 

SWC during the 4th and 5th years after plantation establishment, although previous research at this site indicated that the presence of organic residues resulted 

in slightly greater SWC near the surface in years 2 and 3 (Roberts et al. 2005). 

Overall, the positive effect of OM retention on SWC was smaller in magnitude and 

briefer in duration than that of vegetation control. Although our study design did 

2 Unpublished data and report. On file with: USDA Forest Service, Pacific Northwest Research 

Station, Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512.
Research Paper PNW-RP-568

18

not test the interactive effects of OM presence and vegetation control on soil water, 

we found no additive effect when the treatments were combined. Thus, on sites 

similar to Fall River, operational costs, nutrient cycling, wildlife, and long-term 

productivity are more important factors to consider when deciding how to manage 

OM than the mulching effect that OM has on soil surface drying.

On this highly productive site with high annual precipitation, control of competing vegetation resulted in greater SWC to a depth of 60 cm, particularly from 

July to September during years of below-average growing-season precipitation. 

This depletion of soil water occurred in the rooting zone of the young trees and 

may have had a negative impact on tree growth, although vegetation also may have 

negatively affected tree growth through competition for available soil N. Apparent increases in soil water utilization by trees from years 3 to 5 suggest rapid root 

growth during this interval and an improved ability to compete with vegetation for 

soil water. Increased tree growth associated with vegetation control treatments will 

likely result in earlier crown closure, at which point water losses through transpiration of competing vegetation and soil surface evaporation will decline.

Acknowledgments

We thank Thomas Terry, Weyerhaeuser Company, Centralia, Washington, for installing the study and allowing us access for data collection. We also thank Rodney 

Meade and Adrian Ares of Weyerhaeuser Company and our current and former 

coworkers at the Olympia Forestry Sciences Laboratory for their assistance, particularly Joseph Kraft, Bryan Wender, Bridget Korman, James Dollins, and Diana 

Livada. We thank David Peter for providing vegetation data and Douglas Waldren 

for instrument calibration.

English Equivalents

When you know:	 Multiply by:	 To find:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Square meters per hectare (m2/ha) 4.37 Square feet per acre

Degrees Celsius (C) 1.8 C + 32 Degrees Fahrenheit

Cubic meters (m3) 1.308 Cubic yards

Grams per cubic centimeter (g/cm3) 62.43 Pounds per cubic foot

Kilopascals (kPa) 0.145 Pounds per square inch
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

19

Literature Cited

Aber, J.D.; Botkin, D.B.; Melillo, J.M. 1978. Predicting the effects of different 

harvesting regimes on forest floor dynamics in northern hardwoods. Canadian 

Journal of Forest Research. 8: 306–315.

Blanken, P.D. 1997. Evaporation within and above a boreal aspen forest. University 

of British Columbia, Vancouver, Canada. 220 p. Ph.D. dissertation.

Brix, H. 1979. Effects of plant water stress on photosynthesis and survival of four 

conifers. Canadian Journal of Forest Research. 9: 160–165.

Cienciala, E.; Lindroth, A.; Čermák, J.; Hällgren, J.; Kučera, J. 1994. The 

effects of water availability on transpiration, water potential and growth of Picea 

abies during a growing season. Journal of Hydrology. 155: 57–71.

Cole, E.C.; Newton, M. 1986. Nutrient, moisture, and light relations in 5-year-old 

Douglas-fir plantations under variable competition. Canadian Journal of Forest 

Research. 16: 727–732.

Delta-T Devices, Ltd. 2001. User manual for the profile probe type PR1. Version 

PR1-UM-01-2. Cambridge, United Kingdom: Delta-T Devices. 36 p.

Delta-T Devices, Ltd. 2002. PR1 performance and calibration in high salinity 

and high clay content soils. Rev. December 2002. Cambridge, United Kingdom: 

Delta-T Devices. 6 p.

Devine, W.D.; Harrington, C.A.; Terry, T.A. 2005. Volumetric soil water 

content in a 4-year-old and a 50-year-old Douglas-fir stand. In: Harrington, C.A.; 

Schoenholtz, S.H., tech. eds. Productivity of western forests: a forest products 

focus. Gen. Tech. Rep. PNW-642. Portland, OR: U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station: 161–164.

Drever, C.R.; Lertzman, K.P. 2001. Light-growth responses of coastal Douglasfir and western redcedar saplings under different regimes of soil moisture and 

nutrients. Canadian Journal of Forest Research. 31: 2124–2133.

Fleming, R.L.; Black, T.A.; Adams, R.S. 1996. Site preparation effects on 

Douglas-fir and lodgepole pine water relations following planting in a pinegrassdominated clearcut. Forest Ecology and Management. 83: 47–60.

Flint, L.E.; Childs, S.W. 1987. Effect of shading, mulching and vegetation control 

on Douglas-fir seedling growth and soil water supply. Forest Ecology and 

Management. 18: 189–203.
Research Paper PNW-RP-568

20

Franklin, J.F.; Dyrness, C.T. 1973. Natural vegetation of Oregon and Washington. 

Gen. Tech. Rep. PNW-8. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Range and Experiment Station. 452 p.

Haase, D.L.; Rose, R. 1993. Soil moisture stress induces transplant shock in 

stored and unstored 2 + 0 Douglas-fir seedlings of varying root volumes. Forest 

Science. 39: 275–294.

Hendrickson, O.Q.; Chatarpaul, L.; Robinson, J.B. 1985. Effects of two 

methods of timber harvesting on microbial processes in forest soil. Soil Science 

Society of America Journal. 49: 739–746.

King, J.E. 1966. Site index curves for Douglas-fir in the Pacific Northwest. 

Weyerhaeuser Forestry Paper No. 8. Centralia, WA: Weyerhaeuser Co., Forestry 

Research Center. 91 p.

Lopushinsky, W.; Klock, G.O. 1974. Transpiration of conifer seedlings in relation 

to soil water potential. Forest Science. 20: 181–186.

Meinzer, F.C.; Brooks, J.R.; Bucci, S.; Goldstein, G.; Scholz, F.G.; Warren,

J.M. 2004. Converging patterns of uptake and hydraulic redistribution of soil 

water in contrasting woody vegetation types. Tree Physiology. 24: 919–928.

Morris, L.A.; Moss, S.A.; Garbett, W.S. 1993. Competitive interference between 

selected herbaceous and woody plants and Pinus taeda L. during two growing 

seasons following planting. Forest Science. 39: 166–187.

Nambiar, E.K.S.; Sands, R. 1993. Competition for water and nutrients in forests. 

Canadian Journal of Forest Research. 23: 1955–1968.

Nilsson, U.; Örlander, G. 1999. Vegetation management on grass-dominated 

clearcuts planted with Norway spruce in southern Sweden. Canadian Journal of 

Forest Research. 29: 1015–1026.

Nnyamah, J.U.; Black, T.A. 1977. Rates and patterns of water uptake in a 

Douglas-fir forest. Soil Science Society of America Journal. 41: 972–979.

O’Connell, A.M.; Grove, T.S.; Mendham, D.S.; Rance, S.J. 2004. Impact of 

harvest residue management on soil nitrogen dynamics in Eucalyptus globulus

plantations in south western Australia. Soil Biology and Biochemistry. 36: 

39–48.

Pabst, R.J.; Tappeiner, J.C.; Newton, M. 1990. Varying densities of Pacific 

madrone in a young stand in Oregon alter soil water-potential, plant moisture 

stress, and growth of Douglas-fir. Forest Ecology and Management. 37: 267–283.
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

21

Petersen, T.D.; Maxwell, B.D. 1987. Water stress of Pinus ponderosa in relation to 

foliage density of neighboring plants. Canadian Journal of Forest Research. 17: 

1620–1622.

Petersen, T.D.; Newton, M.; Zedaker, S.M. 1988. Influence of Ceanothus 

velutinus and associated forbs on the water stress and stemwood production of 

Douglas-fir. Forest Science. 34: 333–343.

Piatek, K.B.; Allen, H.L. 1999. Nitrogen mineralization in a pine plantation 

fifteen years after harvesting and site preparation. Soil Science Society of 

America Journal. 63: 990–998.

Powers, R.F. 2002. Effects of soil disturbance on the fundamental, sustainable 

productivity of managed forests. In: Verner, J., ed. Proceedings of the 

symposium on the Kings River Sustainable Forest Ecosystems Project: progress 

and current status. Gen. Tech. Rep. PSW-183. Berkeley, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station: 63–82.

Powers, R.F.; Reynolds, P.E. 1999. Ten-year responses of ponderosa pine 

plantations to repeated vegetation and nutrient control along an environmental 

gradient. Canadian Journal of Forest Research. 29: 1027–1038.

Roberts, S.D.; Harrington, C.A.; Terry, T.A. 2005. Harvest residue and 

competing vegetation affect soil moisture, soil temperature, N availability, and 

Douglas-fir seedling growth. Forest Ecology and Management. 205: 333–350.

Rose, R.; Ketchum, J.S. 2002. Interaction of vegetation control and fertilization 

on conifer species across the Pacific Northwest. Canadian Journal of Forest 

Research. 32: 136–152.

Sands, R.; Nambiar, E.K.S. 1984. Water relations of Pinus radiata in competition 

with weeds. Canadian Journal of Forest Research. 14: 233–237.

SAS Institute, Inc. 2005. The SAS System for Windows. Version 9.1. Cary, NC.

Terry, T.A.; Harrison, R.B.; Harrington, C.A. 2001. Fall River long-term site 

productivity study: objectives and design. For. Res. Tech. Note 01-1. Centralia, 

WA: Weyerhaeuser Co., Western Timberlands R&D. 10 p.

Troncoso, G. 1997. Effect of soil compaction and organic residues on springsummer soil moisture and temperature regimes in the Sierra National Forest. 

Corvallis, OR: Oregon State University. 56 p. M.S. thesis.

Tsujimura, M.; Tanaka, T. 1999. Evaluation of evaporation rate from forested 

soil surface using stable isotopic composition of soil water in a headwater basin. 

Hydrological Processes. 12: 2093–2103.
Research Paper PNW-RP-568

22

U.S. Department of Agriculture Natural Resource Conservation Service,

National Water and Climate Center, Oregon State University Spatial

Climate Analysis Service [USDA, NWCC, and OSU]. 2006. Parameterelevation regressions on independent slopes model (PRISM) [Database]. http://

www.wcc.nrcs.usda.gov/climate/prism.html. (28 April).

Wilson, K.B.; Hanson, P.J.; Mulholland, P.J.; Baldocchi, D.D.; Wullschleger,

S.D. 2001. A comparison of methods for determining forest evapotranspiration 

and its components: sap-flow, soil water budget, eddy covariance and catchment 

water balance. Agriculture and Forest Meteorology. 106: 153–168.
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

23

Appendix A: Soil-Specific Calibration for the PR1 

Profile Probe

Because soil properties influence readings from the Profile Probe (Delta-T Devices, 

Ltd. 2001)1, we performed a soil-specific calibration to convert the recorded data 

from the Profile Probe (volts direct current) to soil water content (SWC; m3•m-3). 

Initial attempts at soil-specific calibration followed the laboratory procedure 

described by Delta-T Devices, Ltd. (2002). However, we found that this calibration procedure was not suitable for the soils in this study. The procedure resulted 

in disturbance of the soil structure, which consequently reduced water-holding 

capacity of the soil samples. Thus, the calibration equation that it produced was not 

applicable to soil under field conditions.

As an alternative to the laboratory calibration procedure, we performed a 

calibration based on probe readings and samples collected in the field. We took 

readings (n = 16) at two to six depths (10 to 100 cm) at six locations across the study 

site with the Profile Probe. We then collected two 348-cm3 soil samples adjacent 

to the probe sensor where each of the readings were taken and dried the samples 

to constant weight at 105 °C to determine gravimetric and volumetric SWC. The 

relationship between Profile Probe readings and sample SWC (mean of two soil 

samples per reading) was the calibration equation that we used to convert volts 

(V) to SWC (m3•m-3):

SWC = (0.574V) + 0.217

The generalized mineral soil calibration that is provided with the Profile Probe is:

SWC = -0.113 + (1.62V) – (3.56V2) + (8.63V3)

Our soil-specific calibration produced SWC values that were higher than those 

produced by the generalized mineral soil calibration, except in very moist soil 

where the soil-specific calibration produced lower SWC values (fig. A-1). The SWC 

values calculated from our calibration equation were consistently within 0.05 m3•m-3

of values produced by frequency- and time-domain reflectometry sensors at the 

study site (CS616, CS620, and TDR100 (Campbell Scientific, Inc.)). 

1The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.
Research Paper PNW-RP-568

24

Volts

0 0.1 0.2 0.3 0.4 0.5

0

0.2

0.4

0.6

0.8

1.0

Soil-specific calibration

Generalized mineral 

soil calibration

Soil-specific calibration

SWC (m3  m-3)

Figure A-1—Soil-specific and generalized mineral soil calibration curves based on equations used to 

convert output from the Profile Probe type PR1 (volts) to soil water content (SWC; m3•m-3).
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

25

Appendix B: Procedure for Checking Consistency 

Among the Six Sensors of the PR1 Profile Probe

1. We assembled two soil calibrators following the procedure described by 

Delta-T Devices, Ltd. (2002).12 A calibrator is a sealed, 10-cm-diameter 

cylinder containing a soil sample with a known water content. An openended tube runs through the cylinder so that the calibrator may slide along 

the length of the probe, allowing readings from any of the six probe sensors. Our calibrators were filled with soil from the study site and had soil 

water contents (SWC) of 29 and 53 percent, allowing us to verify sensor 

consistency at the high and low ends of the SWC range in this study.

2. For each calibrator, we inserted the probe so that the sensor of interest was 

in the center of the calibrator cylinder.

3. We took three readings with the sensor, rotating the probe 120° between 

readings. We then averaged the three readings.

4. We repeated steps 2 and 3 for all six sensors on the probe.

5. We compared average readings among the six sensors. If any sensor 

returned readings that were different from the other sensors, given the 

instrument’s specified level of precision (Delta-T Devices, Ltd. 2001), we 

returned the probe to the factory for recalibration of sensors.

Twice during our use of the Profile Probe, we found that, when tested on the 

same calibrator, the sensor at the 100-cm location was reporting readings 0.04 

to 0.05 volts higher than the other five sensors on the same probe. This was an 

indication that the probe required factory recalibration of the sensors. Our Quality 

Assurance/Quality Control Guidelines require bimonthly checks for consistency 

among sensors as a part of all research involving this instrument. 

1The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.

2Because this check of sensor consistency was not part of a soil-specific calibration, the 

problems with the calibrators described in appendix A were not relevant to this procedure.
Research Paper PNW-RP-568

26

Appendix C: Soil Water Content at Six Depths in Years 

3, 4, and 5

10 cm

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.25

0.30

0.35

0.40

0.45

0.50

0.25

0.30

0.35

0.40

0.45

0.50

20 cm

30 cm

40 cm

60 cm

100 cm

Month (year 3)

Oc

M

tober

ay

June

July

August

September

Oc

M

tober

ay

June

July

August

September

SWC (m3  m-3)

BO - VC BO + VC TTP + VC

Figure C-1—Soil water content (SWC) under vegetation control and organic matter removal treatments measured at six soil 

depths, during year 3 (2002). 
Effects of Vegetation Control and Organic Matter Removal on Soil Water Content in a Young Douglas-Fir Plantation

27

Oc

M

tober

ay

June

July

August

September

Oc

M

tober

ay

June

July

August

September

SWC (m3  m-3)

BO - VC BO + VC TTP + VC

10 cm

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.25

0.30

0.35

0.40

0.45

0.50

0.25

0.30

0.35

0.40

0.45

0.50

20 cm

30 cm

40 cm

60 cm

100 cm

Month (year 4)

Figure C-2—Soil water content (SWC) under vegetation control and organic matter removal treatments measured at six 

soil depths, during year 4 (2003). 
Research Paper PNW-RP-568

28

Figure C-3—Soil water content (SWC) under vegetation control and organic matter removal treatments measured at six 

soil depths, during year 5 (2004).

Oc

M

tober

ay

June

July

August

September

Oc

M

tober

ay

June

July

August

September

SWC (m3  m-3)

BO - VC BO + VC TTP + VC

Month (year 5)

10 cm

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.25

0.30

0.35

0.40

0.45

0.50

0.25

0.30

0.35

0.40

0.45

0.50

20 cm

30 cm

40 cm

60 cm

100 cm
Pacific Northwest Research Station

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	 	 Pacific Northwest Research Station

 P.O. Box 3890

	 	 Portland, OR 97208-3890
U.S. Department of Agriculture

Pacific Northwest Research Station

333 SW First Avenue

P.O. Box 3890

Portland, OR 97208-3890

Official Business

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