Soil Water Content and Growth of
Young Douglas-fir are Influenced by
Vegetation Control
Warren D. Devine1, Constance A. Harrington1,
Thomas A. Terry2
1
USDA Forest Service PNW Research Station, Olympia, WA
2 Weyerhaeuser Co., Centralia, WA
Background
• Soil water ⇒ atmosphere via plant transpiration &
evaporation from soil surface
• Increase soil water for crop trees by:
– Control of competing vegetation
– Barriers to evaporation (Troncoso 1997, O’Connell et al. 2004)
• Tree water stress reduced or growth increased by
vegetation control for Douglas-fir, ponderosa pine,
loblolly pine, radiata pine, etc. (e.g., Sands and Nambiar
1984, Peterson and Maxwell 1987, Pabst et al. 1990, Morris et al. 1993,
Harrington et al. 1995, Petersen et al. 1998)
Objectives
• Evaluate effects of vegetation control on
volumetric soil water content (VSWC) for:
– Soil depths of 10-100 cm
– Growing season, plantation ages 3, 4 & 5
• Assess relationship between VSWC during
growing season and annual growth of
Douglas-fir
Study Area
• Fall River Long-Term Site Productivity
Study
• Coast Range of southwestern Washington
• Western hemlock vegetation zone
(Franklin and Dyrness 1973)
• Elevation 300 m
• Site class II+, DF site index 42 m
• Mean annual precipitation 2260 mm
Study
Precipitation
• Mean June-September precip. 241 mm
• 2002-2004: 112, 120, 322 mm
Long-term mean precip. (mm)
400
350
300
250
200
150
100
50
0
Jan Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec
Study Area Soils
•
•
•
•
Boistfort silt loam
Typic Fulvudand
Andic influence
Low bulk density
(0.60 - 0.83 g cm-3 )
• Well-drained
• High water-holding capacity
A
SiL
AB
SiL
2Bw1
SiCL
2Bw2
SiCL
Study Design
• RBD, 12 treatments, 4 blocks
– Plots 30 x 85 m
• Douglas-fir (1+1) planted March,
2000 (2.5- x 2.5-m)
Treatments
• Bole-only harvest (>10 cm), no veg. control (BO-VC)
• Bole-only harvest with veg. control (BO+VC)
– Herbicide broadcast & spot applied annually (not operational)
• Total-tree harvest plus removal of woody debris with
veg. control (TTP+VC)
-VC
Total-tree plus
Bole-only
+VC
Data Collection
• PR1 Profile Probe & HH2 meter (Delta-T Devices)
• Soil-specific calibration ( 3%)
Probe
Access tube
Data Collection
• Volumetric soil water content (VSWC)
–
–
–
–
Years 3-5 (2002-2004)
Measured ~8 times during growing season
Soil depths = 10, 20, 30, 40, 60, 100 cm
30 tubes in yr 3-4; 58 tubes in yr 5
• Tree height, diameter measured annually
Data Analysis
• Treatment effects on VSWC
– ANOVA w/ repeated measures
• Correlated annual growth (mean of 4 nearest trees)
w/ VSWC for various dates during growing season
• Correlated veg. cover (1-m radius) w/ VSWC
Vegetation Control Treatment
>80% coverage
<10% coverage
Mean July VSWC
Soil Water
0
31%
35%
• VSWC increased with depth
• Coefficient of variation
within treatment decreased
with depth:
38%
39%
43%
– 10-30 cm: 35%
– 60 cm: 14%
– 100 cm: 4%
47%
0
10
20
30
40
VSWC (%)
50
Soil Water Content: Year 3
55
10 cm
BO-VC
BO+VC
TTP+VC
50
45
40
VSWC (%)
35
60 cm*
30
25
30 cm*
50
45
40
35
100 cm
30
25
May
Jun
Jul
Aug
Sep
Oct
May
Jun
Jul
Aug
Sep
Oct
* BO-VC significantly (P<0.05) lower than other treatments
Soil Water Content and Daily
Precipitation: Year 3
30 cm
45
VSWC (%)
40
40
30
BO-VC
BO+VC
TTP+VC
35
20
30
10
25
0
May
Jun
Jul
Aug
Sep
Daily precipitation (mm)
30 cm
Soil Water Content: Year 4
55
50
BO-VC
45
BO+VC
TTP+VC
10 cm
40
VSWC (%)
35
60 cm*
30
25
30 cm*
50
45
40
35
100 cm
30
25
May
Jun
Jul
Aug
Sep
Oct
May
Jun
Jul
Aug
Sep
Oct
* BO-VC significantly (P<0.05) lower than other treatments
Soil Water Content: Year 5
55
BO-VC
BO+VC
TTP+VC
50
45
10 cm
40
VSWC (%)
35
60 cm*
30
25
30 cm
50
45
40
35
100 cm
30
25
May
Jun
Jul
Aug
Sep
Oct
May
Jun
Jul
Aug
Sep
Oct
* BO-VC significantly (P<0.05) lower than other treatments
Soil Water Content and Daily
Precipitation: Year 5
45
VSWC (%)
40
40
35
BO-VC
BO+VC
TTP+VC
30
30
20
25
20
10
15
0
May
Jun
Jul
Aug
Sep
Daily precipitation (mm)
30 cm
Summer change in % VSWC
Change in VSWC During Summer:
Year 3 (15 May - 22 August)
4
2
0
B
-2
-4
n.s.
B
B
B
A
BB
BB
A
n.s.
BO-VC
BO+VC
TTP+VC
A
A
-6
10
20
30
40
Soil depth (cm)
60
100
Summer change in % VSWC
Change in VSWC During Summer:
Year 4 (1 May - 21 August)
4
2
0
B
-2
B
-6
10
n.s.
B
B
B
-4
n.s.
BB
B
A
BO-VC
BO+VC
TTP+VC
A
A
A
20
30
40
Soil depth (cm)
60
100
Summer change in % VSWC
Change in VSWC During Summer:
Year 5 (10 May - 19 July)
4
2
0
n.s.
B
-2
-4
BB
A
BB
A
B
A
B
A
B
AB
B
A
BO-VC
BO+VC
TTP+VC
-6
10
20
30
40
Soil depth (cm)
60
100
Year 3
2
0
-2
BO-VC
BO+VC
TTP+VC
-4
10
20
30
4
40
60
Change in VSWC
During Summer
100
Year 4
Soil depth
(cm)
2
0
-2
BO-VC
BO+VC
TTP+VC
-4
-6
10
20
Summer change in % VSWC
-6
Summer change in % VSWC
Summer change in % VSWC
4
30
4
40
60
100
Year 5
Soil depth
(cm)
2
0
-2
BO-VC
BO+VC
TTP+VC
-4
-6
10
20
30
40
Soil depth (cm)
60
100
Correlation Between Vegetation Cover
(1-m radius plot) and Late-Summer VSWC
Soil depth (cm)
10
20
30
40
60
Year 3
-0.43* -0.39 -0.69 -0.70 -0.66
Year 4
-0.39
Year 5
n.s.
-0.42 -0.60 -0.66 -0.61
n.s.
n.s.
-0.30 -0.47
100
n.s.
-0.40
-0.35
n=30 in yr 3 & 4; n=58 in yr 5
* Pearson’s correlation coefficient, value shown if sig. at P<0.05
Results: Tree Growth
Height growth (cm)
BO-VC BO+VC TTP+VC
Diameter growth (mm)
BO-VC
BO+VC TTP+VC
Year 3
54
60
55
9
15
14
Year 4
63
98
98
12
19
19
Year 5
94
112
122
18
21
21
Correlations Between VSWC and Annual
Diameter Growth of Nearest 4 Trees (Year 3)
30
40
60
Date of VSWC reading
9 Jul
24 Jul
22 Aug
n.s.*
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
0.46
0.47
0.47
n.s.
n.s.
0.43
0.47
0.46
n = 30
* Pearson’s
correlation
coefficient, value
shown if sig. at
P<0.05
Daily precipitation (mm)
Soil
Depth
(cm)
40
9 Sep
Year 3
30
20
10
0
Apr
May
Jun
Jul
Aug
Sep
24 Sep
Correlations Between VSWC and Annual
DBH Growth of Nearest 4 Trees (Year 4)
30
40
60
Date of VSWC reading
26 Jun
28 Jul
21 Aug
5 Jun
4 Sep
0.55
0.57
0.68
0.66
0.61
0.53
0.54
0.61
0.61
0.59
0.47
0.49
0.56
0.61
0.63
n = 30
* Pearson’s
correlation
coefficient, value
shown if sig. at
P<0.05
Daily precipitation (mm)
Soil
Depth
(cm)
40
Year 4
30
20
10
0
Apr
May
Jun
Jul
Aug
Sep
Correlations Between VSWC and Annual
DBH Growth of Nearest 4 Trees (Year 5)
30
40
60
Date of VSWC reading
6 Jul
19 Jul
9 Aug
9 Jun
2 Sep
n.s.*
n.s.
n.s.
n.s.
n.s.
n.s.
0.33
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n = 58
* Pearson’s
correlation
coefficient, value
shown if sig. at
P<0.05
Daily precipitation (mm)
Soil
Depth
(cm)
40
Year 5
30
20
10
0
Apr
May
Jun
Jul
Aug
Sep
Conclusions
• Growth appears to be water-limited at a
relatively high water potential
Matric potential (bars)
-16
-14
Soil water characteristic curve
-12
-10
-8
-6
-4
-2
0
-0.65
bars
-65
kPa
0.20
0.25
0.30
0.35
0.40
0.45
Soil water (m3 m-3)
0.50
0.55
Conclusions
• On a site with high soil water capacity & content
veg. control increased soil water
– To 60 cm
– Effects were greatest in late summer
• Increased soil water correlated w/ increased
diameter growth
• Other important / interacting factors? (e.g. N
availability)
– Doug-fir foliar N (w/o veg. control) was ≥1.8% (Roberts
et al. 2005)
– In year 5 (wet), VSWC & growth not well correlated
Acknowledgments
• AF&PA-USDA Forest Service Agenda
2020 grant
• Weyerhaeuser Co.
• Co-workers at Olympia Forestry Sciences
Laboratory, especially Dave Peter &
Douglas Waldren