The impacts of mechanized cut-to-length forest operations on soils Eric R. Labelle1 and Dr. Dirk Jaeger2
1PhD
student, Faculty of Forestry and Environmental Management, University of New Brunswick, phone: 506-447-3132, email:
[email protected] 2Associate Professor, Faculty of Forestry and Environmental Management, University of New Brunswick, phone: 506-453-4945, email:
[email protected]
I N TRO DUCTI O N
RESULTS and DI SCUSSI O N
Unlike bound surfaces i.e. roads and highways, forwarding trails are not engineered to withstand high loads. Today’s forestry involves heavy equipment to process and transport trees. More specifically, forwarders used in the cut-to-length system range from 20 to 40 tons loaded and require several passes within the same trail to transport wood, thus increasing the risk of compaction and reduced plant productivity.
► High soil bulk density increase associated with equipment tracks at every studied depth. ► In total, 210 out of 231 track soil bulk density measurements (91%) increased between pre and post impact by an average of 19.2%. ► 80% of all outside-track measurements increased between pre and post impact by an average of 6.7%.
Research objectives
Why is soil compaction problematic?
A
Bulk density (g cm-3)
1-Determine and quantify the impacts of forest machinery on forest soil bulk density (absolute and relative density). 2. Quantify the effect of a slash mattress on the forwarding trail to mitigate the impact of forest machinery on soil bulk density.
B
0.4
0.4
0.6
0.6 0.8
0.8
a a b b
1
1
10 cm Pre harvest
0.4
10 cm Pre harvest
10 cm Pre harvest
10 cm Post harvest
0.6
10 cm Post harvest
10 cm Post harvest
a b c d
20 cm Post harvest
1 1.2
1.4
1.4
1.4
-1
-0.5
0
0.5
1
1.5
-2
2
-1.5
-1
-0.5
0
0.5
1
1.5
a
20 cm Post harvest
1.2
-1.5
20 cm Pre harvest
20 cm Pre harvest
0.8
20 cm Pre harvest
1.2
-2
Heavy payloads up to 20 tons Machine in direct contact with growing medium Large area of contact between machine and soil
C
2
-1.5
-1
-0.5
0
0.5
1
b c
30 cm Pre harvest
d
30 cm Post harvest
1.5
Cross section of forwarding trail (m)
Fig 5. Pre- and post-impact soil densities across the forwarding trail. A. Gagetown, cycle 1, B. Gagetown cycle 3, and C. Black Brook A
B
1.7
Cycle 1 curve, y = -1E-05x3 – 0.0026x2 + 0.1324x
Proctor points (Cycle 1)
Proctor 1.7 curve (Cycle 1)
R2 = 0.94
- Increase surface runoff - Increase rutting potential - Unacceptable rates of erosion
0.9 0.7
► Considering the usual 15-20 m trail spacing in CTL operations, 3.8-4.6% of the total area would get compacted above 80% RBD following just a few forwarding cycles.
Table 1. Forwarding machinery specifications
Research Load Ground pressure Equipment sites capacity (kg) (kPa) loaded Gagetown Timberjack 610 8,000 88.3 Black Brook Rottne solid F9 9,000 82.0 3 forwarding cycles
30
50
70
90
slash uncovered covered
slash covered uncovered
B
Distance from 35 36 main landing (m) 70 71
30 31
75 76
3.3 m 50 cm
5m
1m
50 cm
uncovered 30 31
C
Poly. Proctor Cycle 1
50
70
90
110
D 1.7
35+36 m lines, y =
-4E-06x3 R2
1.3
–
1.5 0.0008x2 +
0.0683x
= 0.93
3
1.5 3
lines)
R2 = 0.95
1.1 0.9 0.7 0.5 0.3 10
30
50
70
90
Watercontent content (%) (%) Water
110
Poly. Proctor Cycle 3
Proctor points (30+31 m lines) 80 % of MBD (30+31 m lines) FBD (30+31 m lines)
D Proctor 1.7 curve (30+31 m
30+31 m lines, y = -5E-06x3 – 0.001x2 + 0.0775x
80 % of maximum dry 1.3 (30+31 m lines) density
Proctor points (35+36 m lines)
1.1 Humboldt readings (30+31 m lines) 0.9
80 % of MBD (35+36 m lines) FBD (35+36 m lines)
0.7 curve (35+36 m Proctor lines) 0.5
100 % Saturation
80 %0.3 of maximum dry density (35+36 m lines) 10 30 Humboldt readings (35+36 m lines)
50
70
90
110
Poly. Proctor 30+31 m lines Poly. Proctor 35+36 m lines
Water Watercontent content(%) (%)
Fig 6. Soil moisture-density relationships along with FBD in relation with 80% RBD for A. 100 % Saturation Gagetown pre impact, B. Gagetown post impact, C. Black Brook pre impact, and D. Black Brook post impact.
► A 21.2 kg m-2 slash mattress composed of white spruce [Picea glauca (Moench) Voss.] branches helped to reduce soil compaction by 40% at a 5 cm depth up to 60% at a 30 cm depth compared to a thinner 10.2 kg m-2 slash mattress.
uncovered 7071
Machine tracks
Location of soil bulk density and water content readings recorded on uncovered soil Location of soil bulk density and water content readings recorded on slash covered soil Control points (undisturbed)
Fig 3. Forwarding and sampling layout
lower air-exchange
= Machine tracks Direction of timber transport
► When considering all field density measurements exceeding 80% RBD after harvesting, 75% of these were associated with bare compartments or with the thin 10.2 kg m-2 slash mattress.
maximum dry density
80%
=
100 % Saturation
Watercontent content(%) (%) Water
100 % Saturation
1m
3 forwarding cycles
110
Water Watercontent content(%) (%)
Fig 2. Rottne Solid F9 forwarder
50 cm
1 forwarding cycle
FBD (Cycle 3)
0.7
40
1 forwarding cycle
1m Direction of timber transport
80 % of MBD (Cycle 3)
n=4
20 m
30 m
Proctor points (Cycle 3)
0.9 Proctor curve (Cycle 3)
Zero air void
Lower plant growth optimum water content
Fig 4. Moisture-density relationship
► While the mattress helped to reduce density increase over all layers, it helped in particular to keep the density of the 20 to 30 cm layer post-impact below the critical 80% RBD threshold.
Mean bulk density increase (%)
A
FBD (Cycle 1)
Humboldt readings 1.1 (Cycle 1)
0.3 readings Humboldt 30 (Cycle 10 3)
Dry (g/cm3)) density(g/cm Drydensity
Fig 1. Schematic of undisturbed and disturbed forest soil
80 % of MBD (Cycle 1)
1.5 80 % of maximum dry density (Cycle 1) 1.3
80 % of maximum dry 0.5 (Cycle 3) density
0.5
Dry (g/cm3)) density(g/cm Drydensity
► On a spatial level, 20 out of 53 track locations (38%) exceeded 80% RBD due to machine impact at both research sites.
M ETHO DO LO GY
higher mechanical resistance
1.1
10
- Impede root elongation - Negative impact plant / tree growth - Slow down organic matter decomposition process
►We then compared these field bulk densities (FBD) to the standard Proctor densities (maximum density). This method is called the relative bulk density (RBD). A RBD greater than 80% has been associated with
R2 = 0.88
1.3
0.3
Decrease gas & nutrient exchange
►Two research sites (Gagetown and Black Brook, NB) were selected for field testing where in-place soil density was recorded pre- and post-impact with a nuclear moisture and density gauge.
Cycle 3 curve, y = 3E-06x3 – 0.0013x2 + 0.0798x
(g/cm33)) density(g/cm Drydensity Dry
Decrease infiltration rate
3
► RBD enabled us to compare bulk density increases from different sites, while taking into consideration inherent soil properties i.e. organic content, texture, etc.
Dry (g/cm)3) density(g/cm Drydensity
1.5
Increase bulk density
20 cm Post harvest
30
20
10
Depths (cm) Lines
0
10.2 kg m-2
21.2 kg m-2
5
5
10 30 30
10 30 31
18.5 kg m-2
5
10 30 70
17.7 kg m-2
5
10 30 71
Fig 7. Mean increase of track soil density per line and depth between pre- and post-impact measurements along with corresponding slash densities in Black Brook.