SOIL COMPACTION ON A MECHANIZED
TIMBER HARVEST OPERATION
IN EASTERN OREGON
By
Richard R. Zaborske
A PAPER
submitted to
Department of Forest Engineering
Oregon State University
Corvallis, Oregon
97331
in partial fulfillment of
the requirements for the
degree of
Master of Forestry
AN ABSTRACT OF THE PAPER OF
Richard R. Zaborske for the degree of Master of Forestry in
Forest Enqineerinq presented on May 26, 1989.
Title:
Soil Compaction on a Mechanized Harvest Operation in
Eastern Oreqon
Abstract approved:
Henry A. Froehlich
The use of mechanized harvest equipment has been
increasing as an economical method to harvest small diameter
timber.
While the use of this equipment is increasing,
little is known about associated soil compaction.
In
particular, little information exists on soil compaction
caused by feller--bunchers.
This study measured soil compaction caused by a
mechanized harvest operation using 2 swing-boom, tracked
feller-bunchers and 2 rubber-tired grapple skidders.
The
study took place on the LaGrande Ranger District of the
Wallowa-Whitman National Forest, located in eastern Oregon
on volcanic ash soils.
USDA Forest Service definitions of
detrimental compaction (20% or greater increase in soil
density over pre-activity levels) were used as a guideline
to determine if detrimental compaction had occurred.
Results show that 54% of the total area was impacted by
either the feller-bunchers, skidders or both.
Feller-
bunchers impacted 19% of the total area and caused a
statistically significant increase in soil bulk density of
8.8% when compared to before logging densities.
Main skid
trails covered 12% of the total area and had a statistically
significant increase in soil density of 36.3%, when compared
to before logging densities and was considered detrimentally
compacted.
trails.
These main skid trails were also feller-buncher
Thus, 12% of the total area was impacted by both
feller-bunchers and skidders while 7% of the total area was
impacted only by feller-bunchers.
Twenty three percent of
the total area received 1 to 4 skidder passes and was not in
identifiable skid trails.
Even these non-skid trail areas
showed a statistically significant increase in soil density
of 9.6% when compared to before logging densities.
An
additional 12% of the total area received 5 to 8 skidder
passes and was also not in identifiable skid trails.
This
area showed a statistically significant increase in soil
density of 19.8% and was considered detrimentally compacted.
Regression analysis showed that slash significantly reduced
compaction caused by feller-bunchers and skidders.
APPROVED:
-
Dr. Henry A. Froehlich - Major Professor
Forest Engineering
Dr. William A. Atkinson - Department Head
Forest Engineering
Paper presented May, 1989
DEDICATION
To my wife,
Vera Ellen
For all the love, support and encouragement
which made this paper possible.
ACKNOWLEDGMENTS
I would first like to thank the USDA Forest Service for
providing me the opportunity to participate in the two-year
advanced Logging Systems Program here at Oregon State
University.
Without this opportunity, this project would
not have been possible.
From the LaGrande Ranger District, I would especially
like to thank Ranger Gail Kimbell for allowing me to do this
study on her District.
I would also like to thank John
Wilkens, Operations Forester in Timber Administration and
the Sale Administrators John Szymoniak and Bruce Rynearson
for helping me locate the study area, providing me with the
supplies I needed and offering encouragement during the data
collection process;
Dan Harkenrider and Art Kreger, Soil
Scientists, who provided me with soils information; and Bob
Dipold, Fleet Manager, who always had a vehicle available
for me.
From the Timber Sale Purchaser, Crisstad Enterprises
Inc., I would like to thank Jack Barrymore, subcontractor.
I would especially like to thank the 2 feller-buncher
operators, Greg Jensen and Homer Gorbett and the skidder
operator, Jerry Gorbett, for their cooperation, information
and advice.
I would particularly like to thank the members of my
graduate committee, Professors Henry A. Froehlich, Loren D.
Kellogg and Department Head Dr. William A. Atkinson.
Their
guidance, help and suggestions were crucial to the success of
this project.
Finally, I would like to thank Don Studier for his help
in the data collection.
Also, thanks to Don and my fellow
graduate students in this program for all of the
encouragement, ideas and support during the past 2 years.
TABLE OF CONTENTS
Paqe
1.0
2.0
3.0
INTRODUCTION
1
.
1.1
OBJECTIVES
2
1.2
SCOPE
3
LITERATURE REVIEW
4
2.1
METHODS OF MEASURING SOIL COMPACTION
4
2.2
AMOUNT OF AREA IMPACTED
7
2.3
INCREASES IN SOIL DENSITY
8
2.4
EFFECTS OF MULTIPLE PASSES
2.5
EFFECTS ON PRODUCTIVITY AND RATE OF RECOVERY
FIELD STUDY DESIGN
11
.
14
19
3.1
STUDY AREA SELECTION
19
3.2
STUDY AREA DESCRIPTION
19
3.3
EQUIPMENT USED IN LOGGING
21
3.4
EQUIPMENT USED IN DATA COLLECTION
22
3.5
HARVEST PATTERN
24
3.6
3.5.1
FELLING PATTERN
24
3.5.2
SKIDDING PATTERN
25
DATA COLLECTION
27
3.6.1
MACHINE PASS GROUPINGS
29
3.6.2
SAMPLE POINT PREPARATION
32
3.6.3
BEFORE LOGGING
34
3.6.4
AFTER FELLING AND BUNCHING
35
3.6.5
AFTER SKIDDING
37
3.6.6
AMOUNT OF AREA IMPACTED
37
TABLE OF CONTENTS (continued)
P aq e
3.7
4.0
LABORATORY ANALYSIS
3.7.1
SOIL MOISTURE DETERMINATION
3.7.2
CALIBRATION OF PROBE
39
.
.
.
39
39
RESULTS
41
4.1
NUMBER OF SAMPLE POINTS TAKEN
41
4.2
BULK DENSITY MEASUREMENTS
42
4.2.1
4.2.2
4.2.3
COMPARISONS WITH BEFORE LOGGING
MEASUREMENTS
43
CHANGES IN BULK DENSITY WITH
INCREASING DEPTH
46
CHANGES IN BULK DENSITY WITH
INCREASING NO. OF PASSES
48
4.3
SLASH DEPTH
49
4.4
AREA IMPACTED
51
4.5
REGRESSION ANALYSIS
55
4.6
SOIL MOISTURE ANALYSIS
59
5.0
DISCUSSION OF RESULTS
6.0
MANAGEMENT IMPLICATIONS AND RECOMMENDATIONS
7.0
FURTHER RESEARCH
68
8.0
REFERENCES
71
61
.
.
.
66
APPENDICES
APPENDIX A - PERTINENT TIMBER SALE CONTRACT CLAUSES
APPENDIX B - CALCULATION OF NUMBER OF SAMPLE POINTS FOR
BEFORE LOGGING AND CONFIDENCE INTERVALS
FOR THE SAMPLE OBTAINED
LIST OF FIGURES
Page
Figure 2.1: Relationship Between Increases in Bulk
Density and the Number of Machine Trips
15
.
Figure 3.1: Map of Unit 1A Showing Access Road
and Landing Location
20
Figure 3.2: Theoretical Ground Pressure Distribution
Under a Loaded Feller-Buncher
23
Figure 3.3: Schematic Diagram of Felling Pattern
26
.
Figure 3.4: Schematic Diagram of Skidding Pattern
.
.
28
Figure 3.5: Schematic Diagram Showing Relationship
Between Machine Pass Categories
31
Figure 3.6: Schematic Diagram Showing Relationship
Between Skid Trail Types
33
Figure 3.7: Map of Unit Showing Areas Not Sampled
by Machine Type
36
Figure 4.1: Graph of Average Soil Density by Condition
Figure 4.2: Graph of Percent of Area Impacted and
Percent Increase in Soil Density for the
0 - 12 Inch Soil Layer
.
44
56
LIST OF TABLES
Paqe
Table 4.1: Number of Measurements Used for Results
.
.
.
42
Table 4.2: Average Soil Densities for Each Grouping
Sampled
43
Table 4.3: Soil Density Comparisons With Before
Logging Values
46
Table 4.4: changes in Soil Density With Increasing
Depth
47
Table 4.5: Changes in Bulk Density With Increasing
Number of Passes
49
Table 4.6: Slash Depth
51
Table 4.7: Area Impacted by Harvesting Activities
.
.
52
Table 4.8: Relationship Between Area Impacted
and Density Measurements
55
Table 4.9: Regression Analysis Results
58
Table 4.10: Soil Moisture Results
60
1.0
INTRODUCTION
Mechanization of felling and skidding offers
considerable economic advantages over conventional logging
methods used to harvest small diameter timber.
Mechanized
harvesting has the potential to not only increase production
but also to reduce labor costs
resulting in lower total
logging costs.
The use of mechanized harvest equipment has been
increasing.
Silversides (1984) traced the use of mechanized
forestry equipment from World War II to 1984.
He noted that
early development of mechanized equipment focused on
chainsaws and ground skidding equipment while later
development included mechanical fellers and multi-function
machines.
He reported that widespread use of chainsaws
began in earnest following the war and, while crawler
tractors had been used for skidding, it wasn't until 1956
that the first stripped-down-four wheel drive truck was
tested as a skidding tractor.
The first production model of
a mechanical feller was introduced in 1965.
From these
beginnings, Schuh and Kellogg (1988) reported, from a 1985
western US industry survey, that more than 140 timber
companies and logging contractors were using one or more
pieces of mechanized logging equipment ranging from fellerbunchers to delimbers and chippers to various types of
skidders.
While research has shown that soil compaction reduces
2
future timber yields (Froehlich, 1979; Froehlich, Miles and
Robbins, 1986; Laing and Howes, 1983), there is currently
little information as to the effect that this new mechanized
harvest equipment has on forest soils.
Most current
research has focused on soil compaction caused by the
skidding phase of the operation, and the majority of these
studies have taken place in the southeastern United States
where soils and climatic conditions are considerably
different from the Pacific Northwest.
Few studies have
focused on soil compaction caused by feller-bunchers and
even fewer have followed a mechanized logging operation from
start to finish.
It is the purpose of this study to
evaluate soil compaction caused by the felling and skidding
phases of a mechanized harvest operation in eastern Oregon.
1.1
OBJECTIVES
The objectives of this research project are:
For a mechanized harvest operation using tracked, swing boom
feller-bunchers and rubber-tired, grapple skidders;
Estimate the amount of compaction and percent of
area impacted by feller-bunchers.
Estimate the amount of compaction and percent of
area impacted by skidders.
Compare the results of this study with respect to
USDA Forest Service, Region 6 guidelines for acceptable
levels of compaction.
3
1.2
SCOPE
This field study reports on soil compaction resulting
from logging a beetle damaged stand of lodgepole pine in
eastern Oregon with feller-bunchers and rubber-tired,
grapple skidders.
It is a case study limited to one site,
using logger selected pieces of equipment.
The most
important timber sale contract specifications which apply to
this study are summarized below.
These complete timber sale
clauses are presented in Appendix A.
CONTRACT CLAUSE CT6.O - OPERATIONS
"Trees shall be skidded or yarded to the landing fulllength with one end suspended.
Skid trails shall be at
least 60 feet apart, center to center, except when
converging.
Single tractor passes off of skid trails are
allowed as long as rutting or detrimental compaction (refer
CT6.4# (Option
3))
is avoided."
CONTRACT CLAUSE CT6.4* (Option
3) -
CONDUCT OF LOGGING
If designated skid trails are detrimentally compacted,
they shall be ripped through the compacted zone but not to
exceed a depth of 20 inches.
Ripping shall be done with
winged subsoilers during the normal operating season.
Detrimental compaction is defined as a 20% increase in bulk
density from undisturbed levels."
4
2.0 LITERATURE REVIEW
Review of pertinent literature will focus on 5 major
categories.
They are
Methods of measuring soil compaction.
Amount of area impacted.
Increases in soil density.
Effects of multiple passes.
Effects on productivity and rate of recovery.
Inherent in topics 3 and 4 above is the significant
effect soil moisture has in the amount of compaction caused
by harvest operations.
2.1
METHODS OF MEASURING SOIL COMPACTION
Alexander (1985) defined soil compaction as a reduction
of soil volume due to a decrease in pore space.
Compaction
can be measured directly by changes in soil bulk density, or
indirectly by changes in soil strength or changes in
macropore space.
Alexander (1985) and the American Society
of Agricultural Engineers (1958) report that changes in soil
strength can be measured by a cone penetrometer.
Changes in
macropore space can be measured by changes in infiltration
rates.
Both of the above references have identified 4 methods
for measuring changes in bulk density.
The first 3 methods
involve sampling a volume of soil while the fourth method
allows determination of bulk density in place.
A brief
5
explanation of each method follows.
Core Method - This method involves driving a
cylindrical core of known volume into the soil and digging
the core out to obtain the sample which is then taken to a
lab where it is oven dried and weighed.
Bulk density is the
dry weight of the core divided by it's volume.
This method
is fairly simple and can give accurate results.
Care must
be taken in collecting and transporting samples and cores
are difficult to collect in gravelly soils or in soils with
large amounts of roots.
Clod Method - This procedure involves collecting
clods of soil in the field, coating them with paraffin or
Saran to prevent moisture loss and transporting them to a
laboratory for analysis.
In the lab, weight, volume and
moisture content of the clod is determined and bulk density
calculated.
This method works well in soils which are too
hard or stony for collecting core samples.
However, it can
be quite time-consuming and tends to overestimate bulk
density by underestimating the amount of pore space, since
clods generally correspond (at least partly) to soil ped
faces and thus the void space between clods is not measured.
Irreqular Hole Method - This method involves
excavating a shallow hole, obtaining dry weight of soil
removed and using a material such as sand or water to
determine volume of soil removed.
This method works well on
near-level surfaces, although Flint and Childs (1984) have
6
developed a method using styrofoam beads which works well on
steep slopes.
This method is time consuming, with
difficulty in measuring the volume of a hole in the side of
a soil pit.
The equipment used in this method can be
cumbersome in the field.
4)
Transmission of Gamma Radiation - This is the
method of measuring soil bulk density used in this study.
Vomocil (1954) describes this method as "a means of
making rapid measurements of bulk density of soil
.
does not require the removal of a large sample."
It
.
.that
involves use of gamma rays, since the amount of gamma
radiation absorbed by the soil is proportional to it's mass.
Measurement is carried out by placing a radiation source
(usually Cesium 137) in the soil a fixed distance from a
radiation detector and counter.
The source is usually a
probe inserted in the soil at a known depth with the Cesium
source at the tip.
The detector is either located in the
body of the device on the soil surface, or in a different
probe inserted in the soil at the same depth as the source
at a fixed distance.
The detector counts the number of
pulses it receives and, by suitable calibration, soil
density can be determined (Van Bavel
1958).
In this study,
the number of pulses recorded will be referred to as the
number of counts.
This method is quick and can give precise results.
is important to determine soil moisture content and the
It
7
probe must be calibrated for the soil types being studied.
Since the gauge measures amounts of radiation absorbed by
all material between the radiation source and detector, it
is important that the sample spot be properly prepared by
removing all litter and debris which could affect
measurements.
Also, reported bulk densities include gravel
and stones found in the soil.
Thus, results from very stony
or gravelly soils may not be accurate in terms of reporting
fine earth bulk densities.
2.2
AMOUNT OF AREA IMPACTED
Studies which have investigated effects of logging on
soils have frequently reported the percent of harvested area
disturbed.
Soil disturbance includes mixing of litter with
soil in place and removing litter and/or duff to expose bare
mineral soil.
Froehlich and McNabb (1983) note that Pacific
Northwest forest soils are generally low in bulk density and
strength and are highly porous.
Because of these factors,
many Northwest soils are easily disturbed and compacted
during timber harvest activities.
In northeast Washington, Laing and Howes (1983)
reported that 41.8% of an area logged with feller-bunchers
and rubber-tired skidders had suffered soil damage.
bunchers covered 20 - 25% of the area logged.
skidders raised the impacted area to 41.8%.
has found similar results:
Feller-
Clam bunk
Other research
8
Dyrness (1965) - 26.8% of a tractor logged stand in the
-
western Cascades of Oregon was compacted.
Aulerich, Johnson and Froehlich (1974) - 16 - 27% of
-
an area tractor-logged near Corvallis, Oregon was in skid
trails.
-
Steinbrenner and Gessel (1955) - 26% of an area
tractor logged in southwestern Washington was in skid
trails.
-
Hatchell, Ralston and Foil (1970) - 32% of an area
tractor-logged on the Atlantic Coastal Plain was in skid
trails.
-
Campbell, Willis and May (1973) - 23% of an area
in Georgia logged with rubber-tired skidders had been
disturbed.
Preplanning skid trails may be a way to reduce the
amount of area impacted by logging.
In the Oregon Coast
Range, Sidle and Drlica (1981) found that 13.6% of the total
area yarded with an FMC skidder was in skid trails, location
of which had been preplanned.
Of this area in skid trails,
25% showed less than a 10% increase in soil density, 38%
showed a 10 - 27% increase in soil density and 37% of the
impacted area showed an increase in soil density greater
than 27%, when compared to before logging densities.
2.3
INCREASES IN SOIL DENSITY
Much research has been done which investigated the
9
relationship between logging and soil compaction.
Adams and
Froehlich (1981) point out a number of factors which
contribute to how much a soil is compacted.
These factors
include the amount and type of pressure and vibration
applied, depth and nature of surface litter, and soil
texture, structure, and moisture content.
Since bulk
density increases with compaction, changes in bulk density
are often used to describe soil compaction.
In Arkansas, Moehring and Rawls (1970) found that wet
weather logging significantly decreased soil macropore space
and increased soil density in the 0 - 2 inch layer, compared
to adjacent, undisturbed sites (alpha = 0.05).
They also
reported that 5 years after logging, traffic on 3 or 4 sides
of trees significantly reduced tree growth (alpha = 0.05).
Logging traffic on 1 or 2 sides of trees did not influence
tree growth.
They found that while dry weather logging did
scarify the soil surface, it did not affect tree growth,
soil density or macropore space (alpha = 0.05).
In North Carolina on a loblolly pine plantation, Gent,
Ballard and Hassan (1983) found a 23% increase in soil
density at a depth of 12 inches in primary skid trails, when
compared to pre-harvest densities.
Rubber-tired feller-
bunchers and skidders were used to whole-tree and treelength yard the unit.
Other studies have found similar
results:
-
Stokes and Sirois (1982) - logging corridors
10
showed a 15% increase in soil density at the 2 inch depth
and an 11% increase in soil density at the 4 inch depth.
These increases were significantly higher (alpha = 0.05)
than densities found in undisturbed soils.
The study was
done in southeast Louisiana immediately after a unit had
been felled, limbed, bucked and piled by a tracked machine,
but prior to skidding.
-
Steinbrenner and Gessel (1955) - an average
increase in soil density of 34.9% in skid trails.
Densities
in areas adjacent to logged areas ranged from 0.479 to 1.117
gm/cc.
Densities in skid roads ranged from 0.763 to 1.394
gm/cc.
-
Dyrness (1965) - 48% increase in soil density.
Before logging densities were 0.657 gm/cc while after
logging densities were 0.975 gm/cc
-
Campbell, Willis and May (1973) -
average
increase in soil density of 16% in disturbed areas versus
undisturbed areas.
-
Hatchell, Ralston and Foil (1970) - soil densities
increased from 0.75 gm/cc in undisturbed soils to 0.92 gm/cc
in secondary trails, and to 1.08 gm/cc in primary trails, a
22% and 44% increase, respectively.
-
Aulerich, Johnson and Froehlich (1974) - reported
the following increases in soil density on tractor thinning
of young-growth Douglas-fir:
11
BULK DENSITY INCREASE (%)
0-6 in.
Major skid trails
Secondary skid trails
Lightly used trails
2.4
21
16
13
6-12 in.
17
12
6
EFFECTS OF MULTIPLE PASSES
Research has shown that only a few passes over the same
area are needed to increase soil densities to near their
maximum.
On a volcanic soil, Lenhard (1986) reported that
soil density reached a maximum after 4 trips with an
unloaded skidder traveling at slow speeds.
While soil
density remained statistically constant with up to 32 trips,
pore size distribution changed with increasing number of
passes between 4 and 32.
In a vehicular compaction test,
Hatchell, Ralston and Foil (1970) reported that an average
of 2.5 trips resulted in soil densities within 10% of the
maximum attained.
-
Other studies have found similar results:
Froehlich (1978) -
reported the following results
with increasing number of passes on 3 sites where a loaded,
low ground-pressure, torsion-suspension skidder was used:
Mt. Hood Site -
At the 2 inch depth, soil density
increased in the first few passes and changed little after 6
passes.
At the 6 inch depth, density increased with
increasing number of trips up to 6, at which point it
reached its maximum and remained virtually the same, even
after 20 trips.
At the 10 inch depth, soil density changed
12
little regardless of the number of passes.
Soil moisture
content was 43% and surface litter ranged from 2 to 8 inches
in depth.
Soil densities reported were:
SOIL DENSITY (GM/CC)
Depths j (inches)
SOILS
Undisturbed
Skidder Trail
1-3 trips
6-10 trips
15-20 trips
50-60 trips
2
0.65
0.87
0.88
1.03
0. 89
1 . 06
1 .01
1.12
1.24
0.96
0.96
0.92
0.95
0.90
1.01
Umpqua Site - Soil density changed little at any depth,
This lack of change was attributed to
even after 20 trips.
high initial densities, thick litter layer (5 to 9 inches)
and lower soil moisture (29%).
Soil densities reported
were:
SOIL DENSITY (GM/CC)
Depths of (inches)
SOILS
Undisturbed
Skidder Trail
1-3 trips
6-10 trips
15-20 trips
90-100 trips
2.
1.06
1
.
09
1.08
1.09
1.33
LQ
1.16
1.06
1.06
1.08
1.04
1.17
1 .04
1 .08
1.18
1.13
1.16
1.34
1 . 29
1 .33
1 . 07
Malheur Site - Density of surface soils increased in
the first few trips with little change thereafter.
There
was a lesser increase in density at depths of 4 and 6
inches.
Densities did not change at the 10 inch depth.
Soil moisture was 13% and the litter layer ranged from 0.2
13
to 2.5 inches deep.
Reported soil densities were:
SOIL DENSITY (GM/CC)
Depths of (inches)
SOILS
Undisturbed
1.09
1.05
1.03
1.06
Skidder Trail
1-3 trips
6-10 trips
15-20 trips
1.21
1.24
1.25
1.12
1.12
1.17
1.11
1.11
1.13
1.01
1.01
1.08
-
2
Burger, et al. (1985) - using an unloaded rubber-
tired skidder and crawler tractor on a clay soil found that
neither machine had an effect on soil density at the 6 to 8
inch depth.
Density of the top 2 inches sharply increased
with the first 3 passes with smaller increases for up to 9
passes.
The level of soil moisture significantly affected
the degree to which the top 2 inches of soil were compacted
(alpha = 0.05).
At 18% soil moisture content,
overall
density increased by 0.10 gm/cc while at 21% moisture the
increase was 0.18 gm/cc.
Despite the fact that the crawler
tractor had a mean ground contact pressure 3.7 times that of
the rubber-tired skidder, the effect of the 2 machines on
soil density was the same.
-
Froehlich et al.
(1980) - developed a regression
model which only contained terms for number of trips and
cone index.
This model explained 54% of the variation in
change in bulk density.
They found that about 70% of the
increase in bulk density had occurred by the fifth trip.
density curve produced from their prediction equation is
A
14
given in Figure 2.1.
Their results are based upon data
collected on the Tahoe National Forest on 4 soil types,
ranging in texture from loam to gravelly loamy sand, and 3
different skidders (rubber-tired, crawler tractor and a
torsion-bar suspension machine).
2.5
EFFECTS ON PRODUCTIVITY AND RATE OF RECOVERY
Compaction alters soil properties such as strength,
density pore space, and size.
These soil properties control
root penetration and aeration, which in turn affect the
ability of plants to uptake moisture and nutrients, and
ultimately its growth rate.
Thus, altering these soil
properties can reduce future growth and forest productivity.
Growth loss studies have used weighted averages of trees
found throughout the width of skid trails in determining
growth reductions.
While it may be possible that growth
losses could be minimized by careful placement of planted
seedlings (planting all seedlings near the edges of skid
trails and none up the middle or not planting at all in skid
trails and planting more seedlings in undisturbed areas), it
cannot be said with certainty what effect, if any, this
would have on minimizing growth losses due to soil
compaction.
There are many other factors besides soil
compaction which may affect growth reductions due to logging
activities.
Some of these factors may be puddling of the
soil surface resulting in reduced infiltration rates and
2
4
6
8
15
Number of Vehicle Trips
10
20
Figure 2.1 RelationshIp between Increases in bulkdensity and the number of
machine trips (Froehlich et al. 1980)
5
30
16
removal or displacement of the nutrient rich surface layer.
Froehlich, (1979) reported that 16 years after logging
on the Ochoco National Forest in Oregon, soil densities in
skid trails at 3 and 6 inch depths were 18% higher than
undisturbed soils.
Densities at 9 and 12 inch depths were
9% higher than undisturbed soils.
Ponderosa pine (Pinus
ponderosa) growth rates were reduced by 6% on moderately
impacted trees and 12% on heavily impacted trees over this
same 16 year period.
Cascades,
On a tractor logged site in the Oregon
Wert and Thomas (1981) found that heavy
compaction existed over 25% of an area logged 32 years
earlier.
Only the top 6 inches showed some recovery from
compaction.
New stand volume was reduced by 11.8%.
Other
studies have shown similar results:
-
Froehlich, Miles and Robbins (1986) - studied
ponderosa pine and lodgepole pine (Pinus contorta) sites
logged 23 and 14 years earlier, respectively, in central
Washington.
They reported that on the ponderosa pine sites
soil densities in skid trails were 15% higher when compared
to undisturbed soils.
Over the past 5 years, trees in skid
trails had a 5% reduction in height growth, 8% reduction in
diameter, and a 20% reduction in volume growth, when
compared to trees in undisturbed areas.
Skid trails in
lodgepole pine sites had densities 28% higher than
undisturbed soils.
They found, however, that increases in
soil density were not significantly related to growth of
17
this species.
-
Hatchell, Ralston and Foil (1970) - using
regression, estimated it would take 18 years for soils
under log decks to return to soil densities found in
undisturbed soils.
-
Laing and Howes (1983) - found an average increase
in soil density of 36% when compared to pre-activity levels.
They predicted a 35% reduction in volume on these areas
over the next rotation.
-
Froehlich, Miles and Robbins (1985) -
reported
that on a volcanic soil in Central Idaho, soil densities in
skid trails at depths of 2, 6 and 12 inches had not returned
to densities found in undisturbed soils 23 years after
logging.
The differences in soil densities between skid
trails and undisturbed soils were significant (alpha =
0.01).
-
Helms and Hipkin (1986) - intensively studied a
1.2 acre site within a 16 year old pine plantation in
California.
Bulk density increased 30% in a skid trail
compared to soils of lowest bulk density measured.
They
also stated that in this skid trail, 15 years after planting
establishment, a 29% reduction in mean tree volume, combined
with 37% mortality, resulted in a reduction of 55% in volume
per unit area.
In areas adjacent to the skid trail, bulk
density was 18% higher and had a reduction of 13% in volume
per unit area.
18
Youngberg (1959) - studied growth rates on planted
-
Douglas-fir (Pseudotsuqa menziesii (Mirb] Franco) seedlings
in Lane Co., Oregon.
he reported the following changes in
bulk density:
Bulk Density (gm/cc)
Condition
Plantation 1
Depth (in.)
Plantation
Cutover
0 - 6
6 - 12
0.87
0.98
0.88
0.88
Skid Roads
0 - 6
6 - 12
1.58
1.73
1.52
1.59
Road Berms
0 - 6
6 - 12
1.01
1.05
0.89
0.88
He also reported the following average height growth
rates for 2 year old seedlings.
The differences in height
growth between cutover and both skid road and road berms was
significant (alpha = 0.01).
Condition
Cutover
Skid Roads
Road Berms
-
Heiqht Growth (inches)
Plantation 1
Plantation 2
6.38
4.73
5.71
7.01
3.16
4.98
Cochran and Brock (1985) - studied 2 small
clearcuts on the Deschutes National Forest.
Eight years
after logging, 37% of the combined total area had increases
in soil density of 15% or more when compared to soils
outside of the clearcut boundaries.
Using regression, they
concluded that compaction reduced ponderosa pine height
growth for the 5-year period after planting.
19
FIELD STUDY DESIGN
3.0
All measurements were made during the period July, 1988
to August, 1988, on cutting unit 1A of the Hoosier Timber
Sale, LaGrande Ranger District, Wallowa-Whitman National
Forest, located in eastern Oregon.
3.1
Study Area Selection
Several cutting units of this timber sale were to be
harvested using feller-bunchers and grapple skidders during
the summer of 1988.
This particular unit was chosen because
it was easily accessible and logged when enough time was
available to do the study.
Field data were collected during
the months of July and August, 1988.
3.2
Study Area Description
The unit was 26 acres in size.
Midpoint elevation was
5700 feet above sea level.
Average slope was 12% with a few
areas having slopes of 25%.
Figure 3.1 shows the outline of
the unit, access roads and landing location.
Unpublished soil survey data classifies this unit as
a Grand-fir/Huckleberry/Douglas-fir plant association.
A US
Forest Service timber cruise estimated an average net volune
of 41 cunits per acre.
Almost three-quarters of this volume
was dead, both standing and down, lodgepole pine killed
within the past 10 years by bark beetle attacks.
diameters were generally less than 10 inches.
The
Tree
0
Feet
400
Approximate Scale
Access Road
Landing
Unit Boundary
Figure 3.1 Map of Unit 1A Showing Access Road and Landing Location
800
-
-
KEY
21
silvicultural prescription called for removing all live and
dead lodgepole pine.
All healthy Douglas-fir trees were to
be left, and areas of existing regeneration impacted as
little as possible by logging
information).
(USDA Forest Service
Data on average tree diameter, piece size and
trees per acre were not available.
Soil inventories for the unit showed soils derived from
volcanic ash overlaying a clayey subsoil.
Total depth to
bedrock was 20 to 40 inches.
Thickness of the ash layer
ranges from 14 to 20 inches.
Texture of the ash layer is
silt loam with generally less than 5% cobbles.
of the surface was covered with stones.
Less than 1%
The soil is well
drained, has moderate permeability rates throughout it's
depth and has moderately high erosion potential (Unpublished
soil survey information).
Soil moisture contents found
during the course of this study ranged from 22% to 27%
throughout the surface 12 inches of soil.
3.3
Equipment used in locqinq
Two feller bunchers were used to fell the stand; a
Mitsubishi MS-180 and a Hitachi UHL 7-7.
Both machines are
tracked, swing boom, converted backhoes (shearing head and
additional guarding added) with hydraulic shears capable of
shearing a tree up to approximately 20 inches in diameter at
ground level.
Both machines had 30 foot booms and weighed
approximately 53,000 pounds, unloaded.
Tracks were
22
25 inches wide and 12 feet long (ground contact) with 1 inch
grousers.
Static, unloaded ground pressure on level ground
is approximately 7 pounds per square inch, assuming uniform
pressure distribution.
Figure 3.2
shows the theoretical
ground pressure distribution under the feller-buncher.
Two rubber-tired, grapple skidders were used to skid
the unit.
One was a John Deere 548D Turbo and the other a
John Deere 540.
Detailed tire information such as footprint
size, inflation pressure and tire type was not collected so
ground contact pressure could not be calculated.
The 548D had a gross weight of approximately 25,000
pounds, tire width of 29 inches, and tire diameter of five
and one-half feet.
The 540 had a gross weight of
approximately 20,000 pounds, tire width of 18 inches, and
tire diameter of five feet.
All timber was chipped on site at the landing using a
chain flail delimber/debarker prototype developed by Gibson
Chip Co., and a MORBARK model 23 Total Chipharvester.
3.4
Equipment used in Data Collection
Bulk density was selected as the measure of soil
compaction because it is a direct method and easily measured
in the field.
A single probe nuclear densiometer was used.
The single probe meter gives the "average" soil density for
the soil layers between the probe tip and the soil surface.
This meter does not give point measurements or soil density
Figure 3.2 Theoretical Ground Pressure Distribution Under a Loaded Fefler-buncher (Lysne and Burditt, 1983).
Longer Imes indicate greater pressure.
24
at a particular depth.
Originally, a double probed meter was to be used so
densities at specific depths could be measured and results
compared with other studies.
However, just before the study
began, the double probed meter malfunctioned and no
substitute could be found.
Rather than delay the entire
project, the decision was made to proceed with the single
probe meter.
Bulk density measurements were taken at depths of 4,
and 12 inches.
8
Four inches was considered to be
representative of the surface layer.
The 8 and 12 inch
measurements were taken in an attempt to determine the
pattern of change in density in the surface 12 inches of
soil.
In addition to the single probe meter, a soil corer was
used to take soil samples for gravimetric soil moisture
determination.
This is necessary in order to convert all
density measurements to dry weight basis for comparison.
3.5
3.5.1
Harvest Pattern
Fellinq Pattern
Both operators worked in a similar fashion.
They would
basically stay in 1 spot, reaching up to 20 feet into the
stand in front of them and on both sides of the machine,
collecting material to make a bunch.
Occasionally, they
would need to move forwards or backwards a few feet to
25
reposition themselves or to reach around obstacles such as
leave trees.
The more dense the residual timber, the more
repositioning they would have to do.
When finished in a
spot, they would move forward 10 - 20 feet and repeat the
Again, the more dense the residual
process described above.
timber, the shorter the distance between spots.
They would
cut trails in the unit approximately 40 feet apart, laying
the trees in bunches next to their trail with the butts
pointing towards the landing.
along a unit boundary.
The first trail would be
When a convenient stopping place
such as a dog leg in the unit was reached, the operator
would back out on the trail just cut, move into the unit
approximately 40 feet and begin cutting a new trail.
Due to
the heights of the trees, the previously cut trail would be
covered with the boles or tops of trees now being cut.
bunch contained approximately 15 trees.
Each
Since the material
was being chipped, all sound material which could be bunched
was taken.
Thus, considerable time was spent
bunching down material, which was jackstrawed.
Because of
this, the operators estimated it took them approximately 40%
longer to cut and bunch this unit compared to a unit of
similar size having predominately standing timber.
Figure
3.3 is a schematic diagram of the tree bunch pattern.
3.5.2
Skiddinq Pattern
Both skidder operators worked in a similar fashion.
Figure 3.3 Schematic Diagram of Felling Pattern
*
Direction
To Landing
Tree Bunch
Trail Cut
Order in Which
Feller-buncher Trail
Unit Boundary
KEY
27
They would operate on every second or third feller buncher
trail, depending upon the number of tree bunches.
Where
the stand volume was high, every other feller-buncher trail
was used.
In more open areas, every third trail was used
for skidding.
taken.
Generally, one bunch at a time would be
When skidding a portion of the unit, one trail would
be their "main" trail over which all bunches in that portion
of the unit would pass over.
They would take all bunches
along the "main" trail and then back off this trail over one
or two other feller-buncher trails to get those bunches.
As
much as possible, they would attempt to follow the same path
each time they backed off the "main" trail.
Thus, the
skidder operators would be traveling over every second or
third feller buncher trail a number of times, making it a
skid trail.
Occasionally two small bunches would be
combined into one large load.
To combine bunches, the
operator would pull the first bunch along the "main" trail
and then back up and pick up the second bunch.
Bunches
would then be combined along the "main" trail and taken to
the landing.
Figure 3.4 is a schematic diagram of the
skidding pattern.
3.6
Data Collection
The intent of this study is to assess the impacts
caused by a mechanized harvest operation.
For this reason,
exact number of passes for each machine at each sample point
E
-
D
Direction
To Landing
Loaded
Skidder Path
Skid Trail
Trail
Feiler-buncher
Tree Bunch
KEY
Figure 3.4 Schematic Diagram of Skidding Pattern: Skidders back up to tree bunch
and pull it out over same path
NL'
/0
Nfr
29
were not recorded.
Rather, groupings of number of passes for
each machine (which reflected normal operating procedure)
were made and the area in each of these groupings was
sampled.
The groupings were based upon observations made
during the harvesting of unit 1A and the unit harvested
prior to unit 1A.
Both units were harvested by the same
operators with the same equipment.
Work began in unit 1A as
soon as the previous unit was logged.
For this study, one
pass is defined as the passing of the machine, either loaded
or unloaded, over a specific point.
3.6.1
Machine Pass Groupinqs
Feller-buncher,
1
- 5 passes.
This was for cases where
the machine basically progressed forward in cutting it's
strip.
Occasionally it would have to back up to reposition
itself or to move around clumps of regeneration, leave trees
or other obstacles.
Feller-buncher, 6 - 10 passes.
This occurred in areas
of dense timber with a large amount of down material.
The
operator would have to reposition the machine many times in
order to make bunches.
Feller-bunchers were not observed to have more than 10
passes over any area.
Skidder,
1
- 4 passes.
This occurred in areas where
the skidder backed off the trail across one feller-buncher
trail to pick up tree bunches.
Very little repositioning
30
by the operator was required to position the tree bunch in
the grapples.
No combining of bunches was done.
Skidder, 5 - 8 passes.
This occurred when the operator
backed off and across two feller-buncher trails, when
bunches needed to be combined, or when the operator had to
do considerable maneuvering in order to correctly position
the bunch in the grapples.
In the field, there was no way to visually distinguish
between the areas having 1 - 4 and 5 - 8 skidder passes.
The soil surface appeared the same in regards to slash
depth, tire tracks and impact on existing vegetation.
Skidder, 50+ passes.
This was for major skid trails
where the skidder passed over at least 50 times.
Figure 3.5 shows the relationship between the machine
pass groupings.
There were also short secondary skid trails and
portions of main skid trails where the number of skidder
passes was between 8 and 50.
The length of these short,
secondary trails was generally less than 50 feet.
The
number of these secondary trails varied according to the
shape of the unit; the more irregular the unit shape, the
more secondary trails there would be.
The length of trail
at the ends of the main skid trails having 8 - 50 skidder
passes varied from approximately 100 feet in dense timber to
over 200 feet in sparse timber.
Both of these areas were
not sampled, as an accurate count on the number of machine
Direction
To Landing
Tree Bunch Location
Skidder - 1 to 4 Passses
Skidder - 5 to 8 Passes
1 -Sand6-lOPasses
Feller-buncher Trail
Skid Trail - 50+ Passes
Figure 3.5 Schematic Diagram Showing Relationship Between
Machine Pass Categories
h
KEY
32
passes could not be made.
Figure 3.6 shows a representation
of the layout of skid trails in relation to each other.
Soil density measurements were taken before logging,
after the feller-buncher, and after skidding, in order to
access the changes in bulk density caused by logging.
Stratified sampling was used, meaning that p'ost activity
measurements would only be taken where the machine had
passed.
For the feller-buncher,
1
- 4, and 5 - 8 skidder
passes, sample points would be taken in the machine track or
tire track, as it was felt that this would be the area of
maximum impact.
For the main skid trails (50+ skidder
passes), sample points would be located across the width of
skid trail, as it would not be possible to determine the
exact path of tire travel in these areas.
Slash depth measurements would also be taken at each
post-activity measurement point to determine if slash had
any effect on reducing soil compaction caused by logging
equipment.
Total slash plus litter depth would be measured
at each point by laying a shovel across the slash and
measuring the perpendicular distance from the shovel handle
to bare mineral soil.
3.6.2
Sample Point Preparation
For all sample point locations (before and after
logging) a square area approximately 1.5 feet by 1.5 feet
was cleared to bare mineral soil.
A spot slightly larger
Figure 36 Schematk Diagram Showing Relationship Between Skid Trail Types: Shows Skid Trails Sampled
and Not Sampled. Feller-buncher Trails Not Shown
Sampled
50+ SkIdder Passes
Main Skid Trails
Not Sampled
8 -50 SkIdder Passes
Secondary Skid Trails
Sampled
1 -8 SkIdder Passes
Ends of Skid Trails
Unit Boundary
KEY
34
than the probe base was carefully smoothed and a probe
access hole driven to a depth of 12 inches.
A minimum of
three, 15 second counts were taken at each depth (4, 8, and
12 inches).
At positions where the readings varied by more
than 5%, one or two additional readings were taken.
The
maximum number of possible readings for any depth was 5.
The mean of the counts was determined for each depth at each
point.
After all measurements were taken, soil samples were
obtained by using the soil corer.
the 0 to 6 and 6 to 12 inch depths.
Samples were taken from
The samples were sealed
in plastic bags and marked for later soil moisture content
determinat ion.
3.6.3
Before Loqqinc
Prior to logging, an "average" bulk density for the
unit was obtained.
This was done to have a "base-line" or
before treatment bulk density for comparison purposes.
The
points were selected using a grid placed over a map of the
area, with 30 sample locations selected using a random
number table.
These 30 points were then located in the
field using a hand compass and pacing distances.
This
number of sample points equates to an approximate sampling
intensity yielding 90% confidence that the average bulk
density obtained is within
density for the stand.
.1 gm/cc of the true mean bulk
Ninety-five percent confidence
35
intervals for the before logging mean bulk densities
measured are:
0 - 4 inch layer, .672 to .726 gm/cc; 0 - 8
inch layer, .669 to .711 gin/cc;
0 - 12 inch layer, .626 to
.676 gm/cc (Appendix B).
After Fellinq and Bunchinq
3.6.4
The two feller-buncher machines were operating at
opposite ends of the unit.
One machine was selected and the
number of passes it made along the trail was recorded.
When
a trail had been cut out, and before it was covered with
trees from the next trail, sample points were randomly
located in the track marks and the required data were
obtained.
After the trail was sampled, the second feller-
buncher was observed and sampled using the same process.
Data collection for the feller-bunchers ended when
approximately 90% of the unit was felled and bunched.
This
was necessary because skidding in the unit had already
begun.
Figure 3.7 is a map showing the area where feller-
buncher data were not collected.
The location of each bunch of trees was marked by
spraying paint on a nearby stump, rock or leave tree.
This
was done so the number of passes for major skid trails could
be estimated by counting the number of paint marks which led
into the trail.
0
Feet
400
® No Feller-buncher Data
No Skidder Data
- Access Road
O Landing
Unft Boundary
Figure 3.7 Map of Unit Showing Areas Not Sampled By Machine Type
8D0
Approximate Scala
KEY
37
After Skiddinq
3.6.5
Due to the fact that when skidding began, data were
still being collected for the feller-bunchers, skidding data
were only taken on the northern one-half of the unit.
Figure 3.7 shows the area where skidder data were not
collected.
skidder.
Most sample points were taken behind the 648D
Density measurements were made once the number of
passes was determined over a given point in a tire track.
For major skid trails, the number of bunches passing
over the trail was estimated by counting paint marks Sand
determining the location where at least 50 passes had
occurred.
Sample points were then randomly located from
that point up to the landing.
These sample points were
located along the entire width of the trail and not just in
the tracks.
3.6.6
Amount of Area Impacted
Total area in machine trails was desired, including the
area beneath the machine between tracks or tires.
This
could be measured for skidders by recording the area in tire
tracks and area showing evidence of trees being dragged
along the surface.
For feller-bunchers, the area beneath
the machine between tracks was not touched and thus showed
no signs of disturbance.
Rather than try to "estimate" this
area beneath the feller-bunchers, only area in feller-
38
buncher tracks was recorded.
Upon completion of skidding, transects were run to
determine the amount of area impacted by each machine.
total of 15 transects were taken.
feet in length.
A
Each transect was 100
The starting point and compass direction
for each transect was randomly located throughout the unit.
For each transect, a 100 foot tape was stretched and the
number of feet in each of the five categories below was
recorded.
No disturbance - No visible signs of disturbance.
Feller-buncher track - Imprints of feller-buncher
tracks were visible.
Skid track - Imprints of skidder tires were
visible.
Tree bunch sweep - No evidence of track or tire
imprints but evidence of trees being dragged over
the ground.
Evidence looked for was pieces of
slash on the ground, broken seedlings or shrubs or
the appearance of the ground being "swept over" by
tree tops.
Major skid trail - Obvious major skid trail, as
though a machine had cleared the trail down to
bare soil.
39
3.7
Laboratory Analysis
3.7.1
Soil Moisture Deterniination
Each soil saniple was weighed and then dried in an oven
at 105 degrees Celsius for 24 hours.
The samples were
weighed again and the percent of moisture on a dry weight
basis was calculated using the following formula:
MOISTURE
(WET SOIL WEIGHT - DRY SOIL WEIGHT)
CONTENT (%) =
(DRY SOIL WEIGHT)
X 100
Calibration of Probe
3.7.2
The probe was calibrated for this soil type by using
soil brought back from the unit harvested.
This was done by
packing a known weight of soil into a calibration box of
known volume.
Bulk density was calculated by dividing the
weight of soil by it's volume.
Bulk densities of .599,
.9, 1.0, and 1.2 gm/cc were produced.
.8,
These compared to the
range of bulk densities encountered in the field during this
study.
The number of counts in 15 seconds for 4, 8, and 12
inch depths were recorded in the same manner as the field
counts.
Regression analysis was then used to develop an
equation for estimating bulk density based upon the average
probe count for each depth.
R-squared for these 3 equations
was .98 or greater.
The equations derived were used to estimate wet bulk
density.
Wet densities were converted to a dry weight basis
40
for the moisture content for each point and depth using the
following formula:
WET BULK DENSITY
DRY BULK DENSITY = (MOISTURE CONTENT + 1.0)
(as a decimal)
41
4.0
RESULTS
Data were originally separated by model of fellerbuncher and skidder.
Since so few points were sampled for
the John Deere 640 skidder (7 out of the 60 total taken for
the skidding operation), results for both skidders were
combined.
Results for both models of feller-bunchers were
also combined.
These machines had identical dimensions,
weights, and operating pattern.
A comparison of average
bulk densities for both feller-bunchers showed no
statistical difference for all 3 soil layers (alpha = 0.01).
Results for both feller-bunchers,
1
- 5 passes (62 sample
points) and 6 - 10 passes (12 sample points), were also
combined.
There was no statistical difference between the
average bulk densities for all 3 soil layers (alpha = 0.01).
4.1 Number of Sample Points Taken
During the course of this study, a total of 165 points
were sampled.
With 3 measurements taken at each point (4,
8
and 12 inch depths), a total of 495 bulk density
measurements were taken.
Of this total, 21 were discarded
from the data set for various reasons such as organic matter
in the soil sample resulting in high soil moisture content,
lost soil samples so that soil moisture content could not be
determined, extremely rocky soils, and atypical sample point
location which did not represent the conditions intended to
be studied.
Results presented here are based upon the
42
remaining 474 measurements (Table 4.1).
Table 4.1 - Number of Measurements used for Results
No. Discarded
Original no.
No. Used for
Taken
Results
Layer (in.)
Layer (in.)
Layer (in.)
0-4 0-8
0-12
0-4
0-8
0-12
Cateqory 0-4 0-8 0-12
Before
27
30
30
30
3
2
2
28
28
Logging
After
FellerBuncher
75
75
75
1
1
1
74
74
74
After
26
Skidder
1-4 passes
26
26
1
0
0
25
26
26
14
After
Skidder
5-8 passes
14
14
3
2
2
11
12
12
After
20
Skidder
50+ passes
20
20
1
1
1
19
19
19
4.2 Bulk Density Measurements
Results presented are average dry bulk densities for
each of the 5 categories (before logging, after the fellerbuncher, after 1 - 4 skidder passes, after 5 - 8 skidder
passes and after 50+ skidder passes) for all 3 soil layers
studied (0 - 4 inch layer, 0 - 8 inch layer and 0 - 12 inch
layer).
Pooled t tests are used to determine if differences
in soil density are significant.
It needs to be kept in
mind that the bulk density measurements reported are average
densities for the 3 soil layers and not point measurements
taken at specific depths.
Table 4.2 presents bulk density
43
measurements for each of the 5 categories sampled.
Table 4.2
AVERAGE SOIL DENSITIES FOR EACH GROUPING SAMPLED
SOIL LAYER
NO. OF
SAMPLES
(INCHES)
AVERAGE
BULK DENSITY
(GM/CC)
BEFORE LOGGING
0-4
0-8
0-12
27
28
28
0.699 (0.065)
0.690 (0.055)
0.651 (0.063)
74
74
74
0.717 (0.064)
0.750 (0.057)
0.708 (0.062)
AFTER FELLER--BUNCHER
0-4
0-8
0-12
AFTER SKIDDER,
1
- 4 PASSES
0-4
0-8
0-12
25
26
26
0.694 (0.074)
0.735 (0.075)
0.712 (0.078)
AFTER SKIDDER, 5 - 8 PASSES
0-4
0-8
0-12
12
12
0.796 (0.064)
0.799 (0.057)
0.780 (0.081)
19
19
19
0.874 (0.058)
0.854 (0.039)
0.887 (0.108)
11
AFTER SKIDDER, 50+ PASSES
0-4
0-8
0-12
(standard deviation)
4.2.1
Comparisons with Before Loqqinq Density Measurements
Table 4.3 compares bulk densities for the different
conditions sampled with before logging measurements for all
3 soil layers.
Figure 4.1 displays the average soil layer
CD
0
o
a)
C
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
Soil Layer Depth (inches)
0-8
0-12
Figure 4.1 Graph of Average Soil Density by Condition
0-4
H
H
H
I
Skidder 50+ Passes
Skidder 5-6 Passes
Skidder 1 -4 Passes
Feller-Buncher
Before Logging
KEY
45
density for each of the 5 categories for all 3 soil layers.
The feller-buncher caused a significant increase in
soil density for the 0 - 8 and 0 - 12 inch layers (alpha =
The fact that the 0 - 4 inch layer did not show a
0.01).
significant increase in density may be explained by the
churning action of the grousers on the feller-buncher
This churning may be mixing the top 1 to 2 inches
tracks.
of soil, thus breaking up the compaction caused by the
machine.
Churning will greatly affect the density of the
top 4 inch layer of soil because 25 to 50% of this layer
will be affected by the grousers.
On the other hand,
churning will have very little effect on the density of the
0 - 8
and 0 - 12 inch layers because the top 1 - 2 inches
make up a smaller proportion of these soil layers.
One to 4 skidder passes also caused significant
increases in soil density for the 0 - 8 and 0 - 12 inch
layers (alpha = 0.01).
The fact that the 0 - 4 inch layer
did not show a significant increase in density may be
explained by a churning action similar to that described
above for feller-bunchers.
Here, instead of grousers, the
tire tread and sweeping action of trees being dragged along
the surface may be mixing the top few inches of soil, thus
breaking up the effects of compaction caused by the machine.
As previously discussed, this mixing of the top few inches
will greatly affect density of the top 4 inches of soil and
have little effect on the 0 - 8 and
0 - 12 inch layers.
46
Five to 8 and 50+ skidder passes caused a significant
increase in soil density in all 3 layers (alpha = 0.01).
Apparently the surface is compacted enough that the mixing
due to tire tread and dragging tree bunches is no longer
occurring.
I
Table 4.3
SOIL DENSITY COMPARISONS WITH BEFORE LOGGING
VALUES
SOIL
LAYER
(INCHES)
NET CHANGE
FROM BEFORE
LOGGING
PERCENT CHANGE
FROM BEFORE
LOGGING
SIGNIF.
(GM/CC)
(¼)
LEVEL
+2.56
+8.73
+8.81
0.11
>0.01
>0.01
-0.69
+6.60
+9.34
0.40
>0.01
>0.01
+13.95
+15.84
+19.83
>0.01
>0.01
>0.01
+25.00
+23.81
+36.30
>0.01
>0.01
>0.01
AFTER FELLER-BUNCHER
+0.018
+0.060
+0.057
0-4
0-8
0-12
AFTER SKIDDER,
0-4
0-8
0-12
1
- 4 PASSES
-0.005
+0.045
+0.061
AFTER SKIDDER, 5 - 8 PASSES
0-4
0-8
0-12
+0.097
+0.109
+0.129
AFTER SKIDDING, 50+ PASSES
0-4
0-8
0-12
4.2.2
+0.175
+0.164
+0.236
Chanqes in Bulk Density With Increasinq Depth
Table 4.4 compares soil densities for each of the 3
soil layers within each of the 5 categories to see how soil
47
density changed with depth.
Before logging, the soil had a fairly uniform bulk
density throughout.
There is no statistical difference in
soil density between the 0 -4 and 0 - 8 inch layers (alpha
0.1).
While the decrease in density between the 0 - 8 and
TABLE
4.4
CHANGES IN SOIL DENSITY WITH INCREASING DEPTH
SOIL LAYER
(INCHES)
BULK DENSITY
CHANGE (GM/CC)
PERCENT
CHANGE (%)
SIGNIF.
LEVEL
BEFORE LOGGING
0-4
0-8
-0. 0091
0 - 12
_0.0392
- 1.291
- 5.652
0.29
>0.01
+ 4.601
- 5.602
>0.01
>0.01
+ 5.901
- 3.132
0.10
0.21
+ 0.381
- 2.382
0.46
0.25
+ 2.291
+ 3.862
0.27
0.16
AFTER FELLER-BUNCHER
0-4
0-8
+0. 0331
0 - 12
_0.0422
AFTER SKIDDER,
0-4
0-8
0 - 12
1
- 4 PASSES
+0.0411
_0.0232
AFTER SKIDDER, 5 - 8 PASSES
0-4
0-8
+0. 0031
0 - 12
_0.0192
AFTER SKIDDER, 50+ PASSES
0-4
0-8
-0. 0201
0 - 12
+0.0332
NOTES
1
Change between 0 - 4 and 0 - 8 inch layers
2 Change between 0 - 8 and 0 - 12 inch layers
48
o - 12 inch layers is statistically significant (alpha =
0.01), it is only a difference of 0.04 gm/cc.
For all
practical purposes, densities in all 3 of these layers can
be considered the same.
The increase in soil density between the 0 - 4 and 0 8 inch layers after the feller-buncher is significant (alpha
= 0.01)
The decrease in soil density between the 0 - 8 and
0 - 12 layers is also significant (alpha = 0.01).
4,
After 1
-
5 - 8 and 50+ skidder passes, soil densities are constant
throughout their depths.
There is no significant change in
soil density between the 0 - 4, 0 - 8 and 0 - 12 inch soil
layers within each of these 3 machine pass categories (alpha
= 0.1).
4.2.3
Chanqes in Bulk Density With Increasinq No. of Passes
Table 4.5 compares bulk density changes for each
machine pass grouping to see if bulk density increased with
increasing number of passes.
The amount of compaction caused by the feller-buncher
and
1
- 4 skidder passes is the same.
For all 3 soil
layers, there is not a significant difference in bulk
densities between "after the feller-buncher" and after 1 - 4
skidder passes (alpha = 0.05).
There is a significant
increase in soil density between 1 - 4 and 5 - 8 skidder
passes (alpha = 0.01) for all 3 soil layers.
There is also
a significant increase in soil density between 5 - 8 and 50+
49
skidder passes (alpha = 0.01) for all 3 soil layers.
This
indicates that bulk density does increase with increasing
number of passes, up to 50.
TABLE 4.5
SOIL LAYER
(INCHES)
CHANGES IN BULK DENSITY WITH INCREASING NUMBER OF
PASSES
BULK DENSITY
CHANGE (GM/CC)
PERCENT
CHANGE (%)
SIGNIF.
LEVEL
- 4 SKIDDER PASSES VRS. AFTER FELLER-BUNCHER1
1
0-4
0-8
0 - 12
-0.023
- 3.17
- 1.96
+ 0.48
-0. 015
+0. 004
5 - 8 SKIDDER PASSES VRS.
0-4
0-8
0 - 12
+0. 102
+0. 064
+0. 068
1
0.07
0.13
0.42
- 4 SKIDDER PASSES2
+14. 74
+ 8.67
+ 9.60
>0.01
>0.01
>0.01
50+ SKIDDER PASSES VRS. 5 - 8 SKIDDER PASSES3
0-4
0-8
0 - 12
+0. 078
+0.055
+0.107
+ 9.70
+ 6.89
+13. 74
>0.01
>0.01
>0.01
NOTES
1
1-4 skidder pass density minus feller-buncher density
2 5-8 skidder pass density minus 1-4 skidder pass density
50+ skidder pass density minus 5-8 skidder pass density
4.3
Slash Depth
Slash depth was recorded after the machine had passed
over the sample point.
There is the possibility that slash
may have been deposited or removed while the machine passed
over the point and thus the slash measured was not the
actual depth at the time the machine went over the point.
Since the feller buncher did not drag trees along the ground
50
but stacked them in bunches, the chance of slash being
removed or deposited was slight.
During the process of
skidding, trees were dragged along the ground and the chance
of slash being swept away by tree tops or deposited due to
breakage was greater.
However, since the exact sample point
location could not be determined until after the skidder had
passed, it was felt that the measurement procedure followed
would be the most consistent one.
Table 4.6 shows the average slash depth and range of
slash depths encountered at each sample point location.
There was very little duff present, generally less than one
inch.
The majority of material on the ground was down trees
and broken limbs, especially considering the large amount of
dead material found in this unit.
The most noticeable slash
depth is the 11 inches after 50+ skidder passes.
This
sample point was located just beyond a half-buried log where
a depression had formed and filled in with slash.
Since it
could not be determined if the depression had formed during
skidding or not, it was assumed it had been there prior to
logging and the sample point was retained.
Bulk density
measurements for this point were not abnormally high or low.
In addition, there does appear to be a trend of decreasing
slash depth and more area swept clear of slash as the number
of passes increased.
There is a significant decrease in
slash depth between 1 - 4 and 50+ skidder passes (alpha =
0.01).
There is no significant difference in slash depth
51
between 1 - 4 and 5 - 8 skidder passes or between 5 - 8 and
50+ skidder passes (alpha = 0.1).
Differences in slash
depth between "after the feller-buncher" and all 3 skidder
pass groups in Table 4.6 are not significant (alpha = 0.1).
TABLE 4.6
SLASH DEPTH
CATEGORY
AFTER FELLER
BUNCHER
AVERAGE
SLASH
DEPTH (IN.)
DEEPEST
SLASH
DEPTH (IN.)
NO. OF
POINTS
NO SLASH
PERCENT
POINTS
NO SLASH
2.4 (2.2)
13
11
15
AFTER SKIDDER
1
- 4 PASSES 4.4 (3.0)
10
4
15
7
2
17
11
14
74
(N = 74)
(N = 26)
AFTER SKIDDER
5 - 8 PASSES 2.9 (1.8)
(N = 12)
AFTER SKIDDER
50+ PASSES
1.6 (3.2)
(N = 19)
(standard deviation)
4.4 Area Impacted
Table 4.7 displays the results of the 15 transects run
to determine amount of area impacted by each machine.
The
percent of area impacted was calculated by dividing the
number of feet recorded in each category by the total
distance measured (1500 feet).
in the transects.
There was much variability
All of the 5 impact categories were not
found on every transect.
This may explain the rather large
52
TABLE 4.7 AREA IMPACTED BY HARVESTING ACTIVITIES
TOTAL DISTANCE
RECORDED (FT.)
CATEGORY
PERCENT OF
TOTAL DISTANCE
NO DISTURBANCE
682
45.5 (18.8)
FELLER-BUNCHER TRACK
100
6.7 (8.8)
SKID TRACK
137
9.1 (7.0)
TREE BUNCH SWEEP
395
26.3 (15.2)
MAIN SKID TRAIL
186
12.4 (14.8)
(standard deviation)
standard deviations.
For purposes of discussion and management implications,
a determination of how these 5 categories of area impacted
correspond to the 5 groupings of bulk density measurements
taken was made as follows:
The feller-buncher track area corresponds to the area
impacted by feller-bunchers.
The percent of area in feller-
buncher track seems low considering that the feller-buncher
trails were approximately 40 feet apart.
However, due to
the nature of the skidding operation, all main skid trails
were first feller-buncher trails.
Using this reasoning,
19.1% of the area would have been impacted by the fellerbunchers.
In addition, only area with evidence of actual
feller-buncher track imprint was recorded.
The area
underneath the machine, between the tracks, would also be
considered part of the feller-buncher trail but would show
no sign of disturbance.
In order to accurately measure area
53
impacted by feller-bunchers, transects would have had to
been run after felling and before skidding.
This was not
possible in this study because, for a time, skidding and
felling were going on at the same time.
Also, after
approximately 60% of the unit had been felled, and before
skidding began, there was a brief rainstorm (amount of
precipitation received was unknown) which may have washed
out some feller-buncher tracks.
For these reasons, it is
likely that the area measured as being impacted by the
feller-buncher in this study may be lower than the actual
amount.
There was no visual way to determine how many skidder
passes the skid track and tree bunch sweep categories
correspond to.
Some of this area had 1 - 4 skidder passes
and the rest would have had 5 - 8 passes.
During the
skidding phase, it was observed that approximately twice as
much area had 1 - 4 skidder passes than did 5 - 8 skidder
passes.
Thus, 65% of the total skid track and tree bunch
sweep areas had 1 - 4 skidder passes and 35% of this area
had 5 - 8 skidder passes.
The majority of the area with 1 - 4 and 5 - 8 skidder
passes were not impacted by feller-bunchers.
These areas
were mainly found between feller-buncher trails as the
skidder backed off trails to pick up bunches.
It is
recognized that when the skidder backed across a fellerbuncher trail to pick up bunches, there would be small areas
54
impacted by both feller-buncher and skidder, where the trails
intersected (Fig. 3.5).
Also, at the ends of feller-buncher
trails there would be areas impacted by both machines.
This
area would be the length of trail in front of the last few
tree bunches in a trail.
Both of these areas were included
in the areas sampled for the 1 - 4 and 5 - 8 skidder pass
categories.
Not enough sample points were located in these
areas impacted by both the feller-buncher and skidder to
analyze as separate populations.
The main skid trails were impacted by both fellerbunchers and skidders.
This area is composed of main skid
trails having 50+ passes, which was sampled, and portions of
skid trails having less than 50 passes but having enough
passes to be easily distinguished as a skid trail
(predominately bare ground).
Observations made of skidding
indicates that this happened after 10 - 15 passes.
At the time the transects were run, it was not possible
to determine if the skid trail encountered had 50+ or less
than 50 passes.
However, since a 19.8% increase in density
was found in the 0 - 12 inch layer after 5 - 8 skidder
passes (Table 4.2), it is likely that the portions of the
skid trails with less than 50 passes had at least this much
of an increase in bulk density, and probably more.
These
areas would have had more than 8 skidder passes and the data
indicate that soil density increases with greater number of
passes above 8.
55
Table 4.8 shows the relationship between area impact
categories and bulk density measurement groupings.
These
values will be used for discussion and management
implication purposes.
TABLE 4.8
RELATIONSHIP BETWEEN AREA IMPACTED AND DENSITY
MEASUREMENTS
BULK DENSITY
GROUPING
AREA IMPACTED
CATEGORY
PERCENT OF
AREA IMPACTED
Before Logging
No disturbance
45.5 (18.8)
After fellerbuncher
Fel ler-buncher
6.7 (8.8)
After skidding
1 - 4 passes
Skid track and Tree
bunch sweep
23.0 (11.8)
After skidding
5 - 8 passes
Skid track and Tree
bunch sweep
12.4 (6.4)
After skidding
50+ passes
Main skid trail
12.4 (14.8)
track
(standard deviation)
Figure 4.2 is a summary chart showing the percent of
the unit in each of the 5 impact categories and the average
percent increase in soil density (compared to before
logging) for the 0 - 12 inch soil layer.
4.5 Reqression Analysis
Regression analysis was used to determine how much of
the variability in dry bulk density was explained by the
variables measured (soil moisture content, slash depth and
category of machine passes).
Since groupings of passes and
12.4% of Area
50+ Skidder passes
and feller-buncher
Distibutlon of Area Impacted
5-8 SkIdder passes
12.4% of Area
V
36.3% Increase
19.8% Increase
9.3% Increase
8.8% Increase
No Increase
Bulk Density Increase (%)
0 - 12 Inch Soil Layer
Figure 4.2 Percent of Area Impacted and Percent Increase In Soil Density for the 0 - 12 Inch Layer
for Unit 1A, After Logging
23% of Area
1-4 Skidder pses
Feller-buncher only
6.7% of Area
No impact - 45.5% of Area
57
not individual numbers were recorded, 0,
1
indicator
variables for each grouping of passes (after feller-buncher,
1
- 4 skidder passes, 5 - 8 skidder passes and 50+ skidder
passes) were used.
It was found that for all 3 soil layers sampled, soil
moisture content did not significantly contribute to the
regression (alpha = 0.1).
Only 1% of the variability in dry
bulk density measurements was explained by this variable.
This lack of effect of soil moisture is probably due to the
fact that soil moisture was fairly constant throughout the
course of this study and within the top 12 inches of soil.
Had there been a wide variation in moisture content over
time or within the top 12 inches of soil, it would be
expected that moisture content would be a significant factor
in explaining variations in bulk density.
Also, this stand
was logged when soils were dry (generally less than 25%
moisture content).
Had logging been done when the soils
were wetter, it is possible that increases in soil density
would have been higher and soil moisture content may have
then been a significant factor in the regression.
By far, machine pass category explained the greatest
amount of variability.
For the 0 - 4, 0 - 8 and 0 - 12 inch
layers, machine pass category explained 42%, 38% and 45%,
respectively, of the variability in dry bulk density.
4.9 displays the regression coefficients for the most
Table
58
TABLE 4.9
REGRESSION ANALYSIS RESULTS
VARIABLE*
COEFFICIENT
STANDARD
ERROR
SIGNIF.
LEVEL
0 - 4 INCH SOIL LAYER
Constant
Slash Depth(inches)
FB
Skidl
Skid2
Skid3
0.702
-0.010
0.039
0.034
0.115
0.188
0.012
0.002
0.014
0.019
0.021
0.018
>0.01
>0.01
>0.01
0.07
>0.01
>0.01
0.689
-0.010
0.085
0.091
0.139
0.181
0.010
0.001
0.013
0.017
0.019
0.016
>0.01
>0.01
>0.01
>0.01
>0.01
>0.01
0.651
-0.015
0.093
0.125
0.172
0.260
0.012
0.002
0.016
0.021
0.024
0.020
>0.01
>0.01
>0.01
>0.01
>0.01
>0.01
R - Squared = .50
0 - 8 INCH SOIL LAYER
Constant
Slash Depth(inches)
FB
Skidl
Skid2
Skid3
R - Squared = .47
0 - 12 INCH SOIL LAYER
Constant
Slash Depth(inches)
FB
Skidl
Skid2
Skid3
R - Squared = .56
* NOTES
Slash Depth has no value for before logging
if for feller-buncher, 0 otherwise
Skidl = 1 if for 1 - 4 skidder passes, 0 otherwise
Skid2 = 1 if for 5 - 8 skidder passes, 0 otherwise
Skid3 = 1 if for 50+ skidder passes, 0 otherwise
FB = 1
meaningful regression, the regression using slash depth and
machine pass category.
density in gm/cc.
The regression predicts dry bulk
This is the dependent variable.
The
independent variables are slash depth (in inches) and
machine pass grouping (either 0 or 1).
Thus, the 0 - 12
59
inch regression equation is:
Dry bulk density (gm/cc) =
+
+
+
0.651 - (0.015 * slash depth)
(0.093 * FB) + (0.125 * Skidl)
(0.172 * Skid2)
(0.260 * Skid3)
Slash depth also significantly contributed to this
regression (alpha = 0.01) for all 3 soil layers.
A negative
relationship between dry bulk density and slash depth was
found.
As slash depth increased, predicted dry bulk density
decreased within the same category of machine passes.
For
the 0 - 4 inch layer, slash depth accounted for 8% of the
variability found in dry bulk density measurements.
For the
0 - 8 and 0 - 12 inch layers, the amount of explained
variability was 9% and 11%, respectively.
4.6
Soil Moisture Analysis
Table 4.10 shows the results of soil moisture
determination done in the laboratory.
The one rainstorm
which occurred during the felling added a small amount of
moisture to the surface layer.
The increase in soil
moisture between 1) after skidding, 2) before logging, and
3) after feller-buncher, for the 0 - 4 and 0 - 8 inch soil
layers is significant (alpha = 0.05).
Though this
difference is statistically significant, it is less than a
5% increase and probably had little bearing on results.
Differences in soil moisture for the 0 - 12 inch layer for
60
all
3 categories are not significant (alpha = 0.1).
TABLE 4.10
SOIL MOISTURE RESULTS
CATEGORY
0 - 4
SOIL LAYER (INCHES)
0 - 12
0 - 8
BEFORE LOGGING
22.6*
(8.8)
23.1*
(8.4)
23.7* (8.1)
AFTER FELLERBUNCHER
22.6* (10.8)
22.8*
(10.3)
22.9* (94)
AFTER SKIDDING
ALL PASSES
27.3** (7.2)
26.7**
(6.9)
25.4* (6.9)
(standard deviation)
NOTES
* No statistically significant difference at alpha = 0.10
** Statistically significant difference at alpha = 0.05
61
5.0
DISCUSSION OF RESULTS
Some comparisons of trends in results between this
study and others can be done.
Previous studies found that
soil densities approached their maximum after 4 to 6 machine
passes and changed little with a greater number of passes.
For the feller-buncher, this appears to be the case.
Considering how the feller-buncher operated, it is also
possible that most of the compaction occurred in the first 1
or 2 passes.
As previously explained, the machine basically
sits in one place or moves very slightly.
It rocks back and
forth as it swings around picking up material to make
bunchers.
Much dynamic loading occurs under the tracks
along with vibration caused by the motor and sudden
movement.
Thus it seems possible that 1 or 2 passes would
be enough to cause most of the compaction that would occur.
As there was no significant increase in soil density between
1
- 5 and 6 - 10 passes (section 4.0), the maximum increase
in density may have occurred within the first 5 passes and
changed very little with a greater number of passes.
The number of passes required for the soil density to
reach its maximum under the skidder paths cannot be
determined in this study.
However, it is not likely that it
took 50 skidder passes to compact the soil to near its
maximum.
It is more likely that near maximum density
was reached between 8 and 50 skidder passes and additional
passes caused little additional change in soil density.
62
There was a significant increase in soil density between
1
- 4 and 5 - 8 skidder passes and between 5 - 8 and 50+
skidder passes.
This compares well with Figure 2.1
developed from Froehlich et al. (1980).
Burger, et al.
(1985) and Lenhard (1986) did their studies with empty
skidders driving in straight lines where both tires went
over the exact same point.
Dynamic loading was probably not
changing much in their studies as opposed to this study
where skidder speeds and loading changed with each pass.
Dynamic loading was probably much higher in this study.
Soil density increases with number of passes may also
be influenced by slash.
The regression analysis (section
4.6) found that slash significantly reduced compaction
produced by the logging machinery.
Slash may tend to reduce
the amount of compaction that can occur by spreading the
load beyond the surface area under the wheels of the
skidder, with larger pieces of slash better able to
distribute the load than smaller pieces.
Thus, down logs
and tree limbs are more effective than duff or leaf litter
in reducing compaction, and more passes over slash would be
required to cause the same changes in density than over bare
soil, litter and duff layers.
Hatchell, Ralston and Foil
(1970) did not mention slash depth in their study.
Burger,
et al. (1985) reported an average litter layer of 1.3
inches, considerably less than the litter cover found in
this study.
Froehlich (1978) and Lenhard (1986) reported
63
duff and litter depths in the ranges of those found here but
as discussed above, the larger pieces of slash found here
would be more effective in spreading out the compactive
force.
Soil moisture content may also help explain the results
found in this study.
Generally, a drier soil requires more
force to compact it to a particular density than does the
same soil at a higher soil moisture content.
Soil moisture
contents in this study were approximately 25% (table 4.7).
Hatchell, Ralston and Foil (1970) classified their soil
moisture content as high for the Atlantic Coast Plain, but
provided no figures.
Considering that this study occurred
in August, which is in the dry season in eastern Oregon, it
is quite likely that the soil moistures found in this study
were lower than those found by the Hatchell et al.
study.
(1970)
Froehlich's (1978) Mt. Hood and Jmpqua sites also
had soil moisture contents higher than the soil moisture
contents found in this study.
The literature review also found that effects of
compaction decreased with increasing soil depth, with very
little change in soil density at depths of 12 inches.
Data
in tables 4.2 and 4.4 support this trend.
For the feller-buncher, soil density significantly
increased from the 0 - 4 to the 0 - 8 inch layers and
significantly decreased from the 0 - 8 to the 0 - 12 inch
layers.
This indicates that soil density reached it's
64
maximum in the vicinity of 8 inches and decreased with
greater depths.
It is likely that the density at a depth of
12 inches was changed very little.
This seems possible when
one considers that in order for the 0 -12 inch layer to have
a lower average density than the 0 - 8 inch layer, the
average density in the 8 - 12 inch zone would have to be
lower than the 0 - 8 inch layer in order to reduce the
average density of the 0 - 12 inch layer, since both
measurements passed through the same 0 - 8 inches of soil.
This same trend of decreasing compactive effects with
increasing depth is also the case for 1 - 4 and 5 - 8
skidder passes.
For both of these cases, the highest
average densities were found in the 0 - 8 inch layer.
the differences in densities between the
inch layers were not significant, th
While
0 - 8 and 0 - 12
fact that the 0 - 12
inch layer had a lower average density indicates that the
densities in the 8 - 12 inch zone again had to be lower than
the average density for the layer above, indicating that the
maximum density would be found in the vicinity of 8 inches
in depth.
For 50+ skidder passes, this trend is not apparent.
Density decreases between the 0 - 4 and 0 - 8 inch layers
and then increases between the 0 - 8 and 0 - 12 inch layers.
It is not known what processes are occurring to cause the
unusual readings for the 0 - 8 inch layer.
Perhaps there is
a root zone in the 4 - 8 inch zone which acts as a "sponge."
65
When the skidder passes over, this zone absorbs very little
compactive force but is merely compressed and transmits the
force to the soil below.
66
6.0 MANAGEMENT IMPLICATIONS AND RECOMMENDATIONS
A.s presented in the Introduction (Section 1.2)
detrimental soil compaction for volcanic ash/pumice soils is
defined as an increase in soil bulk density of 20% or more
over undisturbed levels (USDA Forest Service, 1987).
This
definition applies to any soil layer at least 4 inches thick
between the depths of 4 to 12 inches (personal conversation
with Bob Meurisse, Regional Soil Scientist, USDA Forest
Service, Region 6).
From table 4.2 it can be seen that the
feller-buncher and 1 - 4 skidder passes have not caused the
soil density to cross this threshold.
The increases in bulk
density for these 2 cases are 8.8% and 9.3%, respectively
for the 0 - 12 inch soil layer.
Conversely, 50+ skidder
passes did cause detrimental compaction.
The increase in
soil density was 36.3% for the 0 -12 inch layer.
The
situation is not as clear for 5 - 8 skidder passes.
Here,
the increase in bulk density was 19.8%, not quite 20% but
very close.
Since the increase in density for 5 - 8 skidder
passes is so close to the unacceptable level, for purposes
of discussion here it will be taken as having a 20% increase
in density and thus detrimentally compacted.
This seems
likely when one considers that all it would take for the
increase in density to be 20% instead of 19.8% would be for
the average density of the 0 - 12 inch layer for 5 - 8
skidder passes to be 0.001 gm/cc higher than that reported.
This is a very small change when considering all of the
67
variables involved.
Thus, 24.8% of the total area suffered
detrimental compaction.
As previously stated, areas receiving 5 - 8 skidder
passes were predominately located between feller-buncher
trails where the skidder had backed across 2 feller-buncher
trails (Fig. 3.5).
This area shows little evidence of
impact on the surface and is almost identical in appearance
to the 1 - 4 skidder pass areas.
Thus, it would be
extremely difficult, if not impossible, to identify this 5 8 skidder pass area in the field.
For this reason, it is recommended that the skidders
only be allowed to back across 1 feller-buncher trail thus
confining the area suffering detrimental compaction to the
easily identifiable skid trails.
area being in skid trails.
This will result in more
It is estimated that the area in
skid trails would increase from approximately 12% to
approximately 18%.
One benefit would be that the total area
detrimentally compacted would be reduced from almost 25% to
approximately 18%. Another benefit would be that most of the
area suffering unacceptable increases in bulk density would
be easily identifiable in the field when mitigation
measures, as required by the timber sale contract, would be
done.
68
7.0
FURTHER RESEARCH
For future studies, the following changes and
modifications in study design and procedure are recommended
to attempt to answer some of the questions created by this
study.
Use of a double probed density gauge.
This would allow
point measurements at specific depths to be taken resulting
in easier interpretation of results and easier comparisons
with other studies.
The maximum depth the effects of
compaction reached could be determined.
Also, a compaction
curve for the soil could be developed, which may be useful
to the land manager or other researchers.
Attempt to measure areas where the exact number of
feller-buncher passes is known, particularly areas having 1,
2,
3, and 4 passes.
This would help to answer the question
on how many passes it takes to cause most of the compaction
that will occur under the feller-buncher.
Do impact transects similar to those done in this study
after both the feller-buncher and skidder.
This would allow
more precise estimates on the amount of area impacted by the
feller-buncher to be made.
Thus, comparisons between swing
boom and tree-to-tree feller-bunchers could be made in
regards to amount of area impacted.
In addition, even
though the feller-buncher in this study only caused an
increase in soil density of 8.8%, on a different soil or
under different conditions the increase in density could
69
become unacceptable.
Then, knowing the amount of area
adversely impacted would become very important to the land
manager for possible mitigation measures.
Attempt to access the amount of area having between 8
and 50 skidder passes and what increases in soil density is
occurring in this area.
As previously noted, the amount of
area in this category was small in this unit.
In other
units or with different operators, this area could be quite
large, especially if there are a lot of short, 20 - 30 foot
long feller-buncher trails.
It is not known from this study
what is going on in this area in terms of soil compaction.
Also, from this study it could not be determined how many
skidder passes it takes to compact the soil to the point
where additional passes cause no significant increases in
soil density.
4,
Suggested groupings of skidder passes are 1
5 - 8, 9 - 16, 17 - 30, and 30+.
-
These groupings might
be better able to answer these questions.
Collect more data in the 5 - 8 skidder pass area than
was collected in this study.
This area appears to be the
critical number of passes for the break between detrimental
and non-detrimental compaction, using the definition given.
This break is critical to land managers in deciding if
treatment is needed in order to mitigate the detrimental
effects of compaction.
Collect more information on the amount of area impacted
by each machine due to the wide variation found in this
70
study.
This would allow more precise estimates of the
amount of area impacted by each machine to be made.
71
8.0
REFERENCES
Adams, Paul W. and Henry A. Froehlich, 1981.
Forest Soils, PNW 217.
Compaction of
Alexander, Earl B., 1985. Soil Disturbance and Compaction
in Wildland Management. USDA Forest Service, Pacific
Southwest Region, Watershed Management Staff, Earth
Resources Monograph 8.
Am. Soc. Ag. Eng. - Soil Sci. Soc. Am. Soil Compaction
Committee Report.
1958.
Concepts, terms, definitions
and methods of measurement for Soil Compaction. Ag.
Eng., 39:173-176.
Aulerich, D. Edward, Norman K. Johnson and Henry Froehlich,
1974.
Tractor or Skyline: What's Best for Thinning
Young-growth Douglas-fir? For. md Mag., 101(12):4245.
Burger, J. A., V. Perumpral, R. E. Kreh, J. L. Torbert and
S. Minaei, 1985.
Impact of Tracked and Rubber-Tired
Tractors on a Forest Soil.
American Society of
Agricultural Engineers, 28(2):369-373
Campbell, Robert G., James R. Willis and Jack T. May, 1973.
Soil Disturbances by Logging with Rubber-tired
Skidders. J. Soil Water Cons., 28:218-220.
Cochran, P. H. and Terry Brock, 1985.
Soil Compaction and
Initial Height Growth of Planted Ponderosa Pine.
USDA
For.
Serv., Res. Note PNW-434, 4 pp.
DeVore, Jay and Roxy Peck, 1986. Statistics, The
Exploration and Analysis of Data.
West Publishing
Company, 701 pages.
Dyrness, C. T., 1965. Soil Surface Conditions Following
Tractor and Highlead Logging in the Oregon Cascades.
J. For., 63:272-275.
Flint, Alan L. and Stuart Childs, 1984.
Development and
Calibration of an Irregular Hole Bulk Density Sampler.
Soil Sci. Soc. Am. J., 48:374-378.
Froehlich, Henry A., 1978.
Soil Compaction from Low
Ground-Pressure, Torsion Suspension Logging Vehicles on
Three Forest Soils.
Forest Research Laboratory, Oregon
State University, Corvallis. Research Paper 36.
12p.
72
Froehlich, H. A., 1979.
Soil Compaction From Logging
Equipment: Effects on Growth of Young Ponderosa Pine.
J. Soil Water Cons., 34:276-278.
Froehlich, H. A., J. Azevedo, P. Cafferata, and D. Lysne,
1980.
Predicting Soil Compaction of Forested Land.
Final Project Report, Coop. Agreement No. 228.
USDA
Forest Serv., Equip.
Div. Cent., Missoula, Mont.
Froehlich, Henry A. and David H. McNabb, 1983. Minimizing
Soil Compaction in Pacific Northwest Forests. A paper
presented at the 6th North American Forest Soils
Conference on Forest Soils and Treatment Impacts.
Froehlich, H. A., D. W. R. Miles and R. W. Robbins, 1985.
Soil Bulk Density Recovery on Compacted Skid Trails in
Central Idaho.
Soil Sci. Soc. Am. J., 49:1015-1017.
Froehlich, H. A., D. W. R. Miles and R. W. Robbins, 1986.
Growth of Young Pinus ponderosa and Pinus contorta on
Compacted Soil in Central Washington.
Forest Ecology
and Management, 15(1986):285-294.
Gent, J. A., Jr., R. Ballard and A. E. Hassan, 1983. The
Impact of Harvesting and Site Preparation on the
Physical Properties of Lower Coastal Plain Forest
Soils.
Soil Sci.
Am. J., 47:173-177.
Hatchell, G. F., C. W. Ralston and R. R. Foil, 1970.
Disturbance in Logging.
J. For., 68:772-775.
Soil
Helms, John A. and C. Hipkin, 1986. Effects of Soil
Compaction on Tree Volume in a California Ponderosa
Pine Plantation.
Western Journal of Applied Forestry,
Vol. 1, No. 4: 121-124.
Laing, Larry E. and Steven W. Howes, 1983.
Detrimental Soil
Compaction Resulting From a Feller Buncher and Rubber
Tired Skidder Timber Harvest Operation:
A Case. Study.
USDA Forest Service, Pacific Northwest Region, Range
and Watershed Management, l3pp.
Lenhard, R. J., 1986.
Changes in Void Distribution and
Volume During Compaction of a Forest Soil.
Soil Sci.
Am. J., 50:462-464.
Lysne, D. H. and A. L. Burditt, 1983.
Theoretical Ground
Pressure Distribution of Log Skidders.
Transactions of
the ASAE, Vol. 26, No. 5, pp. 1327-1331.
Moehring, D. M. and J. K. Rawls, 1970. Detrimental Effects
of Wet Weather Logging.
J. For., 68:166-167.
73
Schuh, Donald D. and Loren D. Kellogg, 1988. TimberHarvesting Mechanization in the Western United States:
An Industry Survey.
Western Journal of Applied
Forestry, Vol. 3, No. 2: 33-36.
Sidle, R. C. and D. M. Drlica, 1981.
Soil Compaction From
Logging with a Low Ground Pressure Skidder in the
Oregon Coast Range.
Soil Sci. Soc. Am. Proc., 45:12191224.
Silvercides, C. R., 1984. Mechanized Forestry, World War II
to the Present.
Forestry Chronicle, 60(4):231-235
Steinbrenner, E. G. and S. P. Gessel, 1955. The Effect of
Tractor Logging on Physical Properties of Some Forest
Soils in S. W. Washington.
Soil Sci. Soc. Am. Proc.,
19: 372-376.
Stokes, Bryce J. and Donald L. Sirois, 1982.
Operational
Characteristics of a Harvester in Intermediate
Cuttings.
A paper presented at the Second Biennial
Southern Silvicultural Research Conference, Atlanta,
Georgia, November 4-5, 1982.
Van Bavel, C. H. M., 1958. Soil Densitometry By Gamma
Transmission.
USDA Forest Service, North Carolina
Agricultural Experiment Station, Journal Paper No. 887,
pages 50-58.
Vomocil, James A., 1954.
In Situ Measurement of Soil Bulk
Density.
Ag. Engr. 35:651-654.
Wert, S. And B. R. Thomas, 1981. Effects of Skid Roads on
Diameter, Height and Volume Growth of Douglas-fir.
Soil Sci. Soc. Am. J., 45:629-632.
Youngberg, C. T., 1959.
The Influence of Soil Conditions
Following Tractor Logging on the Growth of Planted
Douglas-Fir Seedlings.
Soil Sci. Am. J., 23:76-78.
APPENDIX A
USDA FOREST SERVICE TIMBER SALE CONTRACT CLAUSES PERTINENT TO
THIS STUDY.
A-i
PART CT6.O - OPRATI0N5
CTS.42f - Soecial Yardino Objectives. (4/82)
Special yarding metho
shall oc ac:omplisneo oy tne te or yarding equipment listed herein.
Methods other than those specified may be approved in writing If
methods meet the objective to protect the residual stand and to
detrientaj demae to the soj..
uct approva sna
inc uce Increases
appropriate.
in
urrent
ontrac:
ates wnere
Sale Area Map shall be revised to record such approval.
Soecial Yartf no Methods
Pvment Unit
1.
Skid or yard trees full-length to the landing
2.
Skid trees with one end suspension
3.
Skid tra±ls shall be at least 80 feet apart,
center to cen:e; encept where converging.
4.
Skid trail. shal2. be at lea.: 60 feet apart,
center to center, encept where converging
5.
Skid trails shall not be wider than 10 feet.
Single tractor passes off of skid trail. are
allowed as long as rutting or detrinentaj.
ccnpac:ion (refer cT6.4# (OptLom 3) is avoided.
l,2,3,4,3,6,7,8,LA,2A,
3A,4A,5A
3A,4A,5A
Acceotable Tardjn
1.
reve
Eauioment
GA,74
3A,4A,SA
All.
Payment unit
Any tractor or line system capable of
accomplishing the above methods
AU
-.1
P age
A-2
a.-
PART c76.D - OPERATIONS
CT6.4# (Dotfon 3) - Conduct of Locolna. (10/85' Other methods or requfremen
e agreed
in writing between Purc:iaser and Forest Service. The following
hlng measures are applicable for use in addition to those in 8T6.4:
s.,-
Recuirements Aoollcable to Clearcuttino Unt.
(I) Notwithstanding 872.31 all live and dead trees,
except Reserve Trec
subject to CTZ.3#, which exceed 4 Inches in diameter breast high are
designated and required to be cut. Trees reouired to be cut, that
meet the minimum 0311 in AT2, sha}l be felled c:ncurrenty with the
logging operation. Concurrent felling does not apply when Stage
logging is authorized.
Recuirementz Aoolicable to Partial Cuttinc Areas.
The use of roll-out tractors will, not be permitted.
Payment Units of Sale Area where tree length logging is prohibited
are so designated on Sale Area Map. Tree length loaglng is defined
as skidding, yarding or otherwise moving a tree in one piece not
Including the stump or top (that part of the tree less than the
d.i.b. specified in AT2).
(ill) When necessary to meet the terms and cunditions of 3T6.42, the winch
line shall be unspooled from the tractor winch anc manually pulled to
the location of individual logs for skidding purposes.
'c)
General Loacino Reauirements.
(i) If Sale Area Nap designates any cuttIng unit for
stage loggln'a,
timber in such a unit shall be removed in two or more separate
felling and yardina operations in accordance with a plan agreed to by
Purchaser and Forest Service. Stage logging by species will not be
permitted on Sale Area, unless Purchaser and Forest Service acree to
conditions for such an operation, including but not limited to an
increase In performance bond amount.
(if)
If designated on Sale Area Map, landings, Temporary Roads, and/or
tractor skid trails or portions thereof which are detrimentally
compacted as defined below shall be ripped through the compacted zone
but not to exceed a depth of 20 inches. Ripping shall span the width
of compacted area, and the distance between ripped furrows shall not
exceed 36 inches. Ripping shall be done with winged Subsoilers
during Normal Operating Season unless otherjise approved by the
Forest Service.
Detrimental compactton Is defined as a 15 percent increase in soil
bulk density frme the undisturbed level for payment units 6A and 7A
ano a like increase of 20 percent for payment units 1. 2,
7, 8, lA, 2A, 3A, 4A, and 5A
Page
/
A -3
, 4,
, b,
APPENDIX B
Calculation of Number of Sample Points for Before Loqqinq
Conversations with the two Forest soil scientists
indicated that the soils in this unit were fairly uniform in
soil properties such as texture, rock content, parent
material and bulk density.
A reasonable estimate of
average, before logging bulk density was .7 gm/cc.
Assuming most before logging bulk densities would fall
in the range of .55 to .95 gm/cc (standard deviation of .25
gm/cc), and desiring to have 90% confidence that the bulk
density determined would be less than +.1 gm/cc of the true
mean bulk density, the following formula was used to
determine the number of before logging sample points needed
(DeVore and Peck, 1986).
ta ()2
n=
where:
n
t
s
B
=
=
=
=
B2
sample size (solved for)
student's t value for 90% confidence
estimated standard deviation
specified error of estimation
Substituting in values:
n =
(1.645)2
(.25)2
(.075)2
= (5.48)2
:
30
Thus, 30 sample points should be sufficient before logging.
B-i
Calculation of 95% Confidence Intervals for Measured
Before Loqqin Bulk Densities
To determine the actual confidence in the measured,
before logging soil densities, 95% confidence intervals were
calculated with this formula (DeVore and Peck, 1986):
S
CI = X + t * n1'2
where
X
CI
t
n
S
=
=
=
=
=
observed mean bulk density for the soil layer
confidence interval
student's t value for 95Z confidence
number of sample points taken
standard deviation of the mean density
The chart below summarizes the calculations for each
soil layer.
These confidence intervals are for the number
of measurements used for the results.
Soil
Layer
-
(in.)
X (gm/cc)
o - 4
0.699
27
0.065
2.06
.013
0 - 8
0.690
28
0.055
2.05
.010
0 - 12
0.651
28
0.063
2.05
.012
S
S (gm/cc)
n
t
Computed Confidence Intervals
0 - 4 inch layer
= .699 ± .027 = .672 to .726 gm/cc
0 - 8 inch layer
= .690
.021 = .669 to .711 gm/cc
0 - 12 inch layer = .651 ± .025 = .626 to .676 gm/cc
B-2
n1'2