COMPARISON OF POTENTIAL EROSION FOLLOWING CONVENTIONAL AND
CABLE YARDING TIMBER HARVESTS IN THE APPALACHIAN PLATEAU
REGION OF VIRGINIA
William C. Worrella, M. Chad Boldingb, and W. Michael Austc
a
Associate Forestry Extension Agent, [email protected]
Virginia Cooperative Extension Service, Lebanon, VA 24266
b
Assistant Professor of Forest Operations/Engineering, [email protected]
c
Professor of Forest Soils/Hydrology, [email protected]
Department of Forest Resources and Environmental Conservation
228 Cheatham Hall, Virginia Tech
Blacksburg, VA 24061
ABSTRACT – Cable yarding systems have potential environmental advantages over groundbased skidding on steep terrain. Our goal was to compare potential soil erosion losses from cable
yarding and conventional skidder harvests in the steep Appalachian Plateau region of Virginia.
We evaluated erosion on three sites where cable yarding and conventional skidding were
occurring in close proximity by using the Universal Soil Loss Equation as adapted for forestlands
(USLE-Forest) to model soil erosion. We estimated soil loss in a minimum of three locations for
five yarder activities (deck, yarder landing, spur road, corridor, and harvest) and three skidder
disturbance categories (deck, skid trail, and harvest). We used GPS to calculate the area in each
disturbance category and used these values to estimate total overall erosion for the skidder and
cable yarder harvests. Overall, the cable yarder and skidder operations produced similar potential
erosion estimates of 1.86 and 1.70 tons/ac/yr, respectively. This similarity was caused primarily
by the high estimated erosion (>25 tons/ac/yr) of the spur road that was used to connect the cable
yarder landing with the log deck. On our sites, the current cable yarder operation did not offer
clear erosion prevention advantages, but the yarding operation could have been significantly
improved with greater attention to spur road layout and Best Management Practices (BMPs).
Introduction
Conventional harvesting systems on the steep terrain of the Appalachian Plateau region utilize
ground-based skidders operating on skid trails bladed by bulldozers. Alternate harvesting
systems for steep terrain, such as skyline cable yarding, are not as widely utilized or available in
the region, but it is generally believed that such systems could be advantageous for minimizing
area of disturbance as well as soil erosion (Miller and Sirois, 1986).
Best Management Practices (BMPs) for forest harvesting were developed to minimize erosion
and protect water quality and subsequently help protect forest site productivity (Aust and Blinn,
2004). The region of operations, local soil types and weather conditions during the operation,
and forest road and trail layout all influence soil erosion from timber harvesting operations.
Timber harvesting can cause significant changes in soil physical properties in the upper portion
of the soil (Gent et al., 1984). When logs and litter are removed from a forest soil in timber
harvesting and soil compaction occurs, the soil microbial biomass, soil moisture content, and
nutrient levels are reduced (Jordan et al., 1999). Ground cover protects the soil in several ways,
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including energy dissipation of rainfall, wind, and surface runoff. Litter on the soil surface
promotes infiltration, which decreases runoff (Clayton, 1981). The amount of damage to the soil
surface and the litter layers varies for each logging operation; however, alternative harvesting
systems such as cable yarding operations may potentially have less impact on the soil surface
because the timber is partially lifted off the ground with a system of cables and towers. The
ground-based systems create more disturbance of the surface soil and litter layer, which reduces
overall soil quality.
Road construction and distribution over the forest floor is the major factor leading to potential
erosion from harvesting systems. In skyline cable logging, the forest floor is primarily impacted
in the cable corridors, where the logs are suspended above ground on the system of cables and
towers. In these corridors, some soil disturbance will occur because some of the trees will be
partially dragged on the ground, displacing the litter layer and exposing small areas of soil to
erosion. In ground-based logging, skid roads are constructed across the entire harvest area to pull
the trees to a landing and loading area. The ground-based system has the greatest impact on the
surface soil and litter layer because heavy equipment is operating on the soil surface.
Ground-based timber harvesting operations have the potential to cause severe soil disturbances
that can reduce soil productivity, particularly when ground-based operations require construction
of bladed skid trails (Aust et al., 2006). In addition to removal of the litter layer and compaction,
bladed skid trails sidecast the surface soil and create low standard roads that can amount to 10%
of the area (Kochenderfer et al., 1997). These skid trails typically have reduced site productivity
and increased the drainage density of the site, thus contributing additional sediment to streams
(Eisenbies et al., 2007).
Ground-based logging usually produces well-defined skid trails on the forest floor. Soil
compaction negatively impacts soil structure and is less favorable to tree growth. The impact of
skid trail construction on the growth of the next forest crop is dependent upon the number and
distribution of skid trails over the harvest site. One way to reduce the impacts on soil quality
would be to carefully locate and mark all skid trails to be used during the ground-based
harvesting operation (Froehlich, 1957). The impact of conventional logging systems rests in the
number and distribution of the skid roads, so it is very important for forest managers to limit the
number of skid roads that the logging operation uses on the forest site.
Alternative harvesting systems can effectively remove timber from the forest and help maintain
soil quality by having less impact on the entire harvest site. For example, skyline cable logging
caused substantially less site disturbance than rubber-tired cable skidders in research studies in
the mountains of Georgia (McMinn, 1984).
Soil erosion becomes sediment when it enters a stream. Much of the eroded soil will be trapped
in the litter on the forest floor before reaching a stream. Yoho (1980) reported that undisturbed
mixed forests produced up to 0.32 tons/ac/yr year of sediment movement offsite. He also
reported from 0.06 to 0.17 tons/ac/yr of sediment yield in a carefully clearcut forest, while a
carelessly harvested clearcut generated 1.35 tons/ac/yr of sediment. Sediment production from a
timber harvest is increased when stream banks and channels are disturbed by logging equipment
(Yoho, 1980). Cable logging systems suspend or partially suspend the logs off the ground in the
2
skidding process, almost eliminating the possibility of disturbance to stream banks or channels.
However, conventional logging systems that use ground-based skidders must sometimes cross
streams in order to reach all areas of the harvest, and any type of stream crossing will cause some
impact to the stream banks and potentially to the stream channel. Kochenderfer et al. (1997)
found that sediment doubled during the first year after harvest but returned to pretreatment levels
within three years. Swank et al. (2001) found large increases in sediment following road
construction and major storm events. Sediment from logging activities was greatly reduced and
insignificant when the logging was completed (Swank et al., 2001).
Timber harvesting can increase erosion, sediment, and nutrient losses to streams. Aust and Blinn
(2004) found the sediment quantities introduced to streams to be relatively low and below the
levels that are considered acceptable for alternative land uses. Most studies indicate that within
five years of a timber harvest, the water quality recovers, especially when BMPs are followed in
the harvesting operations (Aust and Blinn, 2004). Croke et al. (2001) found that erosion rates
declined by almost one order of magnitude over a five-year period following harvesting.
Our research objective was to compare erosion rates on sites harvested with cable yarders with
erosion rates on similar sites harvested with tracked cable skidders.
Methods
The research was conducted on private forestland in Dickenson County, Virginia. The study
sites were located in the Appalachian Plateau physiographic province, a region characterized by
steep relief. The soil series in the study areas were dominated by the Highsplint-Shelocta and the
Matewan-Gilpen soils. These soils are loams to sandy loams and side slopes ranged to above
100%. The study sites contained a variety of cove and upland hardwood species, including
northern red oak, chestnut oak, white oak, yellow-poplar, and red maple.
The harvesting systems were a cable yarder with a 30-foot tower and maximum corridor reach of
1200 feet. A bulldozer was used to pull the logs from the yarder to a knuckleboom loader for
processing into logs and pulpwood. The mechanical logging was completed with a tracked cable
skidder (bulldozer). The terrain on the sites required bulldozer logging to be completed with
bladed skid trails across the harvest area. Both harvesting systems used manual tree felling.
Potential soil erosion was estimated using the Universal Soil Loss Equation as modified for
forest land by Dissmeyer and Foster (1984). Data were collected on the harvest sites in the
summer of 2009, six to nine months after the harvest was completed. The soil erodibility factor
was determined from the USDA NRCS Soil Survey for Dickenson County. Slopes were
measured with a Suunto clinometer. Distances were determined with a GPS receiver. The area
was calculated from timber sale maps by using the recorded distances and measurements.
The study was analyzed as a randomized complete block design having three blocks of two
treatments. The two treatments were conventional logging and yarder logging. Soil disturbance
was classified into categories for each harvesting system. The locations sampled on the
conventional harvest sites were the deck, the bladed skid trails, and the harvest area. The
locations sampled on the yarder logging sites were the harvest area, the yarder site, the yarder
road used to pull logs from the yarder to the landing, and the corridor. For each location, one
3
sample was collected on the deck; three samples in the harvest area; six samples on skid trails;
three samples on yarder roads; six samples in corridors; one sample on each yarder site; and one
sample in an unharvested area adjacent to the site. All the harvest sites were regeneration
harvests.
Results and Discussion
The timber harvesting operation used both cable yarding and tracked skidding to complete the
entire timber harvest. The components of the harvesting system are shown in Figure 1. The cable
yarding side of operations included a deck used for processing and loading; a spur (bladed) road
used to pull the logs from the yarder landing to the deck; the yarder landing; and the corridors
across the harvest area where the mainline was used to pull the logs to the yarder landing. The
tracked skidder operation used the same deck and consisted of a system of bladed skid trails used
by the bulldozer to pull logs to the deck.
Deck
Figure 1. Components of the timber harvesting operation.
The harvested areas had an average erosion rate of 0.6 tons/ac/yr, while nearby unharvested areas
had an average erosion rate of 0.47 tons/ac/yr (Table 1). Conventional logging with a tracked
cable skidder had 1.86 tons/ac/yr average erosion rate from the three research sites. The greatest
soil erosion rate found on the tracked skidder operation was from the bladed skid trails, at 17.18
tons/ac/yr.
The tracts harvested with the cable yarder system had an average erosion rate of 1.7 tons/ac/yr.
The spur roads had the highest erosion rate, 25.06 tons/ac/yr. The area of the spur roads averaged
4
0.17 acres per site; however, they generated 34% of the sediment from the cable yarding
operation.
The logs were pulled to the deck by either the tracked skidder or the cable yarder. The main
difference between the cable logging system and the mechanical logging system was the method
of in-woods transport of the trees. The cable system utilized the cable yarder and pulled the logs
to the landing through the corridors. The cable skidder used bladed skid trails to pull logs to the
deck. The study showed that the spur road and skid trails had the greatest erosion rates. The skid
trails had an average erosion rate of 17.18 tons/ac/yr, while the spur roads had an average erosion
rate of 25.06 tons/ac/yr. The three sites had an average of 0.97 acres of skid trails and 0.17 acres
of spur roads. There was an average of 1.22 acres of corridors with an average erosion rate of
4.49 tons/ac/yr, which generated 5.14 tons of soil loss.
Table 1. Average predicted soil loss for three Appalachian sites harvested with tracked
cable skidders on bladed skid trails and cable yarder systems by disturbance categories.
Harvest
System
None
Tracked
cable
skidder
Cable
yarder
Disturbance
Category
Nonharvested
Deck
Skid trail
Harvest
Overall
Deck
Yarder landing
Spur road
Corridor
Harvest
Overall
Average
Erosion Rate
(tons/ac/yr)
0.47
1.16
17.18
0.6
1.86
1.16
1.05
25.06
4.49
0.6
1.7
Average
Area
(ac)
--0.11
0.97
11.92
12.67
0.11
0.057
0.17
1.22
5.78
7.33
Average
Total Erosion
(tons/yr)
--0.13
16.68
6.75
23.56
0.13
0.06
4.21
5.14
2.92
12.46
In getting the logs from the stump to the deck, the cable system yielded a lower rate of soil
erosion than did the skid trails. The low erosion rate in the corridors shows the benefit of cable
logging operations. The skid trails generated an erosion rate of 17.18 tons/ac/yr, while the
corridors only yielded 4.49 tons/ac/yr of soil loss. To reduce the amount of soil erosion from
timber harvesting operations, cable yarding systems should be utilized. The cable system allows
the skidding of logs and pulpwood without having to build bladed skid trails on steep mountain
slopes. However, to reduce overall soil erosion from cable logging systems, the high erosion rate
of the spur roads must be decreased.
With improved preharvest planning, the number of spur roads could be reduced, which would
decrease the total erosion from the harvesting site. The spur roads had the greatest erosion rates
on average, yielding 25.06 tons/ac/yr of sediment. This erosion rate is high; however, on
average, the three yarding sites had 0.17 acres of this disturbance area.
5
With regard to the total average soil loss from these harvesting systems, the cable yarding system
yielded 11.1 tons/yr less than the tracked cable skidder system. The tracked skidder sites in the
study involved more acreage, but when the total erosion from the skid trails (16.68 tons/yr) is
compared with the total erosion from the yarder landing, spur road, and corridors (9.41 tons/yr),
it can be seen that the tracked cable skidder system produced 7.27 tons/yr more erosion than the
cable yarding system.
A more complete pre-harvest plan could reduce erosion from cable yarding operations. The key
would be to locate the yarder landing close to the deck so the need for spur roads can be reduced.
If spur roads are needed, then better layout using BMPs could reduce the erosion rate from this
disturbance area. Reducing the number and acreage of spur roads would reduce the amount of
erosion from the harvest site, because the spur roads were found to have the highest erosion rate
in the study.
The data from all three research sites are compared in Table 2. On Site 1, the cable yarder system
and the skidder operation had the same average erosion rates of 1.03 tons/ac/yr. The average
erosion rate for the spur roads was 30.25 tons/ac/yr, while the average erosion rate for the skid
trails was only 15.64 tons/ac/yr. The data for Site 2 show that the average erosion rate for the
cable yarder system was 2.31 tons/ac/yr, while the skidder operation had an average erosion rate
of 1.43 tons/ac/yr. This illustrates the importance of improving pre-harvest planning to improve
the layout and use BMPs for spur roads in a cable logging operation. In Site 2, the erosion rate
for the spur road was more than double the erosion rate for the skid trails, at 24.91 and 9.96
tons/ac/yr, respectively. The data from Site 3 show that the skid trails had a higher erosion rate
(25.95 tons/ac/yr) than the spur roads, which had a rate of 20.02 tons/ac/yr.
Summary
Forest harvesting operations can potentially have tremendous impacts on soil quality. Our study
found that a conventional logging system with a tracked cable skidder created more erosion on
harvesting sites than a cable yarder system. However, only a small decrease in erosion was
found with the cable logging system. The largest erosion rate from all the disturbance categories
in the study was found on the spur roads, with an average of 25.06 tons/ac/yr of soil loss. More
advanced rigging techniques could possibly enhance the effectiveness of the yarder equipment,
which could reduce the need for spur roads by having the yarder landing beside the log deck.
Better pre-harvest planning in cable logging operations could improve the layout of the deck, the
yarder landing, and the spur roads. If the spur road disturbance was reduced or eliminated from
the operation, there could be significant reductions in erosion from cable logging operations.
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Table 2. Predicted soil loss for three Appalachian sites harvested with tracked cable skidders
on bladed skid trails and cable yarder systems by disturbance categories.
Skidder Operations
Site
1
2
3
Yarder Operations
Estimated
Total
Erosion
(tons/yr)
Disturbance
Category
Area
(ac)
Average
Erosion
(tons/ac/yr)
Deck
Skid trail
Harvest
0.1
0.63
12.27
1.48
15.64
0.28
0.15
9.85
3.43
Total
Deck
Skid trail
Harvest
13.0
0.12
1.17
12.71
1.03
1.04
9.96
0.65
13.43
0.13
11.65
8.26
Total
Deck
Skid trail
Harvest
14.0
0.1
1.1
9.8
1.43
0.96
25.95
0.87
20.04
0.1
28.55
8.57
Total
11.0
3.38
37.22
Disturbance
Category
Area
(ac)
Average
Erosion
(tons/ac/yr)
Deck
Yarder landing
Spur road
Corridor
Harvest
Total
Deck
Yarder landing
Spur road
Corridor
Harvest
Total
Deck
Yarder landing
Spur road
Corridor
Harvest
Total
0.1
0.08
0.14
1.57
9.11
11.0
0.12
0.05
0.19
1.24
4.4
6.0
0.1
0.04
0.18
0.86
3.82
5.0
1.48
0.94
30.25
2.76
0.28
1.03
1.04
1.01
24.91
4.89
0.65
2.31
0.96
1.19
20.02
5.83
0.87
2.44
Estimated
Total
Erosion
(tons/yr)
0.15
0.08
4.24
4.34
2.55
11.36
0.13
0.05
4.73
6.06
2.86
13.83
0.1
0.05
3.67
5.02
3.34
12.18
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