Northeast Forestry University
Harbin, China
The author is Associate Professor, Director of Centre for Forest Operations and Environment.
The proportion of animal skidding in forest operations in Heilongjiang Forest Region has increased significantly in recent years. First, the development of animal skidding and machine skidding was demonstrated and analyzed. Then, two methods of ground skidding currently used in this region were evaluated both individually and comprehensively using the following criteria: operation efficiency or operation cost, degree of damage on soil and on residual stands, accident rate, and natural regeneration. Finally, according to the results of synthetic assessments, classification of operational conditions suitable to each skidding method were recommended quantitatively with considerations of multiple evaluation criteria.
Keywords: Animal skidding, ground machine skidding, multiple criteria, assessment, classification of operation conditions.
Heilongjiang Province, with a 460,000 square km area and a population of 35 million, is located in the northeast part of China. Forest land, which is mainly dominated by three large forest areas: Yichun Forest Area (YC), Mudanjiang Forest Area (MDJ), and Daxinanling Forest Area (DXL), covers nearly 36% of total land area. More than 30% of total timber used in China since 1949 has come from Heilongjiang Province.
Table 1. Yearly Harvesting area and volume since 1951.
Forest harvesting in Heilongjiang Forest Region has been changed greatly. As shown in Table 1, timber volume harvested decreased gradually after 1970 while the harvesting area increased rapidly, harvesting operations, such as operation site slope, forest stand volume per hectare, timber volume removed from site per hectare, timber volume per stem felled, and average skidding distance have adversely changed in recent years (Table 2). Now, selective cutting and thinning, representing up to 85% of total harvesting area, dominates in Heilongjiang Forest Region. Clear cutting, which was prevalent (highest about 90%) from 1950 to 1970 has gradually decreased.
Table 2. Changes of operation conditions.
Ground machine skidding in Heilongjiang Forest Region, including crawler, skidder and winch skidding, started in 1950, due to the introduction of tractor T-12 and C-80 from the former Soviet Union. Animal skidding was a traditional measure to transport timber off-road. Development of these two skidding methods from 1951 to 1994 is shown in Table 2.
As shown in Table 3, ground machine skidding increased rapidly but animal skidding decreased sharply from 1955 to 1965. This is introduced because the ground skidding machines showed higher operation efficiency and were more suitable to social demands and operating conditions, which included cutting method and harvesting policy, than the traditional ways. High timber demand for construction in China at that time, on the other hand, stimulated the development of skidding machines. Several type of tractors were continually introduced from the Soviet Union, and three new type of tractors, J-50, DFH-54, and DFH-75, were domestically manufactured during that period to meet the high demand of skidding machines for "busy" forest harvesting and wood production. The best criterion for evaluating forest harvesting within this ten year period was operation efficiency, withless consideration given to environmental factors. Most of the forest harvesting operations took place at sites with high stand density and even timber distribution, flat terrain, and high accessibility.
From 1965 to 1975, the proportion of both machine skidding and animal skidding fluctuated around 80% and 15%, respectively. Then, animal skidding increased steadily to 1994 while machine skidding declined after peaking between 1976 and 1979. Both values might reach 50% this year. The main causes for this development might be signifi- cantly adverse changes of operation conditions and harvesting policies with strong environmental constraints.
Average skidding distance, representing the accessibility to scattered forest, and average slope at sites rose because trees available to be harvested were sparsely distributed on difficult terrain sites. The decrease of stand volume per hectare, timber volume removed per hectare, and timber volume per stem felled shows that the quality of forest in Heilongjiang Forest Region is getting poorer. Results of investigations and feedback information from harvesting practices also show that heavy skidding machines, such as J-50, J-80, and CAT518 have more difficulty than animals to fit operational conditions both today and in the near future. So, under these circumstances, animal skidding expanded again and replaced some machine skidding.
Environmental considerations play an important role in the decrease of ground skidding. Policy makers, administrators, and operators pay more attention than before to other important multiple functions of the forest except timber supply. The Act of Forest of China and the Regulations for Forest Harvesting and Regeneration of China noted clearly the multiple uses.
Table 3. Development of proportion of ground machine skidding and animal skidding.
Investigations were carried out in three major forest areas, covering animal, crawler, and rubber-tired skidding. Winch skidding was not included since it represented less than 1% of all skidding operations.
Effects of Ground Skidding on Soil Properties
Experiments for studying the effects of ground skidding on soil properties were conducted in Dailing Forest Enterprise, Yichun Forest Area. Three blocks with a mixed forest of conifer and broadleaf based on Dark-Brown soil, located at 128°37'46" E and 47°21'26" N, were selected as experimental plots. Other climate and terrain data are shown in Table 4.
Changes of Soil Physical Properties
Caused by Ground Skidding
Soil samples for physical property tests were obtained at skidding roads (trails), operation sites, and reference forests very close to the sites along an S- shaped line by using a soil circle-cylinder. Sample numbers for each block were greater than thirty. Results of the laboratory analysis are shown in Table 5.
The results show that changes of soil physical properties depend greatly on the degree of disturbance. The more the disturbance, the greater are the changes that occur. All criteria of soil physical properties listed in this part are adversely proportional to skidding passes, except soil bulk density. Soil disturbance caused by ground machine skidding is stronger than that by animal skidding, since the difference of each criterion between skidding trail and reference forest caused by machine skidding is significantly larger than by animal skidding. Of course, not all data obtained show the same conclusion due to heterogeneity of soil distribution.
Changes of Soil Chemical Properties
Caused by Ground Skidding
Soil samples used for chemical property tests were obtained at the same time as sampling for physical property tests. Groups of samples would be ready for laboratory testing after a series of processing steps, including classifying, mixing, air-drying, and grinding. Results of laboratory analysis (Table 6) show that chemical properties of soil at the skidding trail also differ significantly from that of site soil and reference soil. It is explicit that many more nutrient elements in the skidding trail soil had been lost due to more heavy disturbance, and that more water-soil run-off had occurred along the trail. According to the results, disturbance by machine skidding is slightly more severe than animal skidding in spite of the heterogeneity of soil distribution.
Analysis of Skidding Operation Costs
Skidding operation costs consist of fixed costs and variable costs. Fixed costs include construction of skidding roads, construction of landing, maintenance and depreciation, fixing fees and management fees. Variable costs are composed of fuel, replacement of damaged equipment components, salary, margin, bonus, and small repair fees. Skidding costs vary from place to place and depend on conditions of terrain, trees removed and maintained, operation system, and organization. Data of fixed cost and variable cost for both animal skidding and tractor skidding are shown in Tables 7 and 8. The values used recommended by the Heilongjiang Forest Industry Bureau and are based on the following parameters of operating conditions: removed stem size 0.2 to 0.6 m3; removed timber volume per hectare 20 to 60 m3; and slope, 15% to 27%. Models using skidding distance as a variable for both animal and machine are derived from Tables 7 and 8 using a simple regression method:
|
CM=11.08 + 0.0017*X R2=0.991 F0=689 | (1) |
|
CA=3.3 + 3.86*EXP(0.00058*X)
R2=0.992 F0=754 | (2) |
Table 6. Changes of soil chemical properties caused by machine and animal skidding. (Depth 0-20cm)
Table 7. Fixed operation cost of machine skidding and animal skidding.
Table 8. Variable operation cost of machine skidding and animal skidding.
From Equation (1) and Equation (2), animal skidding is more economical than machine skidding if the skidding distance X ranges 0<X<1769 m, otherwise machine skidding would be better. Results of field investigations show that rubber-tired skidders, under operation cost criterion, work slightly better than crawlers but not as good as animals in selective cutting and thinning operations.
Analysis of Accident Rate
According to the data from the Heilongjiang Forest Industry Bureau, the accident rates of both animal skidding and machine skidding since 1986 are shown in Table 9. The direct cost of accidents is 0.11 Yuan per cubic m, for animal skidding and 0.16 Yuan per cubic m for machine skidding if only considering insurance and subsidy fees. However, direct costs would be higher if they were not hidden in wood production and social costs.
Table 9. Accident rate (no./million cubic meters).
Accident analysis showed that more than 80% of accidents are caused by improper operations on steep terrain, i.e., not correctly following operation instructions and regulations. A high percentage of accidents (nearly 70%) occurred due to operating at more sharply in the last ten years. Conditions of over-slope limitation. Maximum slope limitation for animal skidding and machine skidding was stated in the Operation Safety Rules of Skidding of the Heilongjiang Forestry Industry Bureau as follows: seventeen degrees for both animal and machine skidding under normal skidding conditions and twenty degrees for machine skidding with the help of a winch during the winter period.
Analysis of Direct Damage of Skidding to Residual Stands and Young Seedlings
Field surveys of the direct damage of skidding operations on residual stands and young seedlings was carried out in three major forest areas before snowfall in 1994 (Table 10). There are at least thirty small squares, each 10 m by 10 m, in each of the samples used. The distribution of small squares follows the lines that cross the test site. All data, which might not be highly accurate, are from these small squares, where timber volume removed ranges from 22 to 38 m3/ha. It is evident that the direct damage rate of machine skidding is much higher than animal skidding due to the machine's big size, high power, and low flexibility.
Analysis of Soil Erosion from Skidding Site
The quantity of water and soil run-off at the operation site caused by skidding depends on site area, slope, cutting method, operation time, period after cutting, operation system, soil, and skidding passes. It is difficult to integrate all factors for evaluating the amount of soil run-off from site to site.
Therefore, the field surveys concentrate on skidding trails. Results of surveys using maximum run-off of soil from skidding trails of three major forest areas are shown in Table 11. Here, in order to comprehensively evaluate soil run-off along skidding trails, and considering other important influencing factors mentioned above, the coefficient of soil run-off from skidding trails is defined as follows:
| RF = S (Li * Wi * Di * 106)/(S * I * Ai * Q * T * CF * TF) | (3) |
where
In terms of Equation (3) and data from field investigations, the maximum RF for the three major forest areas is shown in Table 11. Under soil and water run-off criteria, animal skidding has obvious advantages over machine skidding.
Table 10. Ratio of direct damage to residual stands and young seedlings(%).
Table 11. Maximum water and soil run-off from skidding trail (m3/m).
Analysis of Natural Regeneration at Operation Sites
Skidding is one of the disturbances that is affected by natural regeneration at operation sites, especially at skidding roads and off-road sites close to trails. Sites disturbed only slightly by machines (except heavily damaged main skidding roads) have a better regeneration. For example, results of a field survey in Yichun Forest Area showed that one year after cutting there were 18.5 seedlings per m2 for slightly touched sites, 9.8 seedlings for untouched sites, and 1.1 seedlings for heavily damaged main skidding roads. Most of the animal skidding took place on frozen soil. Therefore, animal skidding had no significant disturbance of topsoil due to snow cover and the high resistance of the frozen topsoil.
Results of field surveys for the three major forest areas (Table 12) show that two or three years after operation, natural regeneration at machine skidding sites is better than at animal skidding sites. It is, however, difficult to predict the further development of that now.
In order to comprehensively evaluate animal skidding and ground machine skidding, a structure model with two hierarchies is presented as follows: using AHP (Analytic Hierarchy Process):
C1 = operation cost;
C2 = accident rate;
C3 = damage to residual stands and seedlings;
C4 = damage to soil;
P1 = animal skidding;
P2 = crawler skidding;
P3 = skidder skidding.
In terms of practical operations in the three major forest areas, the typical classification of operation conditions is suggested here as an example:
1) timber volume removed from site: 20 to 60
m3/ha; 2) average skidding distance: less than 1700 m;
3) average slope: less than 28%;
4) average timber volume per stem: less than 0.5
m3;
5) cutting method: selective cutting;
6) operation period: soil frozen period.
The results of the AHP show that animal skidding is the best choice among the three alternatives under multi-evaluation criteria such as cost, accident rate, damage to residual stands and seedlings, and damage to soil.
Table 12. Natural regeneration at sites machine skidding and animal skidding (number/ha).
Animal skidding is significantly superior to heavy machine skidding in selective cutting or thinning operations under evaluation criteria such as operation cost, damage to residual stands and seedlings, and disturbance to soil, but slightly inferior to machine skidding only considering accident rate and regeneration. The results of a comprehensive assessment under evaluation criteria (operation cost, accident rate, damage on residual stands and seedlings, and disturbance to soil) showed that animal skidding is a first preference among skidding alternatives in the moderate and steep terrain conditions that are popular in Heilongjiang Forest Region. The wheeled skidder had advantages over other ground skidding methods in longer distance skidding.
ACKNOWLEDGEMENT
This paper is supported by the project "Theory and Techniques of Ecologically Harvesting" sponsored by the National Research Funding Agency for Young Scientists. Dailing Forest Enterprise provided part of the survey facilities and local transport. Sun Molong, Li Bingzhu, Yang Xuechun, and Dou Jianwei offered help with field survey and laboratory analysis. I am very grateful to all these people and institutions.
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