De-Ling Yang，Li-Hai Wang，Shu-E Ji，Wen-Shu Lin

(1.College of Engineering and Technology，Northeast Forestry University，Harbin 150040，China;2.College of Automobile and Traffic Engineering，Heilongjiang Institute of Technology，Harbin 150050，China)

1 Introduction

The labor cost of animal skidding increases year by year，and it has disadvantages，such as low resource utilization rate，low bucking percentage，and low skidding productivity，therefore mechanicalskidding ismore preferred than animal skidding in the Northeast forest region of China［1］.However，the traditional skidding machines such as J-50，J-80，and the others ever used for skidding are not suitable due to the disadvantages of scattered working area，difficult terrain，sparse woods，low skidding amount per hectare，and small volume per single tree［2］.A new skidding machine，which is small，flexible，environmental-friendly，and more suitable for actual production，is needed urgently.

The type and main parameter selection of an experimental prototype skidder，as well as its design and installation of the skidding machine have been done before［3］.In this paper，the experimental comparison between the experimental prototype skidder and J-50 including skidding productivity，soil compaction on skidding trails，and damage rate of the residual trees were analyzed.

2 Site Conditions

The selected experimental plots were located in the Daxi Forest Farm which belongs to the Linjiang Forest Bureau，Jilin Forest Industry Group. This area is characterized by temperate monsoon climate，cold and snowy in winter，warm and rainy in summer with annual average rainfall of 660 mm.Average annual temperature is 1.3℃;frost-free period is about 115 d，and the soil is dark brown forest soil.The skidding field belongs to No.1 subcompartment of No.58 compartment in Daxi Forest Farm with area of 10.9 hm2.The altitude of the site is 670 m;slope faces west-south and the average slope gradient is 6°—8°.The average depth of the soil is 20 cm.For middle coverage degree Equisetum as its vegetation cover，and heavy coverage degree Acer ukurunduense as its shrubbery.This is a man-made forest with mixed softwoods and hardwoods.The main tree species are Larix gmelinii， Pinus koraiensis，F(xiàn)raxinus mandshurica Rupr，T.tuan SzyszyL.The average forest age is 48 y;average diameter at breast height(dbh)is 24cm;average tree height is 18 m;canopy density is 0.9，and tree volume is 244 m3/hm2.

3 Methods

In order to analyze the performances of the small track-type experimental prototype skidder more objectively and scientifically， the experimental prototype skidder and J-50 skidding tractor were used，respectively，

for skidding tree length and saw logs in the experimental plot from April to May，in 2010.The skidding productivity，soil compaction on skidding trials，and damage rate of the residual trees，were measured and surveyed［4－6］.The collected data were examined for completeness and usability and then entered into Excel spreadsheets and made the contrastive analysis.The statistical software R was used to identify outliers.The outliers were examined to determine whether the data could be corrected(data entry error)before exclusion.In all cases，the number ofoutliers removed was less than 2% ofthe population.The linear regression tool in the package of Data Analysis in MS Excel was used to examine the relationship between and skidding productivity skidding quantity per cycle for each type of skidding machine.

4 Data Analysis and Results

4.1 Skidding Productivity Comparison

Table 1 shows the basic experimentaldata obtained from the experimental prototype skidder and J-50 skidding tractor on the experimental plots，which includes skidding distance，skidding volume per cycle，and time.

Table 1 Statistics for small track-type experimental prototype skidder and J-50 skidding tractor

Fig.1 shows the comparison of skidding productivity per skidding cycle conducted by the small track-type experimental prototype skidder and J-50 skidding tractor，while Fig.2 shows the comparison of their average skidding productivity.It can be concluded that both skidding productivity per skidding cycle and average skidding productivity of the small track-type experimental prototype skidder are slightly lower than J-50 skidding tractor［7］.

Fig.1 Skidding productivity of small track-type experimentalprototype skidder and J-50 skidding tractor

However，it does not mean that J-50 skidding tractor has better skidding performance.The higher skidding productivity of J-50 skidding tractor is mainly attributable to its higher skidding quantity per skidding cycle.The average skidding quantity per skidding cycle of J-50 skidding tractor is 5.7 m3，while that of small track-type experimental prototype skidder is only 0.8 m3.Fig.3 shows the change trend of skidding productivity and quantity per cycle with small tracktype experimental prototype skidder and J-50 skidding tractor.The skidding productivity of J-50 skidding tractordecreases with the reduction ofskidding quantity percycle(Fig.3). When theskidding quantity per cycle reduces to Point A，the small tracktypeexperimental prototypeskidderhas ahigher skidding productivity than J-50 skidding tractor.It is optimistic to conclude that with the condition of scattered skidding area and low skidding quantity volume percycle，smalltrack-type experimental prototype skidder has a higher skidding productivity compared with J-50 skidding tractor.

Fig.2 Average skidding productivity of small track-type experimental prototype skidder and J-50 skidding tractor

Fig.3 Relationship between skidding productivity and volume per cycle of small track-type experimental prototype skidder and J-50 skidding tractor

4.2 Comarions of Soil Compaction on Skidding Trails

Table 2 shows the contrastive analysis of soil compaction data on skidding trails by J-50 skidding tractor and the small track-type experimental prototype.Figs.4 and 5 are made based on the data in Table 2.

Figs.4 and 5 indicated that no matter on the main skidding trails or assistant skidding trails，the small track-type skidding tractor makes less compaction to the soil on skidding trail in the depth of 0－5 cm，5－10 cm，10 －15 cm compared with J-50 skidding tractor. On the main skidding trails， the soil compaction in each depth made by small size skidding tractor isonly 60.5%，33.2%， and 34.7%［8］compared with those made by J-50 skidding tractor，while the data on the assistant skidding trails are 80%，38.9%， and 25%［9］，respectively.Therefore， the small track-type skidding tractor has less compaction to the soil in the skidding trails.

Table 2 Comparison of soil compaction on skidding trails made by small track-type experimental prototype skidder and J-50 skidding tractor

Fig.4 Compaction to the main skidding trails soil made by small track-type experimental prototype skidder and J-50 tractor

4.3 Comparison of Residual Trees Damages

Fig.5 Compaction to the assistant skidding trails soil made by small track-type experimental prototype skidder and J-50 tractor

J-50 skidding tractor and the small track-type experimental prototype skidder were used for skidding 20 cycles in the same experimental plots with size of 100 m×100 m.The statistics on the damage status of the residual trees is summarized in Table 3.

Table 3 Comparison of damage to residual trees made by small track-type experimental prototype skidder and J-50 skidding tractor

Damage rate of residual trees is calculated by the following formula:

Fig.6 is the contrastive bar chart of the damage rate of residual trees after operations by these two skidding tractors.It shows that the damage rate of residual trees made by small skidding tractor，which is designed environmentally friendly in this project，is only 6.3%，and that made by J-50 skidding tractor is as high as 32.5%.Therefore，with the same working conditions，the damage rate of residual trees made by small skidding tractor is only 1/5 of those made by J-50 skidding tractor［10］.

Fig.6 Damage rate of residual trees after skidding for both smalltrack-type experimentalprototype skidder and J-50 skidding tractor

5 Conclusions

Based on the analytical results，the small tracktypeexperimental prototypeskidderhas ahigher skidding productivity compared with J-50 skidding tractor under the condition of scattered skidding area and low skidding volume per cycle.The small tracktype skidding tractor has less compaction to the soil in the skidding trails in the depth of 0－5 cm，5－10 cm，and 10－15 cm.Under the same work conditions，the damage rate of the residual trees made by the small track-type experimental prototype skidder is only 1/5 of those made by J-50 type skidding tractor.Therefore，all the data above prove that compared with J-50 skidding tractor， small track skidding tractor experimentalprototype is smaller， lighter， more flexible and more suitable to the skidding work in the Northeast forest farms.Itmakeslessdamageto residual trees and can be better to the forest ecological environment.It is noted that more variables should be considered for the comparison of forest roads，including diverse forest types，different cutting intensity，energy consumption，and vehicle trafficability.The research will be conducted in the future while suitable working conditions are available.

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