Soil disturbance caused by timber harvesting in a natural forest, West Sumatera Indonesia

Logging operations in natural forests generally use bulldozers. Bulldozers skid logs from the stump site to the landing site. The bulldozer maneuvers in the felling area cause skid trails and soil compaction. Soil compaction is one of the negative impacts of logging operations. The research aim is to analyze the level of soil compaction in various bulldozer skidding intensities after logging operations. The research occurred at a natural forest area in Siberut West Sumatera, Indonesia. The results of the study showed that there was a relationship between skidding intensity and the level of soil bulk density. The more a bulldozer moves at the same skid trail, the higher the soil bulk density. The range of soil bulk density is from 0.82 g cm−3 to 1.31 g cm−3. The bulk density increases markedly after the bulldozer’s first, second, and third passes. After the fourth pass, the bulk density is relatively constant. The porosity of the soil decreases with the increase of the bulk density. On skid trails where the soil is very compact, seeds that fall on skid trails will have difficulty growing well. Requires planting on primary skid trails. Soil damage in the form of soil compaction by bulldozers needs serious attention in the Reduce Impact Logging technique.


Introduction
Forest operation of natural forests in Indonesia generally uses mechanical devices.Logging operations start with felling trees, then skidding wood from the stump site to the landing site.After arriving at the landing site, the wood will be loaded onto a trailer truck and transported to the log pond or yard via the hauling road.Logging operations generally cause environmental damage in forest soil ecosystems, change diversity, stand damage, and river water quality [ 1,2,3,4].
Extraction of wood in the natural dry land forests of Indonesia generally uses bulldozers.The type of bulldozer used is a track-type tractor.This type generally causes soil damage.If it rains, the eroded topsoil will be carried away by water in the form of runoff and become sediment.The soil surface and under the topsoil were compacted during the moving of the bulldozer.The soil will become compact, the porosity of the soil will decrease, and it will reduce water infiltration.The maneuver of bulldozers as heavy equipment for skidding operations in natural forest areas causes soil disturbance, especially soil compaction, and decreased soil porosity [5,6,7].Each heavy equipment track tends to compact the soil and affect the macropore and organic matter [8,9,10].The negative impacts of using heavy equipment logging operations are increasing bulk density, reducing total pore space and infiltration rate, reducing soil permeability and water holding capacity, changes in soil structure, and needing a long time 2. Material and methods.

Research location and sample plot for tree cutting.
This research was performed in concession areas in West Sumatra (Figure 1).The research in West Sumatera was conducted on Siberut Island on natural forest concession areas covering 48,420 ha.The topography is generally undulating and hilly, with an altitude of 50−340 meters above sea level.The Indonesian Selective Cutting and Planting System was implemented in this location, where harvestable trees are in diameter with a Diameter at the Breast Height (DBH) of > 50 cm.Felling was carried out using a Stihl 070 chainsaw, bucking, and end cutting.Bucking is a process of cutting a fallen tree in the felling site into small segments called logs, and skidding is performed using a bulldozer (Cat D7G).Skidding is pulling logs from the felling site to a log collection point or landing.
We calculated the number of trees felled per hectare, the density of trees per hectare, and the volume of trees cut were calculated.Sample plots were ten plots with an area of 1 hectare each.

Soil bulk density measurement
We measured the soil bulk density at the skid trail after the bulldozer passed the skidding process.The skid trail was the pathway through which bulldozer mobility was tracked from the stump site to the landing for collecting logs.The trajectory of the bulldozer was tracked, followed by marking on each pass on the skid trail.The marking of the passes was performed up to 9 times.Soil compaction measurement was conducted ten times as replications every 5 meters of skid trail length along the tertiary secondary and primary skid or skid trail.Bulk densities were measured by a cylinder tube collected from the skid trail to the laboratory, followed by water content measurement and soil physical properties.Soil density measurement was carried out using a cylindrical tube, and the bulk density value was calculated using the following formula [23].Soil mass density and bulk density were calculated based on the following equation: Bulldozer is used for skidding logs, generally the track type bulldozer or crawler tractor.At the research location, the bulldozer type was Caterpillar D7G.This bulldozer has 200 horsepower or 149.2 kW.The ground pressure of the track is 71.02 kPa.This ground pressure is the primary factor that impacts soil compaction, the specification of Caterpillar D7G [24] in Figure 2.

Results and discussion
Logging was conducted according to the selective cutting rule.The felled trees are commercial trees more than 50 cm in diameter.From 10 sample plots, the minimum number of trees cut was three trees ha -1, and the highest was thirteen treesha -1 , or the mean and standard deviation of 7.20 ± 3.2 trees ha -1 .The volume of trees felled ranged from 38 m 3 ha -1 to 150.27 m 3 ha -1, with the average and standard deviation of the volume of trees cut per hectare being 104.25±53.9m 3 ha -1 (Table 1).The more trees cut per hectare, the more stand damage, and the more intensive bulldozers maneuver along the skid trail to remove logs from the stumps to the landing site [25,26].The number of trees cut per hectare impacts soil damage, forest opening and changes in the diversity of forest vegetation, and other impacts [27,28,29,30].More trees cut causes a felling gap widely [31], changes the floristic of natural regeneration [19,29], and affects carbon stock and forest biomass [18,20].The soil type is ultisol, and the soil texture is clay.Undisturbed condition is a measurement of control of soil compaction.It is a zone without logging activity (non-skid trail).This undisturbed soil's bulk density (in dry) was 0.82 ± 0.05 g cm -3 .After the bulldozer's first pass, the soil's bulk density increases to 0.99 ± 0.04 g cm -3 .There is an increase of 20% in bulk density from undisturbed, and it increases significant compaction during the second to fourth passes the bulldozer passes.After the fifth to ninth passes, it showed the maximum bulk density.The value of the maximum bulk density was 1.31 ± 0.03 g cm -3 .It was an increase of 59% from the undisturbed bulk density.The lowest average soil compaction was in undisturbed conditions, and the highest was from the fifth to ninth passes (Table 2, Figure 3).The drastic soil density intensification during the four bulldozer passes (1.23 ± 0.09 g cm -3 ), and after this critical point, the constant value of soil density performed due to maximum up to the ninth bulldozer passes (Figure 3).
Soil porosity was decreased during the soil compaction.The undisturbed value of soil porosity was 67.5 ± 1.94 %.The value decreases with the increase of bulldozer passes.It decreased by 7.8% to 62.2 ± 1.67 % after the first bulldozer passes.The porosity reduced markedly by 25% to 50.6 ± 1.23 after the fifth bulldozer passes.From the fifth to the ninth bulldozer passes, the soil porosity was constant in value at 50.6 ± 1.29 % (Table 2, Figure 4)   The bulldozer skids logs from the stump site to the landing site.The longest skidding distance is 460 meters.The farthest part from the landing site is the skid trail which is only traversed by the bulldozers 2−3 times.This skid trail section formed because the amount of wood harvested is only 2 to 3 trees.This section of the road is called the tertiary skid trail.The part of the road that the bulldozer maneuvers 4-5 times is the part of the road that is called the secondary skid trail.The primary skid trail with the highest intensity of passing bulldozers is the skid trail.The primary skid trail is about 150 meters long.Bulldozers crossing this road can pass This skid trail more than five times, achieving very high soil compaction or bulk density.The bulk density of the tertiary skid trail was 0.9−1.1 g cm -3 , the secondary skid trail was 1.1−1.21g cm -3 , and the primary skid trail was 1.31 g cm -3 (Figure 5).decreases soil porosity [7].The increase in soil density is in line with the frequency of harvesting machine paths [32,33]; several studies have shown that there is an increase in the density of pus which increases significantly at the initial frequency of the traverse, which causes the bulk density to increase sharply and decreased the porosity [33,34].The use of heavy machines in logging operations causes soil compaction with the bulk density increase and affects the macropore [9,10].Soil compaction is on the surface and to a certain depth.The effect of density on soil depth and driving system factors, number of passes, soil moisture content, and soil texture significantly affect the level of soil density [11,13].
Soil density due to logging operations affects forest soil ecosystems, including forest productivity [1,35].Root damage also occurred due to soil disturbance due to forest operations [12].Soil density is a barrier to seeding growth.Compacted soils may be a barrier to seedling growth [26].Research in Central Amazonia shows that recovery of soil compaction in skid trails takes 28 years after logging [19].A study showed that the increase in soil density in the form of a cone index value occurred at a depth of 21 cm, and after being deeper than 35 cm, there was no significant change [14].Other research shows that the change in the cone index to within 50 cm [5,21].The weight of a heavy machine such as a bulldozer plus load causes pressure on the wheel surface area of the ground contact.Ground pressure was the pressure per unit contact area.Hight ground pressure is one of the causes of the high level of soil density through which the heavy machine passes.
Based on the results of this study and previous research, skid trail planning can be designed to reduce the number of bulldozer tracks to prevent high soil density.If an ex-logging area is planted, the priority is to plant on the primary skid trail (Figure 5) because the soil is very dense, and seeds that fall naturally may not grow properly.Land preparation needs to consider the planting hole to a certain depth.Natural regeneration must be considered on skid trails to reduce forest damage from logging operations.

Conclusions
Timber harvesting in the natural forest effect stand damage and soil disturbance.The more trees cut per hectare, the greater the stand damage and the more intensive the bulldozer maneuvers.The intensity of bulldozer maneuvers causes an increase in soil bulk density.The more a bulldozer moves at the same skid trail, the higher the soil bulk density.The bulk density increases markedly after the bulldozer's first, second, and third passes.After the fourth pass, the bulk density is relatively constant.The porosity of the soil decreases with the increase of the bulk density.Soil density on the primary skid trail is higher than on secondary and tertiary skid trails.Planting is needed on the primary skid trail, and attention is needed on soil compaction in order to reduce impact logging.

Figure 2 .
Figure 2. Specification of Caterpillar D7G Crawler Tractor.Dimension A Length W/Blade 5280 mm B Width Over Tracks 2620 mm C Height to Top of Cab 3250 mm D Length of Track on Ground 2700 mm E Ground Clearance 347 mm F Length W/O Blade 4190 mm

Figure 5 .
Figure 5. Soil bulk density at the tertiary, secondary, and primary skid trail.

Table 2 .
Soil bulk density (dry) and porosity by the passes of the bulldozer Caterpillar D7F