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Article

Assessing the Extent and Severity of the Impact on Forest Soils of Two Different Fully Mechanized Timber Harvesting Operations

by
Eunjai Lee
1,†,
Taehyung Kim
2,†,
Ho-Seong Mun
1,
Jae-Heun Oh
1 and
Sang-Kyun Han
2,*
1
Forest Technology and Management Research Center, National Institute of Forest Science, Pocheon 11187, Republic of Korea
2
Division of Forest Sciences, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon 24341, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2024, 15(6), 985; https://doi.org/10.3390/f15060985
Submission received: 22 April 2024 / Revised: 31 May 2024 / Accepted: 3 June 2024 / Published: 5 June 2024
(This article belongs to the Special Issue The Influence of Forest Operations on the Sustainable Management of Forest Soils)
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Abstract

:
Recently, the use of powerful and heavy vehicles for timber harvesting on flat or slightly sloping terrains has been widely expanded to provide safe working conditions and high productivity. However, soil disturbances during ground-based mechanized operations in South Korea are not fully investigated and difficult to avoid. Therefore, we compared the soil displacement and compaction (bulk density and hydraulic conductivity) between two different operations: cut-to-length (CTL) logging with a harvester and forwarder, and whole-tree (WTH) logging with a harvester and skidder. After clear-cutting, severe visual disturbances and rut depths were more prevalent in the forwarding trails than in the skidding trails. The CTL harvesting method created larger amounts of slash (6.9 kg/m2) along the trails than the WTH harvesting did (1.8 kg/m2). We found a significant difference in the compaction between the reference and the track and a negative correlation between the slash quantity values and the percentage increase in compaction. Our results showed that using skidding extraction can cause more severe impacts than forwarding extraction. Thus, these results may be helpful in understanding the influence of ground-based CTL and WTH harvesting operations and achieving best practices to minimize the environmental impacts on soil.

1. Introduction

Timber is felled and extracted using various technologies with different levels of mechanization, including fully mechanized, semi-mechanized, and motor-manual methods [1,2]. Even though mechanized harvesting technologies can provide clear benefits by improving productivity and safety, ground-based machine traffic can inherently impact forest ecosystems, particularly forest soils [3,4,5] in the form of compaction, rutting, and displacement [6,7]. The severity and extent of soil disturbance during mechanized operations depend on several factors such as the harvesting system, silvicultural treatments, terrain slope, soil type, and texture [3,8]. Machine operating trails for forwarding and skidding can markedly impact a forest’s productivity, biodiversity, and water and nutrient cycles, with disturbed forest soil effects lasting for 20 to 30 years [9,10]. Thus, considering the marked and long-term effects of mechanized logging operations, sustainable forest management requires a comprehensive understanding of these effects.
Alterations in soil’s physical properties are the most well-known indicators of soil disturbance, because an increased soil bulk density (BD) reduces water infiltration and root expansion, leading to increased surface runoff and erosion [4,11,12]. The BD increases when mechanical forces compress the soil, reducing the pore space owing to vertical and horizontal soil stresses [3]. However, previous studies on the effect of lateral displacement and water infiltration along ground-based operating trails—forwarding or skidding—have not been sufficient [8,10,13]. These disturbances associated with forest operations can have a significant impact on biological processes such as litter decomposition rates and fine root abundance, as shown in previous studies [14,15]. Therefore, it may be necessary to investigate the impact of ground-based based technologies on forest soils.
Recently, mechanized operations involving harvesters and forwarders on both gentle and steep terrains have led to increased productivity but fewer job opportunities for loggers in South Korea (hereafter, Korea; [16]). A harvester–forwarder system can replace manual tree felling, processing, and shovel and cable extraction; however, mechanized operations are more likely to cause severe soil disturbances [3,7,17]. Several investigations on soil disturbance related to cut-to-length systems in Korea have examined conventional chainsaw felling and small-shovel extraction, as well as mechanized harvesting systems, such as harvester–forwarder operations [7,18]. However, comprehensive research on the degree of detrimental soil disturbances—including visual, physical, and hydrological effects—caused by ground-based cut-to-length (CTL) and whole-tree (WTH) systems remains insufficient. Despite several studies aiming to identify the primary factors contributing to soil disturbance, the specific extent and degree of forest soil disturbance caused by skidding and forwarding operations remain unclear [19,20]. Furthermore, given the diversity of sites, harvesting methods, and soil characteristics, comprehensive data are essential to accurately quantify various soil disturbance types [21]. Therefore, this study aimed to evaluate soil disturbance by quantifying the extent of visual disturbance, rut depth, and the severity of physical and hydrological disturbances resulting from ground-based CTL and WTH systems.

2. Materials and Methods

2.1. Study Site Descriptions

The research site (127°17′52.83″ E, 37°76′30.76″ N: WGS84 coordinate system) was established in the experimental forests of the National Institute of Forest Science, Namyangju-si, Gyeonggi-do, Republic of Korea, in March 2023 (Figure 1). The stand characteristics are presented in Table 1. The site was a 60-year-old evenly aged plantation of Larix kaempferi (Lamb.) Carrière. The 2.8-hectare (ha) study site had a gentle topography, with slopes ranging from 10 to 20%. There were 390 trees per ha (average stand volume 194 m3/ha), predominantly Japanese larch (L. kaempferi), Korean pine (Pinus koraiensis), and pitch pine (P. rigida). The average tree diameter at breast height and height were 26 cm and 15 m, respectively. The soil in the site is characterized as sandy loam, with the proportion of sand particles (0.05–2.0 mm) varying from 49.4 to 61.9%.
Prior to clearcutting, the unit was divided into two sites to compare the soil disturbance characteristics according to fully mechanized harvesting operations (Figure 1). On S1, the CTL operation was conducted using a 6-wheel harvester (HL20-2, Konrad, Preitenegg, Austria) for felling and processing trees within a mixed coniferous forest. The harvester weighed 26,200 kg. Timber extraction on S1 was performed with a forwarder, LVS-720 (Novotný, Zábřeh, Czech Republic), weighing 8500 kg, with a maximum payload capacity of 16,500 kg. S2 underwent a WTH operation by employing the same harvester used in S1 for felling. The harvester is versatile and is equipped to perform skidding operations with an attached clambunk. Consequently, the WTH operation involved harvesting, felling, and skidding whole trees to the landing area. Additionally, two machine operators, each with 5 years of experience, operated the forwarder and harvester at both sites.

2.2. Field Data Collection and Analysis

One week after completing the harvesting, a comprehensive collection of data was conducted throughout the trial, including measurements of visual disturbance, rut depth, slash amount, BD, and hydraulic conductivity (K). Data on the visual disturbance and rut depth resulting from soil displacement during harvesting operations were collected at 10 m intervals, with 25 sampling plots across each stand. A rapid assessment method for forest soil monitoring was reported by the United States Department of Agriculture Forest Service [22]. Based on this method, the soil surface disturbances were ranked into four classes (Table 2): undisturbed soil surface (class 0), faint and shallow tracks (class 1), moderate track marks (class 2), and evident and deep track marks (class 3). The rut depths were also measured in the plots where disturbances were observed. The rut depths on the left and right tracks of each trail were measured with a ruler using cross-sectional analysis, and the average was recorded. For the cross-sectional analysis, a pole was placed across the rutted trail to determine the height of the original soil. Then, the straight distance was measured between the pole and the bottom of the rut. Similar soil disturbance assessment methods were introduced by Reeves et al. [23] and Lee et al. [12].
Because most logging residues are concentrated on trails during ground-based harvesting activities, the slash amounts were measured along these trails using the downed woody debris survey method with the allometric equation used by Brown [24] and Kizha and Han [25]. Downed woody debris was recorded using 11 m line transects with five 1/25 ha fixed-radius circular sampling plots within each harvesting activity. Our observations were on a green ton basis, and we estimated that the slash had a ~50% moisture content, which was modified to kg. For example, Poltorak et al. [26] and Choi et al. [27] reported that logging residue produced immediately after logging contains high levels of moisture, ~50 to 60% of the total weight.
The soil’s BD and K were measured using a 90.6 cm3 slide soil corer with a 5.0 cm inner diameter and length of 5.0 cm (Eijkelkamp Soil & Water, Giesbeek, The Netherlands) and a mini-disk infiltrometer (3.1 cm tube diameter and 32.7 cm length; Meter Group Inc., Pullman, WA, USA), respectively. Soil core sampling was performed from depths of 0–10 and 10–20 cm at the top of the mineral soil. In addition, samples were collected along the trails, including the track, centerline, and 2 m away from the track’s outer edge—the reference point—at 3 m intervals. The reference point was defined as no passes from either the harvester or the forwarder, indicating no soil disturbance. Thirty plots (180 samples) were identified where the tracked harvester and forwarder were used, and fifteen plots (90 samples) were selected for tracked harvester trails associated with skidding operations. The samples were placed in plastic bags and transported to the laboratory, where they were weighed on the same day and dried at 105 °C to a constant weight.
The infiltration field tests were conducted adjacent to each BD sample point on the soil surface. Mini-disk infiltrometers have been used in several studies to accurately measure K and water infiltration rates [28]. We recorded the water level every 30 s using a 2 cm pressure head value until stable infiltration was observed. Before each measurement, the litter and duff layers were removed, and the sampled soil surface was gently levelled to facilitate the process. Based on the mini-disk data, we converted K using a Microsoft Excel® (2019 version) spreadsheet Macro provided by Meter Group, Inc (Meter Group, Inc., Pullman, WA, USA).

2.3. Statistical Analyses

The R 4.3.2. statistical program (2023 version) [29]—a free and open-source software—was used for data analysis. Data normality was investigated using the Shapiro–Wilk test and the homogeneity of variance using Levene’s test. An independent t-test was conducted to determine the comparative rut depths, BD, and K of the forests in each unit. The association between the harvesting system and disturbance classes was evaluated using a Chi-squared test and Cramer’s V value. An analysis of variance (ANOVA) was used to identify the effect of machine traffic on the BD and K among the three sampling locations. Specially, two-way ANOVA was used to assess the significance of differences in BD and K between different extraction methods, disturbance types, and their interaction. In addition, the Scheffé test was used as a post hoc test (alpha level of 0.05).

3. Results

3.1. Slash Amount after Ground-Based Harvesting Operations

After harvesting, the densities and spacings of the forwarding and skidding trails from CTL and WTH operations were 761.4 and 753.7 m/ha, and 11.9 and 13.5 m, respectively. The mean slash amount for forwarding and skidding tracks was 6.9 and 1.8 kg/m2, respectively. The harvesting method had a significant effect on the amount of slash in the trails (p = 0.0007).

3.2. Visual Disturbance and Rutted Depth after Ground-Based Harvesting Operations

In both S1 (WTH) and S2 (CTL), the percentage of class 2 (track marks of moderate depth) was considerably higher than that of the other classes, and this type of soil disturbance occurred more frequently in S2 than in S1 (Figure 2). In contrast, S1 was observed to have a higher proportion of class 3 (evident and deep track marks) than S2. There was no statistically significant difference in disturbance class distributions between the CTL and WT systems, although a moderate association between the harvesting system and disturbance classes was observed (p = 0.328, Cramer’s V = 0.304). Additionally, undisturbed areas (class 0) were not observed in S1 or S2, because the disturbance assessment was conducted along the machine trails. The mean rut depths of S1 and S2 were 9.05 and 9.01 cm, respectively, and the difference between them was not significant (p > 0.05).

3.3. Soil Compaction after Ground-Based Harvesting Operations

At soil depths of 0–10 and 10–20 cm, the mean BD of the reference points in S1 and S2 was 0.87 and 1.05 g/cm3, and 0.89 and 0.99 g/cm3, respectively. No significant differences were observed between the two units (p > 0.05).
At the trails in S1, the mean BD was notably different between the track and reference points, but there were no statistical differences between the center and reference points at both the 0–10 and 10–20 cm levels (Table 3; Figure 3). In the tracks, the BD increases at 0–10 and 10–20 cm were 44 and 25% higher than the reference values, respectively. At S2, the track had a considerably higher BD than both the reference and center at both depths, and the difference was statistically significant. The increases in BD by depth within the track and center were 59 and 38% at 0–10 cm, and 41 and 28% at 10–20 cm. Consequently, the BD increases inside the tracks in S2 were statistically higher than those in S1.
At the reference points in S1 and S2, the mean K was 2.23 and 2.24 cm/h, respectively, but no significant differences were observed (Table 4; p = 0.6746). In S1, the mean K in the forwarding track was significantly lower than those in the reference and center points (p < 0.01; Figure 4); however, there were no significant differences between the reference and center points. In S2, the Ks in the track and center were significantly higher than that of the reference point (p < 0.05). The percentage decrease in K (65% of the reference point) was the highest in the skidding track compared with the center point (43% of the reference point). In both units, there were significant differences in Ks between the track and the reference.
Our data show that the disturbance type and extraction methods were significant factors in determining the change in BD (Table 5). We found that the interaction of extraction methods × disturbance type only had a significant effect on BD at soil depths of 0–10 cm. In addition, the extraction methods × disturbance type did not significantly affect K.

4. Discussion

Focusing on ground-based mechanized harvesting operations, it is necessary to minimize negative impacts on soil environments that cause soil displacement, rutting, and compaction. If not properly performed, timber harvesting operations with extraction based on forwarders or skidders may lead to severe soil compaction and lateral displacement along the trails. Although the disturbance of forest soils because of mechanical harvesting has been well documented, there is a lack of studies on soil disturbances under different working conditions, particularly in Korea. Our study revealed that in the forwarding trails, severe surface soil displacement and rutting were slightly higher than in the skidding trails, but there were no significant differences among the trails. The greatest changes in BD and K were observed at the track points of the forwarding and skidding trails compared to both the center and reference points. In particular, during skidding operations, both the track and center had statistically higher compaction risks than the reference plots. This information may aid decision-making in order to minimize negative impacts on the soil environment and improve forest production in the future.
The severity of soil displacement and rutting interacts with skidding and forwarding. Marra et al. [18] reported that in skidding trails, less visual disturbance was observed than in forwarding trails, because the ruts were not clearly visible owing to dragging and pulling activities. The skidding operation—characterized by the lifting of the bottom ends of logs and dragging their top ends—exerts pressure and scarifies the soil surface, thus affecting soil displacement [30]. In this process, the soil surface is reshaped, and the ruts left by the skidder are hidden [19]. During forwarding, the log load is carried by the trailer, and ground pressure combined with wheel slip is exerted on the soil surface [19,26]. Forwarding causes a greater rutting depth than skidding, because the ruts are clearly visible, and the passes are concentrated [20]. Our results show that visual disturbances and rut depth were more severe in the forwarding trails than in the skidding trails. We found that disturbance class 3 (evident and deep track marks) was prevalent in the CTL unit because of the wheel slip action of the forwarder traffic on the slash. Han et al. [31] and Marra et al. [19] reported similar results in that after the passage of the forwarder, ruts were more evident than when the skidder was used due to log deck transport. Thus, even if a slash mat is placed on the forwarding trails, slippage under vehicle traffic may occur, leading to severe and deep rut formation compared with skidding trails.
In ground-based timber harvesting, the CTL method is usually performed using a harvester–forwarder system, whereas the WTH method usually involves manual or machine felling with various skidder types. Both these methods cause differences in the degree of soil compaction on machine tracks [3,5]. For example, Labelle et al. [5], Labelle et al. [6], and Poltorak et al. [26] reported that the use of logging residues (treetops, branches, and non-merchantable trees) in a forwarding trail is more generally linked to CTL than to WTH. Brush mats can spread applied loads over a larger area under the mats, reducing peak pressures and friction on the soil surface [6,32]. Our results can be explained by the harvesting system and are consistent with those of previous studies. The forwarding track areas experienced less compaction than the skidding track areas. In the forward trail, statistically significant differences in the mean BD and K between the track, outside track, and center areas were found. We conclude that the BD would be statistically higher and K lower on track areas than on the outside track and center areas on the test plots owing to a lack of brushing (6.9 kg/m2) to distribute the load. This result is compatible with a previous study by Poltorak et al. [26] and Labelle et al. [6], who observed that brush mats of 10–20 kg/m2 covered forwarding trails and played a marked role in reducing soil compaction and displacement. Consequently, the ratio of soil compaction depended on the logging method (CTL or WTH).
We found that the difference between the outside track and center in terms of the BD and K was not noticeable in the forwarding trails, whereas in the skidding trails, there was a significant difference. These differences are because of the different extraction types, and they are partly described by what has already been discussed regarding their effects on visual disturbances and rut depth. For example, Cambi et al. [3] and Labelle et al. [5] studied the top ends of dragged logs that transformed the ground after the skidder passed, hiding the ruts and bulges that were left by the wheels, whereas those caused by forwarder passes were clearly visible. Thus, during skidding, ground contact pressure on both the track and center may be exerted on the soil surface along the entire trail.
Our study shows that any machine (i.e., forwarder or skidder) has an impact on the soil, particularly during the process of the extraction. Although our results can still be matched with previous studies, this study has potential limitations. The limitation is that this study covered only one study area, so it is essential that further comparative studies be carried out using the same methods but on a wider variety of working conditions. Unfortunately, neither data about the wood volumes transported over each trail nor the number of passes in the sampled soil area (i.e., core sampling and infiltration field tests) were available. Without this information, general conclusions may be misleading. For example, Labelle et al. [5] reported a strong influence on the effective impacts on soil disturbance from the proper or improper use of machines, which is influenced by the operational conditions. As a result, minimizing soil disturbance is a challenge, as is the volume transported per route and route-planning.

5. Conclusions

Our findings highlight that the use of skidding and forwarding extraction methods may affect soil disturbances differently. We found significant differences in the BD and K among the extraction methods and disturbance types. The track had a significantly higher BD than both the center and reference due to the direct contact between the wheels and the soil. Compaction may necessarily alter the total pore volume, and thus, our K results were significantly greater for in-track K than off-track K. For this reason, forest managers should consider the potential soil damage from each extraction method in order to justify the management and control of soil disturbance. Skidding has a higher effect on soil compaction than forwarding, and severe visual disturbances and rutting were observed in forwarding operations. Although soil disturbances caused by skidding and forwarding were compared, we do not suggest that one of the extraction methods is less detrimental than the other. Therefore, further studies are necessary to compare the working conditions (e.g., soil moisture and traffic intensity), species, amount of slash, and organic matter to determine the extent of the soil disturbance and its impact on sustainable forest management.

Author Contributions

Conceptualization, E.L., T.K., H.-S.M., J.-H.O. and S.-K.H.; methodology, E.L., T.K. and H.-S.M.; validation, E.L., T.K. and H.-S.M.; formal analysis, E.L., T.K. and H.-S.M.; investigation, E.L., T.K., H.-S.M., J.-H.O. and S.-K.H.; resources, E.L., T.K. and H.-S.M.; data curation, E.L., T.K., H.-S.M. and J.-H.O.; writing—original draft preparation, E.L., T.K., H.-S.M., J.-H.O. and S.-K.H.; writing—review and editing, E.L., T.K. and S.-K.H.; visualization, E.L., T.K. and S.-K.H.; supervision, E.L. and S.-K.H.; project administration, E.L. and S.-K.H.; funding acquisition, E.L. and S.-K.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Korea Forest Service: grant number FO0200-2024-01-2024 and 2023491B10-2425-AA02.

Data Availability Statement

The original contributions presented in the study are included in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 15 00985 g001
Figure 1. Map of study sites (S). S1: Cut-to-length harvesting operation; S2: whole-tree harvesting operation. Red lines: forwarding trails; blue lines: skidding trails.
Figure 1. Map of study sites (S). S1: Cut-to-length harvesting operation; S2: whole-tree harvesting operation. Red lines: forwarding trails; blue lines: skidding trails.
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Forests 15 00985 g002
Figure 2. Visual disturbance percentages of the four categories: undisturbed soil surface (class 0), faint and shallow tracks (class 1), track marks moderate in depth (class 2), and evident and deep track marks (class 3).
Figure 2. Visual disturbance percentages of the four categories: undisturbed soil surface (class 0), faint and shallow tracks (class 1), track marks moderate in depth (class 2), and evident and deep track marks (class 3).
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Forests 15 00985 g003
Figure 3. Soil bulk density (g/cm3) collected from soil samples at the track, center, and reference points.
Figure 3. Soil bulk density (g/cm3) collected from soil samples at the track, center, and reference points.
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Figure 4. Hydraulic conductivity (cm/h) collected from soil samples at the track, center, and reference points.
Figure 4. Hydraulic conductivity (cm/h) collected from soil samples at the track, center, and reference points.
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Table 1. Stand characteristics before harvesting and soil particle size distribution.
Table 1. Stand characteristics before harvesting and soil particle size distribution.
UnitArea (ha)DBH a (cm)Height (m)Tree per haSoil Texture (%)
SandSiltClay
S11.526.015.236555.119.925.0
S21.3 b26.015.042054.819.825.4
a Diameter at breast height; b the area includes a 0.42 ha riparian zone.
Table 2. Description of visual disturbance types in the study area.
Table 2. Description of visual disturbance types in the study area.
Disturbance ClassDescription
0No traces of machines or logs
Organic layer is present and intact		Forests 15 00985 i001
1Tracks are identified but are faint and shallow
Topsoil is exposed or mixed with subsoil		Forests 15 00985 i002
2Visible track marks are moderately deep
Topsoil is partially present or mixed with subsoil		Forests 15 00985 i003
3Evident track marks are deep
Topsoil is removed and subsoil is exposed		Forests 15 00985 i004
Table 3. Mean (±standard deviation) bulk density (g/cm3) collected from soil samples at the track, center, and reference points.
Table 3. Mean (±standard deviation) bulk density (g/cm3) collected from soil samples at the track, center, and reference points.
UnitSoil Depth (cm)nTrackCenterReferencep-Value
S10–10301.26 ± 0.18 a0.94 ± 0.22 b0.87 ± 0.22 b<0.001
10–20301.31 ± 0.13 a1.07 ± 0.20 b1.05 ± 0.22 b<0.001
S20–10151.41 ± 0.13 a1.22 ± 0.22 a0.89 ± 0.25 b<0.001
10–20151.36 ± 0.17 a1.24 ± 0.15 ab0.99 ± 0.21 b0.0034
The same letters indicate no statistical difference at each soil depth within each unit.
Table 4. Mean (±standard deviation) hydraulic conductivity (cm/h) collected from soil samples at the track, center, and reference points.
Table 4. Mean (±standard deviation) hydraulic conductivity (cm/h) collected from soil samples at the track, center, and reference points.
UnitnTrackCenterReferencep-Value
S1300.94 ± 0.57 a2.34 ± 2.19 ab2.23 ± 1.67 b0.0090
S2150.79 ± 0.64 a1.28 ± 1.26 ab2.24 ± 1.77 b0.0101
The same letters indicate no statistical difference at each soil depth within each unit.
Table 5. Two-way ANOVA for combined data from units S1 and S2 showing degree of freedom (DF), F statistics, and p-value for main effects (methods and disturbance types) and their interaction with BD and K (dependent variable).
Table 5. Two-way ANOVA for combined data from units S1 and S2 showing degree of freedom (DF), F statistics, and p-value for main effects (methods and disturbance types) and their interaction with BD and K (dependent variable).
Source Soil Bulk Density
(0–10 cm Depth)		Soil Bulk Density
(10–20 cm Depth)		Hydraulic Conductivity
DFFp-ValueDFFp-ValueDFFp-Value
Extraction methods114.705<0.01118.750<0.0111.258<0.01
Disturbance type245.663<0.01219.685<0.0127.904<0.01
Extraction methods × Disturbance type24.072<0.0122.2580.10921.7540.178
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Lee, E.; Kim, T.; Mun, H.-S.; Oh, J.-H.; Han, S.-K. Assessing the Extent and Severity of the Impact on Forest Soils of Two Different Fully Mechanized Timber Harvesting Operations. Forests 2024, 15, 985. https://doi.org/10.3390/f15060985

AMA Style

Lee E, Kim T, Mun H-S, Oh J-H, Han S-K. Assessing the Extent and Severity of the Impact on Forest Soils of Two Different Fully Mechanized Timber Harvesting Operations. Forests. 2024; 15(6):985. https://doi.org/10.3390/f15060985

Chicago/Turabian Style

Lee, Eunjai, Taehyung Kim, Ho-Seong Mun, Jae-Heun Oh, and Sang-Kyun Han. 2024. "Assessing the Extent and Severity of the Impact on Forest Soils of Two Different Fully Mechanized Timber Harvesting Operations" Forests 15, no. 6: 985. https://doi.org/10.3390/f15060985

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