Timber harvesting operations on gentle terrain are performed with ground-based systems using skidders or forwarders, while cable systems are recommended on steeper terrain (Kellogg et al., 1992). However, in many parts of the eastern US such as the Cumberland Plateau region of Kentucky typified by relatively steep, highly dissected terrain with short distances, the effective use of cable systems has been difficult to establish, and ground-based operations are common. As opposed to gentle terrain areas where skidders can travel relatively unrestricted, steeper areas require constructed skid-trails to facilitate cost-effective and safe operations. Consequently, efficiently locating skid-trails becomes crucial as they directly impact skidding and skid-trail construction costs. Typically, skid-trail networks are designed manually by managers using vegetation and terrain characteristics but more often are constructed on-the-fly by a bulldozer operator without careful planning. Typically, bulldozer operators start building skid-trails either along ridge lines or near stream corridors and subsequently along contour lines. This results in relatively parallel skid-trials, spaced between 45 m and 75 m depending on harvest machinery and crew resources to facilitate reaching all harvestable volume between skid trails.

The heavy traffic of harvesting equipment along skid-trails has also been reported to cause significant soil disturbances that can lead to erosion and compaction (Croke et al., 2001; Williamson & Neilsen, 2000), a shift in vegetation composition (Avon et al., 2013; Buckley et al., 2003), and loss of vegetation productivity (Lockaby & Vidrine, 1984). Best management practices including disking and seeding, subsoiling, re-contouring, and installing water bars are often recommended to ameliorate soil disturbances (Conrad et al., 2012). However, these practices carry additional costs ranging from 500 $ to 8,000 $ ha^-1 that might cause significant economic impacts on timber harvesting operations (Soman et al., 2019; Sawyer et al., 2012). The effort and costs used to ameliorate soil disturbance is partially governed by, and positively related to the traffic-level. Reducing the length of high-traffic skid-trails can help alleviate administrative costs and thus designated skid-trails is typically recommended to also reduce these soil disturbance (Garland, 1983; Han et al., 2006).

There are only a few models to automate the design of optimized skid-trail networks. Halleux and Greene (2003) developed an automated approach to evaluate alternative networks but assumes flat terrain and evenly distributed volume. Gumus & Turk (2016) developed an approach to optimize the design but is also applicable only for flat terrain. Contreras et al. (2016) developed a computerized model to generate an optimized skid-trail network that minimizes skidding and skid-trail construction costs based on terrain, volume distribution, and extraction locations. Despite these developed models, there has been no formal comparison between field implementation of computer-generated and operator-designed skid-trail networks to quantify potential economic benefits. One of the main reasons for the lack of these studies is the required coordination and collaboration with forest companies and logging contractors. Other reasons are the logging contractors’ unwillingness to change tradition, perceived costs associated with tasks such as flagging skid-trails before construction, and an inherit distrust and misunderstanding of computergenerated resources.

In this study, we retroactively compared an operator-designed skid-trail network for a harvest operation conducted in eastern KY, USA in 2008 with the optimized computer-generated skid-trail network using the Contreras et al. (2016) model. This work presents a novel attempt to quantify potential economic benefits of computer-generated skid-trail networks, which can facilitate future more comprehensive ground comparisons and evaluation of model applicability.

The total harvest volume represented by the volume distribution data in the harvest area (Fig. 1c) was 16,021 tons, from which the log-bunching routine identified 1,667 log-piles (Fig. 1d). The average log-pile volume was 9.6 ton, which was near the skidder maximum capacity set at 10 ton. The operator-designed network presented the typical parallel pattern with an average spacing of 56 m (Fig. 1a). Figure 2a shows the complete operator-designed skid-trail network after connecting all log-piles to the closest constructed skid-trails. The number of loaded machine passes ranged from one, for segments connecting log-piles to constructed skid-trails, to 550 for skid-trail segments approaching log-landings.

The computer model successfully generated an optimized skid-trail network (Fig. 2b) connecting all but six log-piles to the three log-landings. These six log-piles were in areas with terrain slope around 75%, which was above gradient allowed for feasible skid-trails. However, as done with the operator-designed skid-trail network, these log-piles were connected to the closest optimized skid-trails and their associated skidding costs were also calculated. The number of loaded machine passes ranged from one to 619 indicating that more traffic was concentrated along fewer skid-trails arriving at the landings. Total skidding cost for the computer-generated network was slightly lower than the operator-designed network ($67,563 vs $69,520, Table 1). The average skidding cost and average skidding distance per log-pile was also slightly lower for the computer-generated network. However, skid-trail construction cost for the computer-generated network was higher ($13,447 vs $8,178) than the operator-designed network. This was because numerous skid-trail segments were located across steeper terrain slope, which increased construction costs. On average in the operator-designed network, skid-trails with fewer than 20 loaded machine passes were placed on areas with terrain slopes of about 29% and skid-trails with more than 20 machines passes were located on areas with terrain slope of about 10%. On the other hand, same traffic level skid-trails, in the computer-generated network, were located on areas with terrain slopes of 43% and 19%. Thus, the resulting total harvesting cost for the operator-designed network was lower than the computer-generated network, approximately $77,700 and $81,000 or 4.8 $ ton^-1 and 5.1 $ ton^-1.

The total length of skid-trails in the computer-generated network was 53.7 km, which is 2.0 km higher than the length of the operator-designed network. This is likely because log-piles in the operator-designed network were connected directly to the constructed skid-trails without terrain slope constraints and feasible skid-trails in the computer-generated network were allowed only when gradient was below 45%. As most of the skidding costs will be accrued while travelling along the high-traffic skidding routes, the correct location of these paths is crucial because of their large impact on total costs. While the computer-generated results provide information for the entire skid-trail network, the ground implementation of correctly identifying skid-trails connecting individual log-pile locations would be relatively difficult. A more practical application of the computerized model can focus on high-traffic or major skid-trails, which can be flagged on the ground to guide operator before construction. In this context, when comparing the length of skid-trails with more than 20 loaded machine passes, the computer-generated skid-trail network was about 28% shorter (9.4 km vs 13.1 km). This indicates that the computergenerated network has a lower density of high-traffic skid-trails throughout the harvest unit concentrating skidding along fewer skid-trails. This becomes evident when comparing major skid-trails, those with more than 20 loaded machine passes (Fig.2c and 2d). For example, the operator-design network has several major skid-trails arriving at the southern and northern landing following a parallel pattern, while the computer-generated presents fewer major skid-trails following a branching pattern.

Several of these major skid-trails in the computer-generated network also follow ridge lines and branch downslope to follow routes along contour lines. The branching pattern of the major skid-trails and of the entire skid-trail network is typical of studies using a network approach to determine routes that minimize skidding and construction costs (Stückelberger, 2008; Ezzati et al., 2015). The computerized model should also incorporate a two-dimensional smoothing routine to help ensure a path that a loaded skidder can efficiently navigate. This would also reduce skid-trail length, which would also reduce skidding and skid-trails construction cost.

Lastly, there was a dramatic difference in procurement area and volume received among log-landings. About 60% of the total volume was skidded to the northern log-landing, only 15% skidded to the middle log-landing, and the remaining 35% to the southern log-landing (Fig. 2b). The uneven volume distribution among log-landings and the relatively long average and maximum skidding distances (Table 1) suggest that overall skidding productivity and cost could have been improved by relocating both northern and middle log-landings farther north to reduce skidding distances.