**1. Introduction**

Productivity is defined as the rate of product output per time unit for a given production system [1]. In forestry, productivity in harvesting operations is a central concern in achieving an optimum balance between profitability and sustainability. When the slope gradient exceeds 40%, ground-based harvesting technology cannot provide good results [2]. Therefore, forest management in steep terrain depends on cable yarding as the primary extraction technology, which is usually deployed on difficult sites [3]. Cable yarding is a well-established practice for timber extraction in mountainous regions of the world where fully mechanized harvesting systems such as harvester–forwarder combinations cannot operate due to the steep terrain [4]. Analysis of 12 years of cable logging studies, from 2000 to 2011, showed that cable system efficiency was the most frequent keyword, with 78 out of 172 scientific references [5]. In the current study, the productivity of mechanized forest harvesting systems in steep terrain was discussed in relation to further improving profitability and sustainability.

Today, most forest harvesting operations are carried out with modern forestry machines, and their efficiency depends not only on the performance of each forestry machine, but also on how they are used in combination and on various site characteristics. In Japan, chainsaws, swing yarders, processors, and forwarders are typically used for felling, yarding, processing, and forwarding, respectively, on steep slopes where the most efficient harvester–forwarder system suitable for gentler slopes cannot be used. Cable yarding has been used for many years for timber extraction in the mountainous forests of Central

**Citation:** Yoshimura, T.; Suzuki, Y.; Sato, N. Assessing the Productivity of Forest Harvesting Systems Using a Combination of Forestry Machines in Steep Terrain. *Forests* **2023**, *14*, 1430. https://doi.org/10.3390/f14071430

Academic Editor: Gianni Picchi

Received: 1 June 2023 Revised: 4 July 2023 Accepted: 10 July 2023 Published: 12 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

European countries [6] and is an efficient and effective harvesting system in steep terrain [7]. Regarding technical development, by the end of the 1990s, tower yarders, combined with a processor (PTY, Processor Tower Yarder) equipped with a radio-controlled carriage that automatically moves and stops, had become the standard for all manufacturers [8]. In Central Europe, system integration through the mounting of a tower yarder, crane and processing unit on a single carrier has been the main innovation path towards maintaining low harvesting costs and improving the productivity of cable yarding operations [9]. As a result, the productivity of forest harvesting operations in Japan is relatively low compared to Central Europe, as represented by Austria, where there have been significant technological innovations in cable-based harvesting machines and systems.

Many previous studies have determined the productivity of variations of the abovementioned forest harvesting operations in Japan. According to the authors of [10], the labor productivity was 1.2 to 2.6 m3/worker/day when a chainsaw, swing yarder, and processor were used for felling, yarding, and processing, respectively. The study described in [11] found that the labor productivity was 3.62 m3/worker/day when a chainsaw, swing yarder, processor, and forwarder were used for felling, yarding, processing, and forwarding, respectively, and identified the use of a swing yarder as the reason for the relatively low productivity. The productivity of forest harvesting operations on moderate slopes was compared for different sizes of forestry machines and different log lengths when a chainsaw, harvester, and forwarder were used for felling, processing, and forwarding, respectively, and ranged from 5.3 to 7.5 m3/worker/h [12]. Labor productivity was 6.64 m3/worker/day when a chainsaw, grapple loader with a small winch, harvester, and forwarder were used for felling, wood extraction, processing, and forwarding, respectively [13]. According to the authors of [14], the overall average productivity of forest harvesting operations in Japan is 7.14 m3/worker/day and 4.17 m3/worker/day for final cutting and thinning, respectively.

While m3/h or m3/day is often used in Japan as a unit of productivity, Productive Machine Hour (PMH) is commonly used as the unit of productivity for mechanized forest harvesting operations. PMH represents the time during which the machine actually performs work, and this excludes time lost due to both mechanical and non-mechanical delays from Scheduled Machine Hours (SMH) that includes all time the machine is scheduled to work [15]. On the other hand, the study in [16] pointed out that Productive System Hour (PSH) must be used for systems consisting of several machines, while PMH has been widely used for systems consisting of a single machine and an operator. PSH is similar to PMH, but PSH includes two or more machines or sequential operations necessary to complete the task [17]. While PMH is commonly used as the unit of productivity for mechanized forest harvesting operations, PSH has been used by many studies, especially in Central Europe [3,4,8,18–27]. In addition, PMH0 and PSH0 do not include delays while PMH15 and PSH15 include delays of up to 15 min.

The PTY systems developed in Central Europe, such as Syncrofalke (MM Forsttechnik, Frohnleiten, Austria), as shown in Figure 1, can efficiently yard and process trees in a single production process. Productivity with the Syncrofalke for uphill and downhill yarding was found to be 11.54 m3/PSH0 and 8.25 m3/PSH0, respectively [6]. The average productivity of Syncrofalke in Central Bulgaria was calculated to be 15.20 m3/PMH [28]. In Romania, the production rate of a PTY system consisting of a Mounty 4100 tower yarder and a Woody 60 processor (Konrad Forsttechnik) was 11.89 m3/h, including delays [29]. In all cases, the productivity of forest harvesting operations with PTY systems was much higher than the average productivity in Japan.

According to the authors of [30,31], the low productivity of forestry operations in Japan is mainly due to the structural characteristics of small-scale forest ownership. We investigated additional reasons for the relatively low productivity of forest harvesting operations in Japan in terms of the way forestry machinery is used in combination. In this study, we evaluated the productivity of Japanese harvesting systems using two indices of the Combined Machine Productivity (CMP) and Combined Labor Productivity (CLP) and compared them with those observed in Central Europe.

**Figure 1.** Syncrofalke, one of the most typical PTY systems developed in Austria.
