Introduction

Forwarders are the most used ground-based machineries to extract the wood around the world (Lindroos et al. 2017; Borz et al. 2021) and represent a mechanized alternative to agricultural tractors and animal logging (Proto et al. 2018a; Borz et al. 2019). These self-propelled forwarders were first introduced in the 1960s for use in the transport of logs as part of harvesting operations (ISO 6814 2009 - Machinery for forestry — Mobile and self-propelled machinery — Terms, definitions and classification) and are used today in partly mechanized systems that integrate motor-manual felling and processing of trees. The actual concept of forwarders has not changed significantly compared to the original ones, but a significant progress in forwarding machines development has been achieved in key areas, such as the ergonomics, environmental impact and automation (Spinelli et al. 2003; Pandur et al. 2009; Stankić et al. 2012; Apǎfǎian et al. 2017; Borz et al. 2021). Modern forwarders are an effective forwarding option for timber harvesting operations that provide the opportunity for higher levels of mechanization and thanks to their ability to carry logs from the forest to the roadside or processing areas, some authors have identified lower environmental impacts in comparison to tree-length skidding options (Spinelli et al. 2014; Proto et al. 2018c; Borz et al. 2023). In the same time other studies, however, have found opposing results. Gondard et al. (2003) assessed the impacts on soil surface of clear-cutting in Aleppo pine (Pinus halepensis) using both forwarders and skidders, observing that skidders created more disturbances than forwarders and the majority of these disturbances were shallow and not long-lasting. Similar results were reported by Deconchat (2001) in mixed oak coppices. In this study, the observed greater soil disturbance was due to skidders, though skidders were responsible for less than 1% of ruts. These studies make the scientific debate on some environmental impacts between skidders and forwarders still unresolved. The use of forwarders requires mainly the cut-to-length (CTL) systems that offer several advantages compared to other ground-based systems: less road construction, lower levels of soil disturbance, smaller landing sizes due to reduced processing requirements, minimal damage to the logs during handling and forwarding and fewer workers (Bettinger and Kellogg 1993; Picchio et al. 2020).

Forwarder productivity has been widely studied in Europe (Cadei et al. 2020). Several studies have demonstrated that the productivity of this system depends on the forest stand, site and operational factors such as ground conditions, slope, operator skill, branch size, operational layout, tree volume, tree form, log assortments processed, numbers of merchantable trees per unit area, hauling distance, undergrowth density and machine design (Stampfer 1999; Spinelli et al. 2002; Apǎfǎian et al. 2017). In recent decades, the fully mechanized cut-to-length timber harvesting system has become widely used in many industrialized European countries where the conditions and the stands are favourable, such as in Sweden (ca. 98%), Ireland (ca. 95%) and Finland (ca. 91%), as compared to motor-manual harvesting (Karjalainen et al. 2001; Mologni et al. 2019; Papandrea et al. 2023). In Austria, where forests are predominantly on mountainous terrains, the distribution of timber harvesting machines from stand to landing in 2009 was: cable yarders – 20%, skidders – 49%, forwarders – 26%, and other means – 5% (Holzleitner et al. 2011). Ten years later, in 2019, this ratio changed in favour of forwarders: cable yarders – 19%, skidders – 37%, forwarders – 43%, and other means – 5% (Prem and Bauer 2020). The important role and diffusion of this forest machine is supported by a large number of studies (Tiernan et al. 2004; Nurminen et al. 2006; Proto et al. 2018b; Cataldo et al. 2022) that are still conducted to test the performance of these machines. Among the first, Sever (1988) determined that travel distance is the most important factor, while Manner et al. (2013) identified log concentration as an influential parameter on the forwarding time consumption; in fact, the movement time during the loading significantly decreases with increasing concentration of the logs. Slugeň and Stoilov (2009) evaluated the slope of 47% as the upper limit for the use of forwarders and harvesters to obtain high productivity in regenerative cuttings and thinning operations, while they found that the productivity between forwarders with different payloads decreased with forwarding distance, both in final felling and thinning. In Bulgaria, as in other European countries (Italy, etc.), although the forest areas are predominantly located on steep terrains with complex shapes, adapted agricultural tractors with logging equipment are predominance and forwarders are not widespread (Proto et al. 2018a; Stoilov 2021a). In particular, in these countries, logs are skidded from the harvest area to the landing on the closest skid road/skid trail using a ground extraction system with a farm tractor and winch or animal force and only after, the forestry trailers, forwarder, or other machines loads the logs from one pile and moves to the next pile, until the load space is fully loaded. Often, this limit is mainly due to the type of cut (thinning, selective, or shelterwood) which does not allow the forwarder to enter in the harvest area/felling site directly. Therefore, these two operations, winching and skidding, are conducted with different systems, machineries and workers. Fortunately, in recent years the development of new technology in forestry mechanization created an alternative operation to ground-based systems with the use of a combi-forwarder for wood forwarding. These innovative machines allow workers to pull the logs from the stump, with a winch (installed at the rear or front of the machine) and then load and unload the platform of the combi-forwarder with the built-in crane. The interest for this combi-forwarders can be explained by the fact that it can operate as both a skidder and a forwarder, thus highly increasing the flexibility of the operator (Schweier and Ludowicy 2020). In Central European countries, combi–forwarders, i.e. forwarders with a built-in single- or double-drum winch, are spreading, but at the moment there are no published papers regarding their productivity potential and the factors that influence their performance. The aim of this research was to carry out early studies of the factors affecting the productivity of combi–forwarders in deciduous forests. The study objectives were to: (i) Analize the operational time consumption of the combi–forwarder with a built-in single-drum winch; (ii) Estimate the productivity in relation to common factors in harvesting operations; (iii) Determine the operation costs for the forwarder studied.

Materials and Methods

Study Site and Work Organization

The study was conducted in Vitinya State Hunting Range, located in the Western Balkan Mountains, in Sofia Province, Bulgaria. The characteristics of the areas chosen for the study are shown in Table 1. The topographical characteristics of the two study areas are very similar, with slopes that do not substantially differ between the two sites. An articulated six-wheel-drive Timberjack-1010D forwarder, equipped with a single-drum winch, as shown in Fig. 1; Table 2, was used for the tests.

Table 1 Characteristics of test sites
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Fig. 1
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The tested Timberjack-1010D forwarder

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The felling and processing of the trees took place with the same chainsaw operators at both sites A and site B (Fig. 2). The timber was hauled from the cutting area by the forwarder’s winch in the form of stems and logs, and the larger ones – in the form of assortments. On the road, after crosscutting, the assortments were loaded by the forwarder’s hydraulic crane and transported to the landing with an uphill forwarding direction.

Fig. 2
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Site map of the study and the schematic layout during the forwarding operations

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Table 2 Technical data of Timberjack 1010D
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Productivity Study and Costs

A detailed study of times and movements was carried out to estimate the duration of the elements of the work cycle and the productivity of the combi-forwarder into account the average value of the two sites under the given conditions. A work cycle was assumed to be composed of repetitive elements (Stokes et al. 1989; Olsen et al. 1998).

The work cycle of of the forwarder with winch was composed of following repetitive components:

  • driving empty - from landing to stand;

  • maneuvering – the time for which the forwarder is as close as possible to the timber to be inhauled;

  • outhaul and hook – the time for which a worker pulls the winch cable and hooks the load by chockers;

  • inhaul – the time for which the logs are drawn to the forwarder with the winch cable;

  • loading – the time for loading the obtained assortments on the forwarder with the hydraulic crane. When stems are pulled out, they are crosscutted during this work cycle element;

  • driving loaded – the time for which the forwarder moves loaded from the stand to the landing;

  • unloading – the time for which assortments are unloaded and piled in large piles on the landing by hydraulic crane;

  • delays – the time for which the forwarder does not perform any work for organizational (waiting for the felling of a certain number of trees, crosscutting of the stems into assortments), technical reasons (mechanical and repair delay time), weather or terrain conditions, or the operators talking with managers or visitors.

During the study, the forwarding distance, winching distance and slope gradient, as well as the number and volume of the assortments were measured. The time-motion study was designed to evaluate the duration of work elements and productivity of the forwarder and to identify those variables that are most likely to affect it. Each work cycle was individually measured by a stopwatch. The productive time was separated from the delay time. Forwarding distance and the slope gradient of the road were measured by GPS device (Macrì et al. 2016), whereas, winching distances were measured by a professional laser range-finder with clinometer. Load volume was determined by measuring the length and the mid-length diameter of all logs from each assortment.

The machine costs were calculated using the COST model (Ackerman et al. 2014). In order to calculate the production cost for 1 m3 timber, the cost analysis employed the following parameters: the number of operators, the hourly cost of an operator, the hourly cost of machines, the volume of extracted timber, and productive machine hours (excluding all delay times). The machine cost per hour was reported both as productive machine hours excluding delays and scheduled machine hours. The purchase prices and operator wages required by the cost calculations were obtained from accounting records (Proto et al. 2018c; Stoilov et al. 2021b). Labor cost was set to 5.60 € SMH− 1 inclusive of indirect salary costs. Diesel fuel consumption was measured by the commonly used method of refilling to full. A salvage value of 10% of the purchase price was assumed and the Value Added Tax (VAT) was excluded.

Cost calculations were based on the assumption that companies worked for 150 working days in the year and a depreciation period of 10 years was used. For forwarding work, this amounts to 130–150 working days per year (20–21 working days per month) at an average of 6–7 scheduled working hours per day (assuming one to two hours spent on lunch, rest and other breaks). Thus yielded annual working hours were 910–1050 SMHs with a 70% use coefficient (Spinelli and Magagnotti 2011; Proto et al. 2018a).

Data Analysis

Regression analysis was performed on the l data in order to develop prediction models for estimating the work cycle time and productivity, this statistical analysis was chosen because the two study sites are very similar to each other and have no influence on the study of the work cycles, therefore the data obtained were combined. The variables used in the modelling approach included forwarding distance L, winching distance l, load volume per cycle V, slop gradient of the skidding road i, and the number of assortments in a load n. The descriptive statistics of the variables were computed and a stepwise backward regression procedure was used to model the variability of the cycle time and productivity as a function of independent variables. The confidence level used for regression analysis was 95% (α = 0.05) and independent variables are considered significant if p < 0.05. To process the experimental data the Statistica 8 (StatSoft Inc., Tulsa, OK, USA) software package was used.

Results and Discussion

Table 3 presents the mean experimental data of monitored combi-forwarder. In particular, the largest share of the working cycle elements occupies the operation “Inhaul of timber” (37% and 32% respectively, excluding and including delays), followed by the “Outhaul of line and hook” (33% and 29% respectively, excluding and including delays), travel loaded (11% and 9% respectively, excluding and including delays), “loading” and “travel unloaded” (both operations cover 7% and 6% respectively, excluding and including delays), “unloading” (3%), and the smallest has “maneuvering” (2%).

Table 3 Mean operational variables for working cycle elements
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The breakdown by main groups of operations in delay-free cycle time shows the predominance of the winching of the load (70%), followed by the movement (18%), and loading and unloading at landing (10%).

The data from Table 3 indicate that productive time was 88% from scheduled time (Fig. 3). The delays (12%) are due to organizational reasons (delays are due to waiting for the felling of trees in the cutting area and crosscutting them into assortments at roadside) (7%), mechanical delays (1%), and those due to adverse weather conditions (rain, snow, thick fog) (2%).

Fig. 3
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Summary statistics of the work elemental time consumption (TU: Travel unloaded; M: Maneuvering; OH: Outhaul of line and hook; I: Inhaul of timber; L: Loading; TL: Travel loaded; U: Unloading; D: Delays)

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The operations of pulling the winch cable, hooking the chockers, inhaul the logs and loading the assortments obtained after crosscutting occupy 77% and 68% of the duration of the work cycle without and with delays, respectively. This is due to the difficult terrain of the stand and the long and massive beech stems, which were inhauled to the machine by the winch. If the unloading time is added, then the winching operations, loading and unloading operations reach 80% and 71% of the work cycle timewithout and with delays. The mean duration of the work cycle is 86.20 and 99.55 min (1 h 26 min and 1 h 40 min), respectively, excluding and including delays, which leads to 0.41–1.51 (average 0.60) turns per hour and an average machine utilization ratio of 0.88.

The total movement time of the combi-forwarder has a share of only 20% and 17% of the duration of the working cycle (turn) without and with delays, respectively. The forwarding distance varies widely between 270 and 1100 m, with an average of 803 m (Fig. 3). The regression analysis was done on the time-study data in order to develop prediction equations for estimating the combi-forwarder cycle time by excluding and including delays (Table 4). The delay-free cycle time Tnet regression equation obtained with significant variables given in Eq. (1) is shown in Table 4.

Table 4 Summary of the work cycle time models
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In Eq. (1), the minimum duration of delay-free cycle time Tnet can be achieved in cases of short winching distances. The regression Eq. (2) for studied combi-forwarder cycle time including delays T is also presented in Table 4. Hence, combi-forwarder cycle time including delays also depends only on winching distance l and its minimum duration may be attaining by minimizing the timber extraction distance.

Travel Speed and Inhaul Speed

The mean travel speed during the skidding phase was 5.83 km h− 1. The mean speeds with and without load were 4.78 km h− 1 and 7.62 km h− 1 respectively. The difference is almost 3 km h− 1, due to loaded movement was downhill, and movement unloaded was uphill with acting grade resistance. Due to the load being entirely on the forwarder, the speed is higher; in fact, Orlovsky et al. (2020) reported mean travel speed of four studied LKT 81T wheel cable skidders of 3.97 km h− 1 (1.87–4.35 km h− 1 unloaded and 2.56–4.05 km h− 1 loaded). Spinelli and Magagnotti (2012) reported empty and driving loaded velocities of a 96 kW agricultural tractor of 8.1 and 7.3 km h− 1, respectively, which were higher than those determined in this study. The mean inhaul speed of winching of stems was 0.01 m·s− 1 (0.36 km h− 1).

Productivity Analysis

Delay-free productivity was defined by the regression Eq. (3) shown in Table 5. From Eq. (3), to enhance delay-free productivity of the studied machine, winching distance l should be reduced.

Table 5 Summary of the productivity models
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The combi-forwarder productivity including delays is expressed by Eq. (4) also shown in Table 5. From Eq. (4), to increase productivity including delays again winching distance should be reduced, whereas the load volume per cycle V and forwarding distance L are found to be insignificant variables.

In Eqs. (3) and (4), as well as in Eqs. (1) and (2), a significant influencing factor is the winching distance because winching operations occupy a leading share of the work cycle. The mean productivity was obtained at mean a timber extraction distance of 802 m, a mean winching distance of 13 m, a mean load volume of 9.12 m3 and a mean of 22 logs per cycle (turn) is 7.09 m3 PMH-1 and 6.11 m3 SMH-1, respectively. For LKT 81T and LKT 81 ILT cable skidders, the latter with a knuckle-boom, operated mostly in beech, beech-fir and beech-oak stands, at mean skidding distance of 300 and 316 m respectively, reported lower performance: mean load volumes of 5.45 m3 and 8.01 m3, and gross production rates of 3.91 m3 and 4.21 m3 SMH-1, respectively (Orlovský et al. 2020). Öztürk (2010a) found that hourly productivity of an MB Trac 900 tractor in beech stands in Black sea region of Turkey were 14.4 m³ h-1 for a timber extraction distance of 55 m and 8.7 m³ h-1 for a timber extraction distance of 105 m, and 11.35 m³ h-1 for a timber extraction distance of 140 m and 7.70 m³ h-1 for a New Holland TD85D. Borz et al. (2015) reported for an TAF 690 OP (shorter winching distance of 8.7 m, two times longer skidding distance of 1706 m, a lower load volume of 4.89 m3 and 6.48 logs per turn) the net and gross production rates were around three time lower (4.41 m3 h-1 and 3.12 m3 h-1, respectively). Due to the lack of studies in deciduous forests, the authors compared the productivity of forwarders in coniferous forests. In comparison for the John Deere 1010 forwarder deployed in coniferous stands and a mean forwarding distance 321 m, Dvořák et al. (2021) reported close mean hourly productivity ranged from 7.4 to 10.3 m3 per machine hour; Borz et al. (2021) evaluated an HSM 208 F 14-tone forwarder under conditions of steep-terrain dominated by Norway spruce stand and found for an average forwarding distance of about 1.5 km, net productivity and efficiency rates at 14.4 m3 h-1 and 0.07 h m-3, respectively. Proto et al. (2018b) found productivities of 14.4 and 15.7 m3 h-1 for forwarding distances of approximately 300 and 600 m, respectively, for two John Deere 1110D and 1010D models, operating on slopes of 26 and 29%, respectively. For steep terrain and similar timber extraction distance, Dinev et al. (2015) found productivities of 44 − 53 m3 per day (5–6 m3 h-1 assuming a work shift of 8 h). Ghaffarian et al. (2007) reported an average forwarding production of about 17.9 m3 PSH0-1 with mean load per trip of 10.04 m3 and average forwarding distance of 97 m.

Therefore, the studied combi-forwarder, unifying skidding and forwarding functions, demonstrates significant efficiency in respect of the productivity in shelterwood cutting in beech stands with a removal intensity of 15%.

Cost Analysis

The hourly costs of the studied combi – forwarder, as well as the labor costs, are summarized in Table 6. As shown, the net costs for uphill forwarding were estimated at 25.33 € PMH− 1 (productive machine hour). In the structure of the gross costs, the fixed costs (8%) were considerably lower than the labor (22%) and variable costs (70%). For the productive time of the machine, the net forwarding costs were estimated at 4.13 € m− 3.

Table 6 Forwarding costs
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For comparison, the results reported by Dvořák et al. (2021) for Czech forwarders in coniferous forests were personnel costs (35 to 66% of the total costs), followed by materials (14.9–27.1%), amortization (12.5–15.7%), and services (3.3–22.1%). The corresponding machine productivity was between 3.5 and 12.3 m3 SMH− 1. Also in Czech Republic, in clear-felling operations in coniferous forests, Jiroušek et al. (2007) found total machine costs of 63.82 € PMH− 1 for forwarders of a class similar to that of the studied combi-forwarder.

In the study in beech stands in the Black sea region of Turkey mentioned above, the cost of skidding for a MB Trac 900 tractor were 3.5 and 8.92 €·m−³ at timber extraction distance ranges between 55 and 105 m and average load volume for every cycle was found to be 1.49 and 2.13 m³, respectively (Öztürk 2010a). The cost for a New Hollad TD85D in the same region for longer timber extraction distances between 140 and 320 m were 4.5 $ m−³ and 8.6 $ m−³ respectively (Öztürk 2010b). The costs of C Holder 870 F tractor during thinning of beech stands were ranges from 69.16 kn m− 3 (approx. 9.19 €·m− 3) for a timber extraction distance of 25 m to 106.66 kn m− 3 (approx. 14.17 € m− 3) for a timber extraction distance of 250 m (Zečić et al. 2005).

Proto et al. (2018b) found that the forwarding costs of a John Deere 1110D and John Deere 1010D were 3.40 €·m− 3 in a Calabrian pine stand and 4.50 € m− 3 in a silver fir stand in the Calabria Region (Italy). The calculated unit cost of forwarder extraction in a selective harvest in Calabria, Italy (with a John Deere 1110E), in a clear-cut on the West Coast of New Zealand (with a John Deere 1910E) and in a larger clear-cut operation in Canterbury, New Zealand (with two John Deere 1910E), forwarders ranged from 2.55 to 4.70 €·m− 3 (Proto et al. 2018a).

In Czech Republic, Dvořák et al. (2021) monitored forwarders LVS 5, John Deere 1010, and John Deere 1110E forwarders in coniferous forest stands with a mean stem volume between 0.10 and 0.84 m3 and forwarding distance between 261 and 560 m, the costs obtained were between 20.95 and 84.39 € PMH− 1. Consequently, the mentioned data show that the studied combi-forwarder, compared to other ground-based machines, is advantageous in terms of unit costs of forwarding operations in low-intensity shelterwood cuttings in beech stands.

Conclusions

In recent years, severe labor shortages have led foresters to use modern, specialized machines to compensate. Combi-forwarders combine equipment typical of skidders with that of forwarders and represent a competitive machine for improving the mechanization of winching logs and assortments, loading the received assortments, transporting them to the landing for sorting and piling phases. The data provided show a clear opportunity to promote this machine during forest operations. The combi–forwarder provides high productivity and cost efficiency compared to many ground-based machines, which makes it very suitable for logging operations in deciduous forests in mountainous conditions. The results obtained from this case study can contribute to the scientific debate and improve knowledge on forestry machinery innovation and at the same help improve the scientific database in mechanized harvesting forestry operations. Over time, further scientific studies of this machine can positively contribute to the practical management of forest planning, logging operation and, consequently, the achievement of a wider cost competitiveness of the wood supply chain.