Estimation of Productivity and Costs of Using a Track Mini-Harvester with a Stroke Head for the First Commercial Thinning of a Scots Pine Stand

The aim of the present work was to estimate the productivity and costs of timber harvesting and forwarding during the first commercial thinning of a Scots pine stand. Three harvesting models were introduced and compared: narrow trail, wide access trail, and schematic extraction. The analyzed harvesting equipment consisted of a track mini-excavator (34 kW) with a stroke harvester head (gripping range 4–30 cm), and a farm tractor coupled to a logging trailer with a hydraulic crane. Merchantable timber (roundwood with a minimum diameter of 5 cm inside bark) was harvested from a 25-year-old planted Scots pine stand growing on a grid of 1.4 m × 1.8 m. The study showed the productivity of the mini-harvester ranged from 3.09 to 3.47 m3/PMH15 (productive machine hours plus 15 min), and that of the forwarding equipment to be 4.07 m3/PMH15. The analyzed model of productivity as a function of tree volume and thinning intensity was statistically significant, but the intensity parameter was significant only on plots located along wide access trails (3.7 m) and insignificant on plots located along narrow access trails (2.5 m). The distance between trees was not found to be significant. The calculated net machine costs for the forwarding equipment and track mini-harvester were EUR 36.12 and 52.47 per PMH, respectively. An increase in the usage rate of the harvesting equipment to 80% would reduce the harvesting and forwarding costs to EUR 22.07/m3.


Introduction
According to the State of Europe's Forests report [1], Poland has a relatively large forest area (9.4 million ha) with a growing stock density higher than the European average (269 m 3 /ha). This translates into a growing stock volume of over 2.5 billion m 3 , offering a vast source of wood for the region. Most of Poland's forests are publicly owned (80.8%), including forests managed by the State Forests National Forest Holding (77.0%). Among the latter, the largest area,~4 million ha, contains stands of 20-80-year-old trees, representing almost 60% of the available wood stock. Taking into consideration different climate change scenarios [2], it is estimated that the highest productivity of Scots pine stands can be achieved with medium thinning (20-40%).
According to Karttunen et al. [3], timber in Scandinavian countries is largely harvested by thinning, which becomes increasingly relevant both to the bioenergy sector and to developing industrial investments. Considering different harvesting alternatives in in the context of the timber supply chain and stand lifecycle, the authors concluded that intensive thinning leads to lower stumpage prices, with harvesting technology having the greatest influence on silviculture productivity. They also noted that whole-tree harvesting leads to the highest biomass production, but also to lower future stumpage prices and decreased forest profitability. Furthermore, a growing body of research is taking into account the logging trailer with a hydraulic crane (outreach of 6.4 m) (hereinafter referred to as "forwarding equipment").
In the first phase of the study, timber was harvested from the sample plots located along wide access trails (AT35). The trees to be cut were selected by foresters and clearly marked with paint at a height of approx. 1.5 m for ease of identification. They were processed into 2.5 m logs (using cut-to-length technology) with a minimum diameter of 5 cm inside the bark, and then placed along the trails. Thinning was conducted using a mini-harvester consisting of a tracked excavator with an Arbro 400 S stroke harvester head (Table 1). Due to the relatively short crane range (6.5 m), the mini-harvester had to move off the trail to perform thinning on the entire area of the designated sample plots. In the second phase, narrow-access trails were made by schematic extraction of single rows of trees (AT27 schemextraction). Due to the relatively low ground clearance of the mini-harvester (310 mm, Table 1), the remaining stumps had to be removed, and the trail leveled by a mounted blade.
In the third phase, thinning was performed on the sample tree stand plots along the new narrow-access trails (AT27) using the same harvesting equipment with a crane having an outreach of 6.4 m ( Table 1). The volume of the transported loads for each work cycle was calculated in cubic meters, including bark, after being unloaded at a landing area near a paved road 700 m away.

Study Description
The study was conducted in a 25-year-old Scots pine stand (50 • 13 43" N; 23 • 12 39" E), which was a 120 m × 240 m rectangle containing two 3.5 m wide access trails (AT35) made during planting that divide the shorter edge into three equal 40 m segments, which is typical for motor-manual harvesting. To facilitate mechanized thinning, three additional 2.7 m wide access trails (AT27) were cut parallel to the existing ones by schematically removing one tree row for each. In this way, the stand had a parallel network of trails spaced every 20 m, which allowed the hydraulic crane full access.
Due to the large number of trees to be removed (more than 1300) and for the sake of observer safety, small 25 m × 20 m sample plots were established 10 m on either side of the access trails ( Figure 1). These plots were demarcated using a Ledha-Geo laser optic-system produced by Jenoptik (Jena, Germany).
The study involved the harvesting of merchantable timber (roundwood with a minimum diameter of 5 cm inside bark) from a planted Scots pine stand in which trees were grown on a 1.4 m × 1.8 m grid at the time of the tending treatment, which could be classified as the first commercial thinning. The growing stock density was approximately 175 m 3 of merchantable timber outside bark with approximately 2200 trees per ha (Table 2). A total  The study involved the harvesting of merchantable timber (roundwood with a minimum diameter of 5 cm inside bark) from a planted Scots pine stand in which trees were grown on a 1.4 m × 1.8 m grid at the time of the tending treatment, which could be classified as the first commercial thinning. The growing stock density was approximately 175 m 3 of merchantable timber outside bark with approximately 2200 trees per ha ( Table 2). A total of 50 rectangular sample plots and 15 sample segments were established along the new access trails (AT27 schemextraction).

Productivity and Cost Analysis
To estimate machine productivity precisely, the present work focused on work time as defined in the IUFRO 1995 classification [32]: main work time (cutting, processing, loading, unloading, and turning) and complementary work time (root extraction, intertree movement, unloading, and turning). As has been mentioned by numerous authors [33,34], a 15 min rest per 8-hour shift was added to productive work time to comply with occupational safety and health standards, which meant that the resulting time was expressed as productive machine hours plus 15 min (PMH 15 ). The duration of the various work elements was measured both in harvesting and forwarding work cycles. Given the purpose of estimating the effects of tree stand characteristics (thinning intensity and tree density) on mini-harvester work time and productivity as well as other operating parameters, the variables were measured separately in cycles for each sample plot. Measurements were calculated to an accuracy of 1 s using a PSION WorkAbout data recorder with dedicated software. In summary, the observation was conducted on 65 sample plots while 1327 timber-processing work cycles were observed.
The machine costs were computed pursuant to FAO guidelines [35], and the harmonized procedure proposed by Ackerman et al. [36] used the basic data presented in Table 3. The costs were computed for the Polish market at the rate of 1 EUR = 4.23 PLN. The calculation included the costs of transport and garage, provided by an external company, and of replacing tracks and tires. Repair costs were adopted as 100% of the purchase price for the mini-harvester and the farm tractor and 60% for the logging trailer.

Statistical Analysis
Tree volume was determined indirectly based on a formula for pine stem volume inside bark [37] using parameters such as stand-specific taper factor, outside-bark diameter at 1.3 m (DBH), and tree height. DBH was determined directly by measuring all trees selected for felling by means of a digital caliper. Individual tree heights were calculated indirectly from a curve determined for the stand based on a measurement of 5% of trees using a Ledha-Geo laser dendrometer. For the purpose of calculating tree volume outside the bark in cubic meters, it was assumed that the bark accounted for 12% of the total volume [38].
Homogeneity of the designated sample plots was evaluated by calculating the thinning intensity as the volume of harvested merchantable timber relative to total merchantable timber, as well as the Hart-Becking index (or relative spacing index, RSI) for the dominant tree height: where AS is the average spacing between trees in meters (assuming positioning on a triangular grid) and H dom is the average height of the tallest 10% [39]. Statistical analysis consisted of descriptive statistics, Fisher's F-test for equality of variances, and Student's t-tests for homoscedastic data (with equal variances) and nonhomoscedastic data. It was assumed that the analyzed relationship would be close to exponential, so the data were transformed into natural logarithms. Among other things, it allowed for the use of a wide range of parametric statistical tests, which are useful in operational research because they involve simple transformations. The model functions were determined using the generalized linear model (GLM) where Y is the dependent variable (productivity); X 1...k is the independent variable (X 1 , merchantable timber volume outside bark; X 2 , thinning intensity; X 3 , relative spacing index RSI); and ε is the estimation error. Due to the relatively small differences in the adopted model, the statistical significance of differences between marginal means was evaluated by contrast analysis in the form of a priori tests for selected average comparisons. Analysis of the residuals (ε) of linearized regression functions included tests for normality of distribution using the Shapiro-Wilk test, which confirmed the lack of bias and random error in the estimated models. Statistical analysis was performed using the StatSoft [40] and R Core Team [41] packages. The following power function model was applied to predict productivity in m 3 /PMH 15 by converting logarithmic values into real values): The analysis of the marginal productivity functions was performed as a simple regression coefficient analysis, separately for each associated variable (access trail, tree volume).

Results
The first step in the analysis involved assessing the homogeneity of sample plots established along the wide (AT35) and narrow (AT27) trails. Statistical tests (Table 4) show that neither individual tree volume nor RSI differed significantly. However, the plots along the wide access trails exhibited significantly higher RSI variance and lower thinning intensity (∆ = 8.97%). Table 4. Homogeneity of sample plots along the wide (AT35) and narrow (AT27) access trails. As can be seen from Table 5, the mini-harvester with an Arbro 400 S head had the lowest productivity (3.09 m 3 /PMH 15 ) for sample plots located along the wide access trails, while it achieved the highest productivity (3.47 m 3 /PMH 15 ) for the schematic extraction of single rows. The productivity of the forwarding equipment (Belarus 952.2 farm tractor with a FAO FAR 842 logging trailer) was found to be 4.07 m 3 /PMH 15 . The general results (Table 6) indicate the absence of statistically significant differences in mini-harvester productivity between sample plots along wide (AT35) and narrow access trails (AT27). Therefore, the two types of sample plots were combined with net cost and productive analyses (Table 7, Figure 2). However, a more accurate analysis of performance models showed up to 12% higher productivity on narrow trails (AT27) for the same tree volume (Table 8).  Table 7. Linear regression analysis of mini-harvester productivity (Equation (1)) with an Arbro 400 S head as a function of average stump volume. Based on these data, the next step of analysis involved estimating the mini-harvester productivity function. In Model 1 (Table 7), when productivity on sample plots was characterized as a function of individual tree volume (R 2 adj. = 0.91), the RSI effect (b 2 ) was not found to be statistically significant (p = 0.220); however, statistical significance was confirmed both for the intercept (b 0 ) and individual tree volume (b 1 ). In Model 2 (R 2 adj. = 0.82), mini-harvester productivity was estimated for schematic tree row extraction during the clearance of new narrow trails and shown to be lower compared to operating on rectangular sample plots. Residual analysis indicates random distribution and absence of bias (average = 0) and random error (p > 0.05 for Shapiro-Wilk test). As can be seen from the functions for real values (Equation (3) and Figure 2), the decrease in productivity amounted to approx. ∆ = 0.58 m 3 /PMH 15 for the adopted average outside-bark tree volume (0.08 m 3 ).  Table 8. Linear regression analysis of the productivity (Equation (1)) of the mini-harvester with an Arbro 400 S head as a function of average stumpage volume (X1) and thinning intensity (X2).

Parameter Value Std. Error t-Stat
Residual, ε (Aver., SD) Based on these data, the next step of analysis involved estimating the mini-harvester productivity function. In Model 1 (Table 7), when productivity on sample plots was characterized as a function of individual tree volume (R 2 adj. = 0.91), the RSI effect (b2) was not found to be statistically significant (p = 0.220); however, statistical significance was confirmed both for the intercept (b0) and individual tree volume (b1). In Model 2 (R 2 adj. = 0.82), mini-harvester productivity was estimated for schematic tree row extraction during the clearance of new narrow trails and shown to be lower compared to operating on rectangular sample plots. Residual analysis indicates random distribution and absence of bias (average = 0) and random error (p > 0.05 for Shapiro-Wilk test). As can be seen from the  Table 8. Linear regression analysis of the productivity (Equation (1)) of the mini-harvester with an Arbro 400 S head as a function of average stumpage volume (X 1 ) and thinning intensity (X 2 ).

Parameter Value Std. Error t-Stat
Residual, ε (Aver., SD) The identified differences in thinning intensity (∆ = 8.97%, Tables 2 and 3) on sample plots located along the wide and narrow access trails (AT35, AT27) were adopted as the basis for estimating further productivity functions while taking into consideration variables such as tree volume and thinning intensity (Models 3 and 4, Table 7). The obtained parameters were statistically significant for Model 3, which described productivity on sample plots along the wide access trails (AT35) with the errors being random. In Model 4, which described sample plots along the narrow access trails (AT27), thinning intensity (b 2 ) was not statistically significant (p = 0.9514).
Analysis of marginal productivity functions (Figure 3) showed higher average productivity for sample plots along the wide access trails (AT35; R 2 adj. = 0.5192, F = 8.304, p = 0.008) at thinning intensity greater than 23%, while at lower thinning intensity higher produc- parameters were statistically significant for Model 3, which described productivity on sample plots along the wide access trails (AT35) with the errors being random. In Model 4, which described sample plots along the narrow access trails (AT27), thinning intensity (b2) was not statistically significant (p = 0.9514).
Analyses showed that during a one-shift work system (8 h/day, 210 days/year) machine usage was rather low and did not exceed 65%. The estimated net machine cost (excluding profit margin, Table 9) for the forwarding equipment was EUR 36.12/PMH, and that for the mini-harvester EUR 52.47/PMH. Under these conditions, the total harvesting cost was EUR 25.11/m 3 . At an average tree volume of 0.08 m 3 , the corresponding estimated mini-harvester cost ranged from EUR 17.7 to 20.9/m 3 ( Figure 2). Furthermore, for a distance of 300 m, the expected productivity of the forwarding equipment was 4.29 m 3 /PMH15 with an estimated net cost of EUR 8.4/m 3 . The lines in Figure 4 were indicate the average productivity and cost for the forwarding equipment as a function of distance by average, constant speed, and fuel consumption (Table 1) (Table 8; Equation (3)).
Analyses showed that during a one-shift work system (8 h/day, 210 days/year) machine usage was rather low and did not exceed 65%. The estimated net machine cost (excluding profit margin, Table 9) for the forwarding equipment was EUR 36.12/PMH, and that for the mini-harvester EUR 52.47/PMH. Under these conditions, the total harvesting cost was EUR 25.11/m 3 . At an average tree volume of 0.08 m 3 , the corresponding estimated mini-harvester cost ranged from EUR 17.7 to 20.9/m 3 ( Figure 2). Furthermore, for a distance of 300 m, the expected productivity of the forwarding equipment was 4.29 m 3 /PMH 15 with an estimated net cost of EUR 8.4/m 3 . The lines in Figure 4 were indicate the average productivity and cost for the forwarding equipment as a function of distance by average, constant speed, and fuel consumption (Table 1).   Using the cost calculator developed by Ackerman et al. [36], it was estimated that an increase in machine usage rate to 80% would lower harvesting and forwarding costs to EUR 22.07/m 3 (Table 10). At an average stumpage price of EUR 35.46/m 3 for timber sold for industrial purposes [42], the net profit would range from EUR 9.8 to 13.4/m 3 depending   Using the cost calculator developed by Ackerman et al. [36], it was estimated that an increase in machine usage rate to 80% would lower harvesting and forwarding costs to EUR 22.07/m 3 (Table 10). At an average stumpage price of EUR 35.46/m 3 for timber sold for industrial purposes [42], the net profit would range from EUR 9.8 to 13.4/m 3 depending on the usage rate (excluding VAT and the internal administration costs of the State Forests National Forest Holding).

Discussion
Based on delimbing speed the Arbro 400 S stroke harvester head (0.4 m/s) and a farm tractor coupled to a logging trailer were obviously less effective than a roller-feed harvester [43] (e.g., KETO Forst Silver 4.0 m/s). Despite this, the productivity of the excavator-harvester head combination was similar to that of some purpose-built fellerbunchers, such as the Moipu 400 E, which achieved 3.16 m 3 /PMH 15 [44]. Importantly, the low overall harvesting costs (EUR 22-25/m 3 ), which were approximately half of those reported for the aforementioned feller-buncher, are attributable to small fuel consumption (2.2 dm/PMH, Table 1), lower machine purchase and regional labor costs, and economical technology, which did not involve chipping, which is a very energy-intensive step in biomass production [45,46]. The forwarding costs calculated in the present work (EUR 7.8-9.13/m 3 ) are lower by EUR 3-6/m 3 than those determined by Spinelli and Magagnotti [47] for a new mini-forwarder model specifically designed for thinning operations.
It seemed obvious that mini-harvester productivity would increase with thinning intensity because a stationary machine can fell several trees within the reach of the crane at a high intensity, especially if the cab can turn 360 • as is the case with the Kubota excavator. The absence of this correlation on sample plots along the narrow access trails is probably attributable to the longer time needed to maneuver the mini-harvester into position since the width of 2.7 m is the absolute minimum for harvesting [9].
However, at a thinning intensity of up to 23%, harvesting productivity was higher on sample plots established along the narrow access trails (AT27) as opposed to those along the wide access trails (AT35). This fact could be explained by the change to multi-tree handing and the forming of new passages with a width of 1 m and a length corresponding to the crane's outreach, which is known as boom-corridor thinning in the literature [43,48]. Multi-tree handling and schematic thinning may increase harvesting productivity by up to 30% and are typically used for felling narrow trees with a DBH of less than 8-10 cm [49,50]. They are less often employed on plots along wide access trails (AT35). In addition, Mederski et al. [51] did not find a simple correlation between thinning intensity and productivity. Given the results, this appears to be a characteristic feature of young tree stands. Interestingly, according to Ackerman et al. [52], an appropriate planting geometry may enhance productivity by approx. 8% and reduce harvesting costs by up to 7%.
The productivity of the mini-harvester and the tractor-trailer equipment varied by 22-52% and 16%, respectively, between plots (Table 5), which is not much compared to the productivity discrepancies (0.64-8.3 m 3 /PSH) reported by Erber et al. [49] in small-diameter hardwood thinning by means of multiple-tree processing.
However, it should be stressed that this paper assessed the productivity of harvesting industrial roundwood having a minimum diameter of 5 cm inside bark. According to a study on the aboveground biomass allocation of Scots pines [53,54], the percentage share of merchantable wood ranges from 73 to 77%, depending on the biosocial position of the trees. As can be calculated from the tree volume tables for this stand of 25-year-old Scots pines [55], the results should, for energy purposes, be multiplied by a factor of 1.36 to ensure mini-harvester productivity in an industrial roundwood system for harvesting fractions of wood with small dimensions.

Conclusions
The aim of this study was to estimate the productivity of harvesting industrial roundwood during the first commercial thinning. The productivity of the track mini-harvester was found to range from 3.09 to 3.47 m 3 /PMH 15 , and that of the forwarding equipment was 4.07 m 3 /PMH 15 and the calculated net machine costs for analyzed machines were EUR 52.47/PMH and EUR 36.12/PMH, respectively. Moreover the estimated models of productivity as a function of tree volume indicated statistically significant findings, but the thinning intensity was significant only on plots along wide access trails (3.7 m). The productivity of schematic tree extraction with the trail clearing showed a smaller rise with the increase in tree volume. The absence of correlation on sample plots along the narrow access trails is probably attributable to the longer time needed to maneuver the mini-harvester into position since the width of 2.7 m is the absolute minimum for harvesting.
Funding: This research was funded by Ministry of Education and Science for the University of Agriculture in Krakow in 2020.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The study did not report any data.