ENERGY EXPENDITURE IN SOME SILVICULTURAL WORKS

Witold Grzywiński
Department of Forest Utilization, Poznań University of Life Sciences, Poland

ABSTRACT

The study presents results from studies of energy expenditure (EE) in work associated with stand establishment and early stand treatment. The performed experiments comprised six operations carried out on seven work stations. The net energy expenditure was determined through measurements of pulmonary ventilation conducted with an MWE-1 energy expenditure meter. The author determined per-minute values of the energy expenditure for individual activities (n=14) as well as the energy cost of an 8-h workday for each work station. The effective work time constituted from 49.4 to 74.1% of the work shift while rest and personal breaks amounted to 12.7–23.5%. The net unit energy expenditure ranged from 7.5 to 31.7 kJ × min^-1, whereas the net energy cost of a workday ranged from 3.9 to 11.9 MJ. A majority of the five examined operations were categorized as very heavy work – and the remaining two (motor-manual cutting of weeds and the job of a helper during planting) with the energy expenditure below 6.3 MJ were classified as moderate.

Key words: silviculture, work time study, energy expenditure, category of work intensity.

INTRODUCTION

Operations connected with stand establishment and early stand treatment constitute the first stages of forest production. During 2004, silvicultural and forest tending activities were carried out on approximately 438 000 ha representing almost 5% of the forest area in Poland [19]. Silvicultural and tending operations represent about 30% of the annual work expenditure of Polish forestry [15]. These activities have a seasonal character peaking in spring and summer.

Practical forest work, especially timber harvesting and silviculture, is considered as heavy or very heavy [10]. The physical work load is most conveniently investigated through measurement of physiological energy exertion. Activities of forestry contain a high proportion of manual operations, requiring considerable physical effort. The observed advances of mechanization have, so far, mainly concerned nursery work and timber harvesting and, to a much lesser degree, silvicultural and tending operations. Most of the latter are still conducted manually or as motor-manual work.

Physical work load, including energy expenditure, in silvicultural and tending operations in forestry was studied in several papers. Most of them concern activities in forest nurseries and in planting. The following planting implements have been studied: planting hoe [22], spade [1,18,22,23,24], planting bars [6] and soil drills [3]. For tending operations, previous studies primarily involve measurements of work load in cleaning using various tools [2,5,11,20,21].

The current study presents observations of the energy expenditure during the most frequent silvicultural and tending operations in Polish forestry.

MATERIAL AND METHODS

Studied operations
The studies include the following six work tasks of silviculture and forest tending:

• Manual scarification – for filling blanks (replacing seedlings which fell out from plantations). The work consists in removing a 40 x 40 cm sod from the mineral soil using a 2.5 kg hoe. The surface was badly weeded and the age of plantation 5 years. Two workers were included in the study.

• Planting using a spade – planting broad-leaved seedlings (birch, beech, oak) on a scarified area (spots). The operation was carried out by two persons: the planter made a hole using a spade and, once the helper placed a seedling in it, he filled it with soil. Then the helper pressed the soil around the seedling with his feet. The weight of the spade was 2.5 kg and the weight of the box with seedlings was approximately 5.0 kg. The work of two such teams was analyzed.

• Manual weeding – weeding using a 2.0 kg forest scythe with a 40 cm blade in a badly weeded, one-year old plantation. One worker was investigated.

• Motor-manual weeding – cutting weeds using a brush-saw with a triangular weeding blade in a two-year old plantation, badly weeded with individual clumps of bush suckers. The weight of the brush-saw is 8.5 kg. Two workers were investigated.

• Early manual cleaning – cutting out small trees with a machete in order to regulate species composition and spacing in a 5-year old plantation. The weight of the machete 1.0 kg. One worker was investigated.

• Late motor-manual cleaning – cutting out trees with a chainsaw to regulate spacing and remove negative trees without harvesting the felled trees. The treatment was carried out in a 15-year old pine stand. The weight of the chainsaw 6.0 kg. One worker was investigated.

The experiments were carried out on lowland areas in four forest districts of western and central Poland.

Subjects
The investigated operations involved seven work stations. Each of the eight studied male workers agreed to take part in the investigations and was informed about the purpose and the design of the experiment. Table 1 presents anthropometric characteristics of the participating workers.

Work time study
For each of the studied operations, a time study was carried out using continuous timekeeping and covering two consecutive working days. Based on this data, the proportion of individual activities was calculated. Because of considerable differences in the real time of work on individual work stations, the proportion of individual activities was referred to the 8-hour workday. In order to compare the analyzed work, all activities were categorized into the following main working times:

• T_e – time of effective work (principal working activities, e.g. planting, cutting weeds etc.),

• T_a – auxiliary time (walking, operational breaks),

• T_t – technical maintenance time (sharpening of tools, regulations, refueling),

• T_r – time of resting (rest and personal allowances).

Energy expenditure
The net energy expenditure of the individual activities was determined through measurements of pulmonary ventilation. This is a simple and accurate method [12,17]. The measurements were carried out using an MWE-1 energy expenditure meter of Polish production (Fig. 1). The measurement of the pulmonary ventilation was performed in accordance with [13,16]. Before starting measurements, each worker worked in the half-mask for a few minutes in order to get accustomed to new working conditions and to achieve a state of functional equilibrium. Next, measurements of pulmonary ventilation were carried out for all activities identified on individual operations. After the measurement, an interval of several minutes was applied and afterwards the measuring cycle was repeated. At least three cycles were carried out for each working activity.

On the basis of the unit value of the net energy expenditure (kJ × min^-1) of individual activities and the relative proportion of each such work element of total time, obtained from the time study, the energy expenditure of an 8-h work shift was obtained.

The classification of physical work load was based on the net energy expenditure per 8-h shift, according to the definitions in [14].

RESULTS

Work time study
The investigated operations are considered to have a fairly low work content (short work cycles) with a high degree of repeatability. This affects the proportion of the effective working time T_e (basic activities directly contributing to the realization of a given task). For the majority of the examined operations, T_e constituted over half of the 8-hour work shift (Fig. 2). During motor-manual weeding, its proportion was 74.1% of the utilized time. The lowest proportions of effective work time were recorded in early cleaning with a machete and for the helper in planting operations where it constituted almost 50%.

The proportion of auxiliary time (T_a) was highest for the operation of planting (filling blanks) and constituted 37.5% for the helper and 30.4% for the planter. A considerable proportion of auxiliary time was found also for the operations manual scarification (25.8%) and late cleaning with chainsaw (22.1%). Both these operations include a considerable proportion of walking as a result of the need to relocate repeatedly. The proportion of the time of resting (T_r) ranged from 12.7% to 23.5% of the work shift.

Unit energy expenditure
The net energy expenditure of activities associated with the studied operations is presented in Table 2. A considerable spread of the minute energy expenditure value reaching 24 kJ was recorded. The lowest recorded minute energy expenditure for the activity of effective work time occurred during motor-manual cutting of weeds (7.5 kJ) and for sorting and loading of seedlings into plastic boxes – 8.7 kJ. Planting (placing seedlings in holes and soil treading) of broad-leaved seedlings during the process of filling blanks required energy expenditure of 13.8 kJ × min^-1 and digging holes for seedlings using a spade required an energy expenditure of 24.8 kJ × min^-1.

The highest unit energy expenditure rates were observed for manual activities in operations using hand tools and equipment requiring force. These activities included: cutting small trees with a machete – 31.7 kJ × min^-1, manual scarification with a hoe – 30.6 kJ × min^-1 and manual weeding with a forest scythe – 28.3 kJ × min^-1.

Energy expenditure of work shift
The net energy expenditure of a work shift (8 h) ranged from 3.9 to 11.9 MJ (Fig. 3). The lowest energy cost – less than 4.0 MJ – for a work shift was found during motor-manual weeding. The energy expenditure was almost 2.0 MJ higher for the helper with seedlings in the operation of planting broad-leaved seedlings during blanks filling. The highest values were found for the following operations: manual scarification – 11.9 MJ, early cleaning with a machete – 11.4 MJ and manual scything weeds in a plantation – 10.1 MJ.

Category of work intensity
Of the seven investigated silvicultural operations, five were categorized as very heavy (Table 3). Motor-manual weeding and the job of the helper in planting, their energy expenditures ranging from 3.9 to 5.6 MJ × 8 h^-1, were classified as moderate.

DISCUSSION

In the examined silvicultural operations, a high proportion of effective work time was recorded, in some cases, exceeded 70% of the utilized time. A similarly high proportion of the effective work time has been previously reported in nursery, stand renewal and tending operations [4,6] and results from the uniform character of these activities.

The study shows that the physical work load, measured as energy expenditure is considerable for a number of operations associated with silviculture. The recorded values of energy expenditure exceeding 30 kJ × min^-1 belong to some of the highest in contemporary Polish forestry [7]. Most of the analyzed operations include workday energy expenditures exceeding 8.4 MJ, which qualifies them to be categorized as very heavy. The energy cost per working day in the manual scarification using a hoe as well as in early cleaning with a machete exceeded the limit of 10.5 MJ net, which is commonly considered as the maximum limit of energy expenditure for professional work carried out by men [14]. Such heavy work may, on a long-term basis result in permanent physiological damage including a number of disorders associated with overloading of the musculoskeletal system. The observed energy cost of silvicultural operations is comparable with the corresponding energy load during motor-manual logging of timber with a chainsaw [8].

The energy expenditure in planting broad-leaved with a spade amounted to 24.8 kJ × min^-1 for the planter, digging holes and 13.8 kJ × min^-1 for the helper, carrying the seedlings. Their daily energy cost of work amounted to 9.3 MJ and 5.6 MJ, respectively. Daily energy expenditures similar (6.8–8.3 MJ) to those observed in our experiments were reported during planting into planting pits [3]. On the other hand, lower values of the minute energy expenditure (16.8 kJ per person) during the planting of several-years old spruce seedlings using the spade are also reported [22]. Making holes for seedlings using a spade is a difficult task and the minute energy expenditure of this operation is comparable with the energy cost of tree felling by a chainsaw [8,9].

Variations in energy expenditure during planting may be associated with different weights of the applied tools and technique of work. Another important factor influencing the work load is the degree of soil preparation for planting. Studies of planting with three types of dibbles revealed that the loosening of the soil in furrows, facilitated the planting operation significantly but, also brought about an increased rate and increased energy cost of work [6].

Early cleaning using a machete was the most arduous of analyzed operations. The minute energy expenditure reached 31.7 kJ. Some researchers report even higher energy expenditure for this operation, up to 36.4 kJ × min^-1 [11]. For comparison, according to the same researcher, cutting small trees with an axe required 27.6 kJ × min^-1, and with a brush-saw – 13.0 kJ × min^-1.

The substitution of manual work for motor-manual work entails a significant decrease of the work energy cost. This is well illustrated by weeding, where the use of a brush cutter resulted in a reduction in energy expenditure to almost a quarter in comparison to the manual weeding with a scythe. The use of a special carrying harness for the brush cutters transfers its weight onto hips, eliminates does the need to constantly bend the body forward and restricts the turning of the torso, allowing the worker to work in a straight position. The example shows that even small-scale mechanization can significantly reduce the arduousness of work and, simultaneously, increase its effectiveness. This is especially important in a case like Poland where the sector of forest services consists, primarily, of small companies with limited capital possibilities.

CONCLUSIONS

1. Effective working time (T_e) in investigated silvicultural operations ranged from 45.9% (planting with spade – helper) to 74.1% (motor-manual weeding) of work shift.

2. A minute net energy expenditure in silvicultural operations ranged widely from 7.5 kJ × min^-1 during motor-manual weeding to 31.7 kJ × min^-1 during cutting small trees with a machete in early cleaning.

3. The lowest energy expenditure of work shift was observed during motor-manual weeding – 3.9 MJ, the highest values were found for the following operations:  manual scarification – 11.9 MJ, early cleaning with a machete – 11.4 MJ and manual scything weeds in a plantation – 10.1 MJ.

4. Among seven analyzed silvicultural operations five were classified as very heavy and the remaining two operations (motor-manual weeding and planting with spade – helper) as moderate.

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Accepted for print: 7.12.2009

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