CABLE CRANE UTILIZATION IN THE CLOSE-TO-NATURE SILVICULTURE IN THE UPPER PIAVE RIVER VALLEY (BELLUNO PROVINCE, NORTHERN ITALY

Raffaele CAVALL, Bruno DE BENEDET,
Department of Land Use and Agricultural and Forest Systems, University of Padova, and Giuseppe MENEGUS, Belluno Province Forest Service, ITALY

Abstract

A study was carried out in four areas in the upper Piave River Valley (Belluno Province of northern Italy) measuring operational parameters of 27 cable cranes. The aim of the study was to determine the volume of logs yarded per unit of length of cable line in order to define if the cable yarding is properly applied in the upper Piave River Valley. The number of trees needed to be felled in order to create the skyline corridor was also investigated. This information was considered useful in planning cable crane utilization because it would allow the reserve to be devoted to the skyline corridor to be defined if the cable crane is mounted after the main marking operation. The data have allowed some indicators to be defined that give an analytical description of the conditions for the use of cable cranes in the areas considered. A correlation was also calculated between the volume of trees to be felled in order to create the skyline corridor and the length of the cable crane line.

The results represent the first stage in the development of knowledge that could bring about effective management of cable crane planning and utilization and provide a useful tool for verifying the proper use of such yarding equipment in the areas considered.

Introduction

The transfer to logging of the typical operational concepts of cable systems designed for the transport of materials and personnel has been most widely developed in countries where vast forest resources located on steep terrain are combined with an economic fabric strictly dependent on the processing and utilization of local wood, i.e. in the Austrian and Swiss Alps (Pestal, 1961; Hafner, 1964; Trzesniowski, 1985; Heinimann and Schmidt, 1990; Aggeler, 1992).

In these countries the development of cable logging techniques can take advantage of a silvicultural reality rich in history and tradition, long used in a general application of theoretical concepts and management procedures from the planning, mapping and operational points of view. The application of forest management criteria, the use of maps containing operative information and the practice of clear-felling with artificial regeneration have provided an ideal situation in which to verify the performance of different technological solutions with cable logging (FPP, 1986; FPP, 1999).

In Italy, where silviculture is based on close-to-nature principles with no clear-felling, forestry enterprises use cable logging machines with a fair technological level and good performance in the environment where they have been developed (Hippoliti et al., 1984; Piegai, 1990; Pollini et al., 1993; Bortoli and Solari, 1997a; Bortoli and Solari, 1997b; Fanari et al., 1999). Nevertheless different conditions to those in Austria and Switzerland, i.e. the impossibility of clear-felling and utilizing a large amount of wood concentrated in limited areas, have limited the adoption of machinery used in the intensive extraction processes on the northern side of the Alps.

The aim of the present study was to verify the operational conditions and the effects of cable crane use in stands managed according to the principles of close-to-nature silviculture located in the upper Piave River Valley (Belluno Province), in the northeastern Italian Alps.

The main objective of the study has been to determine the volume of logs yarded per unit length of cable line. Knowledge of this volume, related to both the field and topographical length of the cable line is useful for defining if cable logging can be adopted successfully in the Alpine stands of the upper Piave River Valley.

Another objective was to quantify the volume of trees per unit length of cable line that must be felled in order to create the skyline corridor. Such information could be a useful tool for cable logging planning because it would be possible to predict the reserved yield to be set aside for skyline corridors when the skyline corridor survey is made after the main marking.

Materials and methods

Forestry area arrangement

The upper Piave River Valley includes the Comunità Montane (associations of municipalities in mountain areas) of Centro Cadore and Comelico-Sappada. According to the data from the Direzione Regionale Foreste ed Economia Montana of the Veneto Region (1998) the forestry surface of the investigated area is 22 009 ha, of which 14 193 ha is reduced area. In the upper Piave River Valley the growing stock and current percentage annual increment are 318 m³ per hectare, 5.55 m³ per hectare per year and 1.8 percent, respectively. These data correspond to a total current increment of 91 303 m³ per year and a planned yield of 37 465 m³ per year, with a yield saving of 53 838 m³ per year. The yield that annually increases the stand growing stock is connected both to the strictly applied saving policy over the last 50 years and the difficulties in harvesting some forestry areas because of the lack of a road network.

Of the forests considered in the study, 66.5 percent belong to the Regole (communities of families with legal status), in the case of the Comunità Montana of Comelico-Sappada, and 63.4 percent to the municipalities in the case of the Comunità Montana Centro Cadore.

According to the data from a survey carried out within the framework of the Interreg Comelico-Osttirol project (Anonymous, 1998), there are 24 forestry enterprises in the upper Piave River Valley, with a total of 74 employees and an average of 3.1 employees per enterprise.

Extraction is done depending on the site characteristics and available machinery, manually downhill (4.6 percent), ground skidding with tractor and winch (75.9 percent) and cable yarding (19.5 percent). Manual downhill extraction is an operation used less and less because of low productivity, high physical energy demands and damage to remaining trees. Ground skidding with tractor and winch, on the contrary, is limited by terrain steepness, low ground bearing capacity or poor road networks. Cable yarding is sometimes the only way that forestry enterprise requirements can comply with stand and soil protection, as stated in the close-to-nature silvicultural criteria.

Cable crane installation features

Fifty percent of the analysed cable cranes belong to the medium mobile tower yarder type (downhill and uphill yarding) and 50 percent to the gravity cable crane type (downhill yarding). The forestry enterprise carries out the survey using a compass; no terrain profile is surveyed nor a project drawn up including calculation of loads.

When felling ends, the cable crane is set up by positioning and installing the sledge winch or the tower yarder, anchoring the skyline, building the supports, installing the carriage and then it is completed by tensioning the skyline.

The length of the assortments generally varies from 2.2 to 8.4 m. Delimbing and cross-cutting are usually motor-manually performed at the felling site, leaving the branches in the wood and limiting organic matter removal.

The productivity of cable cranes working in the surveyed area, measured as wood volume yarded per day by a 3–4 worker crew, is generally lower and varies between 40 and 45 m³ per day. Mounting a mobile tower yarder needs around 3–4 h, while dismounting takes 1–3 h. The time spent on the same operations with a gravity cable crane is usually three times as much, because of the technical features of this kind of cable crane.

Data collection

The study was carried out in 1998–1999 and was concentrated in the most significant places of the analysed area: Costa in San Nicolò di Cadore Municipality (Comelico Superiore), Campolongo in Santo Stefano di Cadore Municipality (Comelico Inferiore), Val Visdende in Santo Stefano di Cadore Municipality and Casera Campo in Vigo di Cadore Municipality.

The study considered 27 cable crane lines, installed on the surface belonging to a total of 10 compartments, located in different areas: Costa (5 lines), Campolongo (5 lines), Val Visdende (7 lines) and Casera Campo (10 lines).

For each line, a preliminary phase was considered in which the trees that were to be felled to create the skyline corridor were marked. Starting from the anchor tree and moving uphill, the species breast height diameter (bhd) and number of trees present in the skyline corridor were registered. The data collected, characterized by compartment, allowed the volume of marked trees (delimbed trunk without treetop) allocated for each corridor to be calculated.

For each cable crane line, the distance between the two anchor points was measured in the field by means of a laser distimeter and the altitude of the two anchor points was measured using an altimeter to calculate the topographical length of the line and its gradient.

For all the compartments the most characteristic parameters of the stands, i.e. total growing stock, unit growing stock, current annual increment, percentage annual increment, number of trees per surface unit and yield, were derived from consulting the working plans. For the compartments included in the felling series, which belong to Municipalities and Regole, the so-called felling plan was analysed, retrieving information on the number of trees to be felled, split by species and diametric class, and the estimated gross volume of felled trees, which serves to establish the value of the area. The felling plan also allowed the surface of the felled area yarded by cable crane to be determined.

The analysis of the data on the volume of marked trees per compartment considered a sample of 24 cable cranes, while the analysis of the data related to the volume and number of trees felled per skyline corridor, considered a sample of 27 cable cranes.

Results

Stand parameters

The cable cranes analysed in the study were set up in stands with different site features, forest management parameters and forestry typologies (see Table 1). The unit growing stock was 484.35 m³/ per hectare on average, from a minimum of 288.6 m³ per hectare (Val Visdende) to a maximum of 839 m³ per hectare (Casera Campo). The current annual increment was very high, 11.13 m³ per hectare per year on average, with the lowest value being 4.4 m³ per hectare per year (Casera Campo) and the highest 21.3 m³ per hectare per year (Comelico Inferiore). Accordingly, the percentage annual increment was also high, 2.5 percent on average, with the lowest value being 1.3 percent (Casera Campo) and the highest 4.3 percent (Val Visdende).

The number of trees per surface unit was 397 trees per hectare on average, with the lowest value being 195 trees per hectare (Comelico Inferiore) and the highest 602 trees per hectare (Casera Campo). The compartment yield achieved by means of the cable crane was 1 360.37 m³ on average and varied from 300 m³ (Comelico Inferiore and Casera Campo) to 3 850 m³ (Val Visdende). Despite the high variability, the stands had some features in common, such as the prevalent protective function and the uneven age structure.

Cable crane line features

Because of the variability of the stands in which the cable cranes were set up, many parameters differed, such as the operational altitude, difference in altitude between anchor points, gradient, field length, topographical length, and number and volume of trees felled to create the skyline corridors (see Table 2).

The mean altitude of the downhill anchor point was 1 355.9 m above sea level (a.s.l.), with minimum and maximum values of 1 000 m a.s.l. and 1 700 m a.s.l., respectively. The altitude of the uphill anchor point was 1 553.1 m a.s.l. on average, with minimum and maximum values of 1 100 m a.s.l. and 1 800 m a.s.l., respectively. The use of cable cranes are generally installed following the maximum gradient allowed differences in altitude to be covered from a minimum of 50 m to a maximum of 475 m and with an average of 197.2 m. The line gradient was 54.4 percent on average and varied between a minimum of 21 and a maximum of 131 percent.

The field length varied from 144 to 1 492 m, the mean value being 457.8 m. The topographical length was 407.7 m on average, with minimum and maximum values of 108 and 1 414 m.

To create the skyline corridors, an average of 49 trees were marked, with a minimum of 7 and maximum of 136 trees per corridor. The mean volume of marked trees was 65.3 m³ per corridor and varied from a minimum of 8.78 m³ to a maximum of 227.67 m³ per corridor.

Definition of indicators connected to cable crane utilization

The data of each cable crane line, measured in the field and collected from the working and felling plans, were processed in order to calculate some indicators useful for an analytical description of cable crane utilization in different operational situations.

The following indicators, reported in Table 3 and 4, were calculated:

1. “Volume of marked trees per compartment/field length of cable crane line” (m³/m)”
2. “Volume of marked trees per compartment/topographical length of cable crane line” (m³/m)”
3. “Volume of marked trees per skyline corridor/field length of cable crane line” (m³/m)”
4. “Volume of marked trees per skyline corridor/topographical length of cable crane line” (m³/m)”

The values of the indicators were then split in relation to the gradient of the cable crane line, the operational altitude of the cable crane line and the stand growing stock (see Table 4). Two classes of gradient were considered: one including the cable crane lines with a gradient of less than 55 percent and one including the cable crane lines with a gradient of 55 percent or steeper. Two classes of operational altitude were involved: one counting the cable crane lines with an altitude of less than 1 500 m a.s.l. and one counting the cable crane lines with an altitude of 1 500 m a.s.l. or above.

To include the cable crane lines in one or other of the two operational altitude classes, the mean altitude was considered, calculated as the altitude of the downhill anchor point plus the semi-difference between the altitudes of the two anchor points. This division between the two operational altitude classes is because of the fact that the main stand features alter at 1 500 m a.s.l.

Two classes were also considered for the unit growing stock: one including the cable crane lines installed in stands with a unit growing stock lower than 400 m³ per hectare, and one including the cable crane lines installed in stands with a unit growing stock equal to or above 400 m³ per hectare.

1. “Volume of marked trees per compartment/field length of cable crane line”

Analysing the indicator values show that an average tree volume of 0.72 m³ was marked per metre of field length of the cable crane line, with a minimum of 0.42 m³ per metre and a maximum of 1.52 m³ per metre (see Table 3).

Table 1. Stand parameters

Table 2. Cable crane features

Table 3. Indicators connected to cable crane utilization

Table 4. Indicators in relation to the gradient of the cable crane line, operational altitude of the cable crane line and stand unit growing stock

Comparing the data related to the gradient classes, the mean marked tree volume per metre of field length of the cable crane line was 0.76 m³ per metre on average for lines with a gradient lower than 55 percent, and 0.83 m³ per metre when the gradient is 55 percent or steeper. The operational altitude data show that the marked tree volume per metre of field length of the cable crane line showed a great difference when the cable crane lines were located beneath or above the altitude limit; in fact the indicator was 1.14 m³ per metre on average for the lines located at an altitude beneath 1 500 m a.s.l. and 0.59 m³ per metre for the lines at an altitude of 1 500 m a.s.l. or above.

Moreover, in stands with a unit growing stock lower than 400 m³ per hectare, an average tree volume per metre of field length of cable crane line of 0.85 m³ per metre was marked, while in stands with a unit growing stock of 400 m³ per hectare or higher the indicator had a value of 0.57 m³ per metre.

The stands with a low unit growing stock are generally affected by less frequent but more intense exploitation, because the higher logging costs must be shared by a large amount of wood to guarantee a favourable stumpage value.

2. “Volume of marked trees per compartment/topographical length of cable crane line”

This indicator was expected to show higher values than the previous one: in fact the volume of marked trees per metre of topographical length of the cable crane line was 0.81 m³ per metre on average, with a minimum value of 0.52 m³ per metre and a maximum of 1.74 m³ per metre (see Table 3).

Considering the gradient of the cable crane line: 0.76 m³ per metre were marked for the lines with a gradient lower than 55 percent and 0.66 m³ per metre for the lines with a gradient of 55 percent or steeper.

The indicator also varied according to the different operational altitude: for the lines located at a height lower than 1 500 m a.s.l., 1.14 m³ per metre were marked, while 0.67 m³ per metre were marked for the lines located at a height of 1 500 m a.s.l. or above.

The data on the volume of marked trees per metre of topographical length of the cable crane line in relation to the unit growing stock also showed some differences. For lines located in stands with a unit growing stock lower than 400 m³ per hectare, the indicator was 0.96 m³ per metre, while it was 0.65 m³ per metre for the lines located in stands with a unit growing stock of 400 m³ per hectare or above.

Even the data of the indicator “Volume of marked trees compartment/topographical length of cable crane line” can be explained given that the stands with low unit growing stock are more intensively felled, but at wider intervals than the stands with higher growing stock. This positively affects the economics of the utilization despite the difficulties of the terrain where the stands are.

3. “Volume of marked trees per skyline corridor/field length of cable crane line”

To create the skyline corridor, an average of 0.15 m³ of wood were marked per metre of field length of the line, with a minimum of 0.02 m³ per metre and a maximum of 0.6 m³ per metre.

Comparing the data related to the gradient classes, the mean marked tree volume per metre of field length of cable crane line were 0.18 m³ per metre on average for the lines with a gradient of less than 55 percent, and 0.15 m³ per metre when the gradient is 55 percent or above.

The operational altitude data show that the marked tree volume per metre of field length of cable crane line was not so different when cable crane lines were located above or below the altitude limit. In fact, the indicator was 0.14 m³ per metre for lines located at a height of 1 500 m a.s.l. or higher, and 0.18 m³ per metre on average for the lines located at a height below 1 500 m a.s.l.

The indicator was 0.15 m³ per metre on average for the cable crane lines installed in stands with a unit growing stock lower than 400 m³ per hectare, and 0.16 m³ per metre for the cable crane lines installed in stands with a unit growing stock of 400 m³ per hectare or higher.

4. “Volume of marked trees per skyline corridor/topographical length of cable crane line”

A mean value of 0.17 m³ per metre was obtained for the relationship between volume of marked trees per corridor and topographical length of cable crane line. The indicator varied from a minimum of 0.03 m³ per metre to a maximum of 1.28 m³ per metre.

For cable crane lines with a gradient lower than 55 percent, the opening of the skyline corridor required 0.19 m³ per metre of topographical length, while with a gradient of 55 percent or steeper, the opening required 0.17 m³ per metre.

It can be noted that the skyline corridor of the cable crane lines installed at a height of less than 1 500 m a.s.l. demanded a volume 0.2 m³ of marked trees for each metre of topographical length, while at a height of 1 500 m a.s.l. or above, 0.16 m³ per metre was required.

The volume of marked trees was 0.17 m³ per metre of topographical length in the stands with a unit growing stock below 400 m³ per hectare, and 0.18 m³ in the stands with a unit growing stock of 400 m³ per hectare or above.

Relationship between the analysed variables

The most significant variables for the analysis were “Field length of cable crane line”, “Topographical length of cable crane line”, “Volume of marked trees per compartment”, and “Volume of marked trees per skyline corridor”. In fact, “Volume of marked trees per compartment” and “Volume of marked trees per skyline corridor” were considered as dependent variables, while “Field length of cable crane line” and “Topographical length of cable crane line” were assumed as independent variables.

No association was found from the processing of data concerning “Field length of cable crane line” and “Topographical length of cable crane line” and “Volume of marked trees per compartment”. The values obtained for “Volume of marked trees per compartment” when “Field length of cable crane line” and “Topographical length of cable crane line” varied, showed great variability as stated by the low values of the correlation coefficient (R) (see Table 5).

Table 5. Values of the correlation coefficient calculated for the analysed variables

An association could exist between both the dependent variables “Field length of cable crane line” and “Topographical length of cable crane line” and the independent variable “Volume of marked trees per skyline corridor”, as assessed by the values of the correlation coefficient (see Table 5).

Figure 1. Volume of marked trees per skyline corridor as function of the field length of the cable crane line

The regression analysis calculated the equations which can describe the relationship between the above-mentioned variables (see Figures 1 and 2). The regression equations were not characterized by a high determination coefficient (R²) that was anyway able to point out the dependence degree among the variables. In fact, increasing the length (both field and topographical) of cable crane line, the volume of marked trees per skyline corridor tended to increase too.

It is therefore possible, through the length of the cable crane line measured either in the field or on a map (Regional Technical Map, Forest Management Map, etc.), to estimate the volume of trees that must be marked and felled for opening the skyline corridor.

Conclusions

The analysis of the operative conditions of cable cranes used in alpine forests, managed according to the close-to-nature silvicultural principles, allows the following conclusions to be drawn:

1. Although the close-to-nature silviculture adopted by the Veneto Region provides for low intensive and frequent exploitations, the use of cable cranes in the upper River Piave Valley is worthwhile from the operational point of view. In fact, data collected in operational areas similar to those considered in the study estimate that a volume of marked trees per metre of field length of cable crane line of between 0.5 and 1 m³ (Piegai, 1984) is necessary for economic cable logging. This volume is obtained in the area as an average of 0.72 m³ of marked trees felled per metre of cable crane line. This figure proves that, despite some limiting factors - such as the breaking up of forest ownership, the limited size of compartments and poor road network - the use of cable cranes is efficient because large quantities of wood are yarded. For this reason, an increased use of cable crane logging in forest harvesting can improve the yield of the forests managed according to the close-to-nature silvicultural principles.

   Figure 2. Volume of marked trees per skyline corridor as function of the topographical length of the cable crane line

2. It has been assessed that through cable crane use a tree volume per surface unit in accordance with the standards of the Veneto Region Forest Law is harvested in the area.
3. The information obtained allows the cable cranes to be installed in the study area to be managed from a preventive (cable crane planning during tree marking), executive (determination of the volume of trees that must be felled to create the skyline corridor) and control (check of the volume of trees felled to create the skyline corridor as regards the length of cable crane line) point of view.
4. If it is true that the main aim of marking is to achieve the best conditions for natural regeneration and to improve the state of the stand, only an efficient harvesting assessment can provide a rational application of the guiding principles of the management plan and criteria adopted during the tree marking. Within this context, the study can offer useful information for a rational use of cable extraction to the managerial and supervisory personnel responsible for harvesting operations.

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