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Dragutin PIČMAN, Tibor PENTEK, Tomislav PORŠINSKY
Department of Forest Harvesting, Forestry Faculty of Zagreb University, CROATIA


The Republic of Croatia is quite diverse regarding terrain, forest stands, climate, ecological, social-economic and other conditions. Globally, we can differ between the Pannonian area of even-aged forests, the Dinaric area of uneven-aged forests and the Eumediterranean and sub-Mediterranean areas of uneven-aged and even-aged forests. Particularly demanding is the opening with forest roads of the Dinara mountainous forest areas of Gorski kotar, Velebit, Velika and Mala Kapela, Plješivica and the Lika central mountains, but also individual mountain massifs of other parts of Croatia.

The principle of functionality is applied in opening mountain forests. Here the basic function of forests is emphasized as a dominant factor, which influences the quality and quantity of forest transport, networks. Further on, attention is given to the method of felling and processing used in exploiting economic forests, i.e. extraction machines used in the second phase of forest harvesting. The optimization of the forest road network (primary and secondary) is done with the assortment method of felling and processing and for ground machines used in skidding roundwood (skidders). Modern technologies and contemporary working methods are used in planning and designing, while ecologically acceptable machines and tools are used in construction.

The use of cables in roundwood yarding in the mountain areas of Croatia is not common. This ecologically acceptable method of wood extraction is used in areas of steep slopes in a rocky terrain, rich of karst phenomena, and firm categories of soils, but also in sensitive, erodible soils. Introducing new working devices instead of the already adopted ground extraction machines, assumes a different approach to opening areas and improving insufficiently opened forests in all work phases. This procedure of introducing new technologies of cables, more acceptable both for a forest and a natural site, has its repercussions on forest communications.

The aim of this paper is to show, as objectively and as extensively as possible, problems of forest opening by forest roads in classical systems of forest harvesting by ground extraction machines on the one hand and in forest openings for the needs of cables on the other hand. We would also like to determine the relationship between forest roads and machines used in wood extraction in a continuous, ecologically aware, intensive management of forest ecosystems.

Key words: cable systems, extraction machines, forest communications, Croatia, mountain area


Forests and forest soil account for about 35 percent of the area of Croatia. The “Hrvatske šume” (Croatian forests) p.o. Zagreb public enterprise manages 78 percent of the total forest and forest soil area in the Republic of Croatia with 16 forest managements across the country. The area of the Republic of Croatia is quite diverse regarding terrain, forest stands, climate, and ecological, socio-economic and other conditions, which makes it quite difficult for forest management.

This paper will deal with the management of selective economic forests of steep mountain areas. Mountain forest harvesting is directly connected to accessibility and forest communications. Šikić et al. (1989) define the following criteria in the ground configuration estimate.

When we speak about economic forests in the mountainous areas of the Republic of Croatia, natural, mixed and uneven-aged forests managed by selection felling, group and individual tree felling with a felling cycle of ten years are significant. The most important market species are fir (Abies alba), spruce, (Picea abies), beech (Fagus sylvatica) and sycamore (Acer pseudoplatanus).

Table 1. The ground configuration estimate

Basic terrain characteristicsThe terrain configuration
Low-lying terrainHilly terrainMountainous terrain
Altitude difference per 1 km of the roadto 20 mfrom 20 to 120 mover 120 m
Inclination of the slopeto 1:10from 1:10 to 1:3from 1:3 to 1:0
Terrain indentationslightmarkedvery strong

The Delnice forest management area is typical of such terrain and forest stand conditions of selective forest stands. Here the average growth is 276 m3, where coniferous (fir and spruce) account for 53 percent and broadleaf 47 percent (primarily beech). The average ten-year increment is 51 m3 per hectare, while the average ten-year harvesting volume is 43 m3 per hectare. The volume of mean cubic tree is 1.47 m3, while the volume of mean cutting tree is 1.96 m3.

Characteristics of forest harvesting in selective forests of hilly mountainous forest stands of fir and beech are as follows:

  1. Felling and processing are completely done by motor chainsaw. In the processing of fir trees, the method of loading length is used where assortment lengths (4 to 12 m) are adjusted to the means of secondary wood transport. The beech trees are processed by the assortment method.
  2. Wood bunching and skidding relies on skidders equipped with single-drum or two-drum winches (LKT 80, LKT 81, Silva, Timberjack 240 C) by which 95 percent of processed assortments are extracted. Adapted agricultural tractors are also used for wood skidding apart from skidders, while cables are only used from time to time.
  3. Secondary (further) transport of round timber is usually done by trucks, while the railway transport is sparse. Various truck types with trailers and semi-trailers, i.e. towing vehicles, are used for the transport of round trees. All trucks are equipped with hydraulic cranes, which are, depending on the vehicle type, either behind the cabin, or on the extended chassis of the truck.

The procedure described of logging roundwood to a landing assumes a high quality built network of primary (forest roads) and secondary (strip roads and skid trails) of forest communications, i.e. sufficient forest openness and an adequate mean value of timber extraction.

The existing situation in the Republic of Croatia

Road division and the existing forest openness

According to the traffic type on forest communications we can distinguish between:

Since extraction machines moving on the ground are the usual means of labour in the second phase of forest utilization in Croatian forestry, and cable use so far has only been used rarely, this chapter will deal with forest roads, strip roads and skid trails.

The existing minimum prescribed openness of the Croatian mountainous areas by forest roads is 15 m per hectare, while the openness of 25 m per hectare is planned until 2010. There is a State land register of forest roads in which the existing forest roads are registered and classified according to certain criteria. Built forest roads are surveyed by GPS and drawn into forest management maps on a scale 1:10000. For individual management units there are also computer GIS databases incorporating built forest roads.

Secondary forest communications are not precisely drawn into forest economic maps and there is not a land register for them. Strip roads and skid trails are partially drawn into working maps when making work site surveys for wood felling and processing, therefore, we cannot speak about average openness of the mountainous area of the Republic of Croatia. In planning and building secondary forest communications, there is a tendency to decrease the average skidding distance to limits where extraction machines used in the second stage of forest harvesting reach their optimum effect.

Opening systems of mountainous forests

In mountainous areas with wider and steeper slopes we design main forest roads that are almost parallel and laid out in layers. Diagonal connecting roads are built between main roads that connect them. In mountain areas with developed hydrography, forest road routes follow watercourses and have the form of veins or pinnate leaves. At the bottom of a valley, forest roads have the form of a fan.

Table 2. Geometrical elements of forest roads in a mountainous area

Road categoryWith 2 lanesWith 1 laneRmin.Nmax.
CR4,5–5,00,75–1,00,3–3,0---40 (S=15)8 (12)
MFR5,0–5,50,75–1,00,3–3,0---20 (S=12)8 (12)
SFR4,5–5,00,75–1,00,3–3,03,5–4,00,750,3–1,520 (S=12)15 (20)
AFR---3,5–4,00,750,3–1,520 (S=12)15 (20)

Comments on Table 2:
CRconnecting roads
MFRmain forest roads
SFRsecondary forest roads
AFRaccess forest roads
Pwpavement width
Swshoulder width
ΔPwpavement extension
Rminminimum radius of a horizontal circular arch
Nmaxmaximum allowed longitudinal gradient
( )exceptional value

Strip roads and skid trails are the forest communications that go deepest into a forest, and in a mountainous area skid trails are on slopes following a terrain fall. The natural terrain fall is usually used in the direction of log skidding where the highest-grade line slopes are even above 30 percent (depending on skidding direction). Strip roads are built by an excavator with a hydraulic hammer and the formation width of a strip road for skidders is 3.50 m and, on exceptionally difficult terrain and in horizontal circular arches of small radius, up to 4.00 m. Skid trails are not built, but are made by a tractor passing several times on the same track.

Planning and designing forest communications

Planning of forest communications (forest roads, strip roads and skid trails) is done on forest management maps on a scale 1:10000. Newly planned communications are fitted into the existing network of forest roads in the form of zero lines. The most favourable variant of zero lines is chosen by analysis, according to chosen parameters, and is later transferred to a terrain. The planning procedure is done by classical methods, i.e. modern technologies, by a computer and computer programmes, if the adequate computer GIS database, which is used for decision making, is designed.

Planning of strip roads is completed when the chosen variant of a zero line is laid down on a terrain (taking into account terrain and forest stand conditions) since this category of forest road is not designed, but is built after the zero line is pegged.

After laying down a zero line on a terrain, a forest road is approximated by axle polygon and polygon points are pegged. Then a classical terrain survey is done by geodetic instruments whose result on the terrain is a laid out route for a future forest road. Terrain data and the main working design are made by a computer programme “CESTA” from a Slovenian firm called SoftData.

Building forest roads

Ecologically acceptable building technologies are used in forest road construction in mountainous areas of Croatia. Bulldozers are used for the work in easier building material categories, on smaller cross falls of a terrain and on erosion-resistant grounds; excavators with a hydraulic hammer are used in various categories of rocks, on higher cross falls of a terrain (sometimes these falls are over 120 percent) and on erodible ground. For some time, explosives have not been used in forest road construction.

Building forest roads is a very demanding task and, owing to hard terrain conditions, there must be structures for surface drainage (outlets, gutters, curbs, sewer pipes, overflows) and road structures (supporting walls, revetment walls, bridges). Steep slopes and small fall width on one hand, and prescribed technical elements of road building on the other hand, result in building higher numbers of serpentines as a characteristic technical element of forest roads on steep mountain terrain by which the forest road route is extended.

In our mountain forests a secondary communication is seldom made by tractor passing along the same track several times; more frequently secondary communications have to be built and the performance technology described in building forest roads is used. Strip roads do not have drainage structures, do not have superstructure or accompanying buildings.

All the above leads to the conclusion that building forest roads in mountain areas (on steep ground) is demanding and very expensive, therefore, they must be planned and designed to yield the highest profit with the lowest investment and a minimum damage to the forest ecosystem.

Comparison of classical (skidders) and contemporary (cables) extraction machines

Slope of the ground, haulage direction, soil bearing capacity and mean skidding distance

The existing combination of a skidder and a truck in primary, i.e. secondary round timber transport, makes forest harvesting rely on the network of forest and public communications. Truck forest roads will shorten a distance and decrease costs of wood haulage; strip roads and skid trails will shorten the distance of wood collection by a winch and enable the movement of extraction machines with cargo on more or less steep mountain terrain.

Skid trails and strip roads in hilly-mountainous selective forest stands are a basic network of secondary forest openings which enable the fastest and shortest way to fell and process trees. The opening up of forests by the system of skid trails and strip roads rationalizes the work in a felling site and enables lower costs for felling, processing and extracting.

Trzesniowski (1998) gives a schematic presentation in which he mentions limits for the use of certain machines for roundwood extraction (on the ground and in the air) regarding the extracting direction (uphill and downhill), the mean extracting distance, slope of the ground and soil strength capacity. This is shown in Table 3 in a rather modified way.

Since the mountainous terrain of the Republic of Croatia has steep slopes, we can say that in our country there is a possibility and need for introducing wood yarding technology that uses cables. Table 4 shows terrain slopes grouped in slope categories according to the categorization Löffler (1984) for one characteristic management unit of the mountain area of the Republic of Croatia. Calculation of slope categories is based on a digital terrain model (DTM).

Table 3. The possible use of some machines for wood extraction on a firm soil and regarding the direction of extraction, mean extraction distance and slope of the terrain

Uphill extraction
Extraction machineAdapted tractorSkidderSkidder (winching)Cable
Slope of the ground/skid trail0 – 10 (15%)0 – 20 (25) %+20 (25)%+ 20 (25)%
Mean extracting distanceto 200 mto 300 mto 50 (100) m300/500/800 m
Downhill extraction
Extraction machineAdapted tractorSkidderSkidder (winching)Cable
Slope of the ground/skid trail0–30%0–45%-+45%
Mean extracting distanceto 300 (500) mto 800 (1 000) m-300/500/800 m

* Data given for terrain of good bearing capacity

Table 4. Slope of the ground category distribution of a characteristic management unit of a mountain area

Slope groupSlopeSlopeAreaPercentage
10 – 60 – 1092.713.69
26 – 1110 – 20566.8022.54
311 – 1820 – 30591.1223.50
418 – 2730 – 50837.2233.29
5+ 27+ 50427.1816.98
Total--2 515.13100.00

Steep and very steep terrain together account for 50 percent of the total area of this management unit and these represent limits where skidders can work, i.e. areas for possible cable use. Baldini and Pollini (1998) mention that in Italy skidders or cables are used on terrain from 31 to 60 percent, and on slopes over 80 percent only cables are used.

Necessary infrastructure

The mountain areas of the Republic of Croatia have good primary openness (about 25 m per hectare) even on the steepest terrain, most difficult for road building. Such openness is sufficient for mobile cable installation that would have to be up to 800 m long. At the same time, after the introduction of cables in Croatian forest harvesting, fewer forest roads and skid trails would be built on the most difficult and steepest terrain (which are the only ones that remained unopened when we speak about mountainous economic forests), where this type of construction is the most expensive and the most damaging for a forest and a forest stand, because of ecological consequences and tree damage.

If necessary, the existing main strip roads, whose greatest longitudinal slope is up to 12 percent, could be reconstructed to become forest roads, therefore, at minimum cost the primary forest openness would increase. Further building of skid trails (in the area of cable use) would not be necessary. All financial means could be directed towards the maintenance of existing forest roads and improvement in their quality, i.e. funds could be directed to the purchase of contemporary extraction machines.

Further building of the forest road network should also be planned in accordance with the chosen technology of the second phase of forest harvesting in sufficiently open steep forest areas in which cables will be introduced. It implies a different approach towards the procedure of forest opening, and different input parameters in the analysis and choice of the best routes for forest roads and different optimum forest openness.

Advantages and failings of cable introduction in Croatian forestry

The basic advantages of cable use in comparison with skidder use on steep mountain terrain are as follows:

With the advantages, we have to mention the disadvantages that will result from the introduction of cable use in Croatian forestry. They are:

Breaking with tradition and breaking down resistance to new things requires effort. Changes have always been met by resistance everywhere, especially in forestry, which is regarded as a traditionalistic profession.

Final considerations

From the above, we can conclude that in the Republic of Croatia there are many steep mountain terrains of economic selective forests that represent the optimum working area for forest cables. Also, Croatia as a country with a long forestry tradition must follow new trends and developments in technology as in other European and the world's leading countries.

The road forest infrastructure in areas with potential cable use offers - without an excessive additional investment - quite a good starting point for their introduction. This introduction of cables must not be disorganized, and scientific studies should be made which will decisively answer the question concerning on which terrain cables should be used, as well as what kind of cables. The experience and expertise of countries that have been using cables for years should be considered and, if necessary, modelled and modified and transferred to our economic, social and ecological environment.

Further possible opening of forests by forest roads, i.e. expansion of the existing transport network, should continue in that direction. In planning and designing forest roads, the principle of forest functionality should be respected; apart from that principle, for a long-term efficient forest transport network, it is also extremely important to have an adequate professional orientation in order to be able to use certain technologies and technical devices in specific conditions.

The use of cables in our low-lying flood areas is also a possibility where the most valuable even-aged forests of bay oak (Quercus robur) grow, but where the limiting factor of forest harvesting is the soil of low or no bearing capacity (which also makes the building of forest roads difficult and increases the cost of their building and maintenance, as well as the building of strip roads that decreases the extraction machine efficacy and increases the cost per cubic metre of processed wood and soil damage, etc.).


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