Possibilities of Application of Relative Openness in Secondary Forest Opening of Slope Forests in Croatia

For achieving a high-quality and efficient management of forest ecosystems (Pentek et al. 2010), it is necessary to provide an optimally distributed network of primary and secondary forest road infrastructure. The openness of forest areas, the need to describe the existing situation and the tendency to rationalize the existing road network were described with traditional openness (density of forest roads). Traditional openness is a well-known parameter that has represented the size of the basis determined by the level of the achieved existing primary and secondary road density for a long time in forestry. In many previous researches, different authors described the current state with traditional openness of the secondary forest road network (Rebula 1983, Zdjelar 1990). Traditional openness is a numeric data that says little about the quality of road distribution on the observed areas. It represents only the quantity and distribution of primary and secondary forest roads. The transitional form of themodern view of the subject matter is a combination of traditional openness andmean skidding distance (Pentek et al. 2010, Pentek et al. 2007). Relative openness has, lately, (Pentek et al. 2010, Pentek et al. 2007) been increasingly used as a variable that describes more precisely and accurately the actual state of forest road infrastructure and defines the need for further opening of forests. The basic terrain characteristics (Šikić et al. 1989) such as relief altitudes, slopes and indented terrains determine the terrain configuration such as: the plain, hilly and mountainous terrains. Generally, relief and terrain can also be divided into plain and bumps, where the plains are – lowlands and plateaus, and bumps are – elevations (hills, highlands, mountains) and troughs (valleys, basins, river basins). The relative openness is both a quantitative and qualitative parameter (Pentek 2010), which gives a good insight into the spatial distribution of forest roads, and expresses the ratio of surface forest area opened by forest roads and the total forest area in percentage terms. It is the parameter that (Pentek 2002), with a good insight into the spatial distribution of forest roads, gives the possibility of establishing open and unopened areas, and offers to designers the option of choosing the most appropriate version of forest road routes. The relative openness is determined by the buffer method and the procedure consists in laying down the buffered areas around the forest road infrastructure. Primary and secondary relative openness can be distinguished depending on the category of forest roads. The secondary relative openness provides information on quantity and spatial distribution of secondary forest road network. The procedure of determining the secondary relative openness is very similar to the process of determining the primary relative openness, but for the calculation of the buffer zone width around skid roads, skid trailsor cable-yarder lines, the mean skidding distance is not taken into account. Depending on applied timber harvesting technologies, the following parameters can be used:• winch rope length,• forwarder cranes,• length of the cable yarder rope for sideward skidding.The primary forest road infrastructure is taken into consideration when calculating the secondary relative openness because the components of the primary forest road infrastructure can be used in winching roundwood. The importance of the quality of spatial distribution of secondary forest roads (skid roads and skid trails) is determined by the fact that with the same density of secondary roads, if their distribution is bad, the distance of timber extraction is twice bigger than when their distribution is carefully defined (Knežević 1980, Rebula 1981, Rebula 1983). The secondary forest opening depends on the relief area in which the procedure of fine opening is carried out, as well as on the applied (possible) timber harvesting technologies. On flat terrains, which allow skid road and skid trail routes to be laid out perpendicular to the contour lines, forest areas are opened in the shape of a rectangle network, i.e. components of secondary forest road network are separated from the primary components at right angle, and the components of lower order are also separated from those of higher order at right angle, within the network of secondary forest roads. Laying out the network of secondary forest roads in the plain area is not a major problem due to the fact that tractors can move in all directions, which makes the performance of the procedure very simple; no excavating is required, but only removing the stumps of cut trees. Here the system of secondary forest roads is represented through skid trails, which are in fact the defined timber extraction routes – the so called – parallel pattern. On subhilly and hilly terrains with developed hydrographic network and indented relief interlaced with coves and ditches, skid roads and skid trails, adapting to and following the terrain configuration, crawl along the waterways and bays, and at final parts of the watercourses they can be branched in fan-shaped pattern, in the so-called plumose pattern and fishbone pattern. In mountainous areas with large slope, skid roads and skid trails are developed on the slopes following them in an appropriate, allowable, positive longitudinal upward inclination – fishbone pattern. In karst terrain with abundant karst phenomena, particularly sinkholes, the routes of skid roads should be laid out on passes between sinkholes, and in other parts of the forest area they must be adapted to the relief configuration. Laying out skid roads in sinkholes is not justified because of uphill timber skidding, especially from deep sinkholes with steep slopes – irregular pattern. The research area was the Management Unit »Bovan-Jelar«, Forest Office Perušić, Forest Administration Gospić. It is an integral part of North Velebitmassif, extending in the east to west direction. Due to its location and altitude, it is considered a high mountain area. The total area of forest management units amounts to 2,413.14 ha. The basic features of forest opening and harvesting are the steep mountainous terrain and indented, shallow soil, rocky substrates and heavy construction material categories. The average slope is 20–40°. These features suggest the need for good primary and secondary forest openness. The annual allowable cut (26.36 m3/ha) is of very good quality. The primary openness of the completeManagementUnit »Bovan-Jelar« is 9.97 m per ha, while the openness of secondary forest roads is only 26.74 m per ha. If only the selected area is taken into consideration, then the primary openness is 27.13 m per ha, while the secondary openness is 45.19 m per ha. Researches were carried out in the selection forests of the Management Unit »Veprinačke šume«, Forest Office Opatija, Forest Administration Buzet. The total area of management units is 1,950.87 ha. TheManagement Unit »Veprinačke šume« is part of the mountain Ćićarija. The basic features of forest opening and harvesting are steep and indented mountainous terrain, rich with karst phenomena, shallow soil, rocky substrates and heavy construction material categories. The average slope is 5–30°. Based on these features, the need for an effective primary and secondary openness is obvious. The annual allowable cut (41.59 m3/ha) is of very good quality. The primary openness is 8.58 m/ha, or 16.78 m/ha, if the old Italian public road with a superstructure made of crushed gravel is taken into account. The secondary openness is 101.94 m/ha. The objectives of this study were defined through the following phases of work:• establishment of the secondary forest road cadastre,• analysis of the secondary relative openness for the selected skidder winch rope length,• designing (development) of the secondary forest road network.In the selected subcompartments of the ManagementUnit »Bovan-Jelar«, 123 skid roads were investigated of a total length of 46,656 m. The research area covers 942.10 hectares and the secondary openness with skid roads is 49.52 m per ha. In the analysis of the secondary relative openness, the value of access to an area of 45 m was chosen based on the skidder winch rope length.Due to the terrain slope and surface barriers, the selected winch rope length was corrected by 10% because of the increased directional felling. The analysis of conditions in the selected subcompartments of the Management Unit »Bovan-Jelar« showed a lack of the secondary relative openness. In order to achieve excellent secondary relative openness, a fine opening of further research areas was done for the selected winch rope length. When improving (upgrading) the existing network of secondary forest roads, the winch rope length of less than 45mwas selected as relevant, with the aim of providing work humanization and adapting to the demanding terrain conditions. For this version, a high number of conceptual routes were designed – 83 new skid roads of a total length of 46,438.91 m. The newly designed secondary forest road network reached the secondary relative openness of 90.47%, by which excellent openness was achieved. Such secondary relative openness was achieved with a density of only 106.88 m per ha of skid roads. The terrain of the observed area is mountainous and this is why the whole management unit was not taken into consideration. The rest of the management unit is extremely steep and has a protective character. The mountainous terrain is quite steep, but only slightly indented and rather unchanging, which explains the smaller traditional openness required to achieve excellent secondary relative openness than was the case in the previous studies (Jeličić 1983, Rebula and Zdjelar 1983, 1990). Asignificant contribution to a more efficient network of the secondary forest roads is provided by the application ofmodern technologies andmethods in the planning of secondary forest roads. Surveys conducted in the Management Unit »Veprinačke šume« cover 1,326.10 ha of forest area and include 629 skid roads of a total length of 152,893.44 m. The reason for such a large number of skid roads lies in the fact that the terrain is mountainous, very indented with a large amount of karst phenomena, and hence a higher number of skid roads is necessary because of large terrain indentation. In the researched area the existing secondary relative openness is very good (according to Pentek 2002), and it amounts to 83.13%. Traditional secondary openness is 134.39 m per ha, and it includes the primary roads: the public roads and forest roads with skidding possibilities. The secondary relative openness was determined for three different skidder winch rope lengths: 30, 45 and 60 meters. The chosen winch rope length was decreased by 10% because of the terrain slope and surface barriers. The surface barriers, spatial distribution of standing trees in the stand and directed trees felling were also taken into consideration. The corrected values were increased by the average value of timber assortments because it is enough to reach only the top of an individual assortment to increase the total area of reach of a tree. Designing new conceptual routes had to be done to improve the existing secondary road network to an excellent level of the secondary relative openness, which was achieved with 42 new skid roads of a total length of 14,332 m. The secondary relative openness in this case amounts to 90.13%, and the road density is 144.12 m per ha. In the Republic of Croatia harvesting is done by ground machinery, cable-yarders are very rare, and helicopters and other forms of air transport are not applied. Therefore, good primary and especially secondary forest openness by forest roads is extremely important. Comparing the results with previous studies in similar conditions (Rebula 1983 and Zdjelar 1990), the optimal secondary relative openness was achieved with less total length of secondary forest roads. A similar result was obtained by research of the terrain with similar characteristics in Bosnia and Herzegovina (Jeličić 1983), according to which secondary openness of 100 m per ha was recommended in even-aged forests and up to 200 m per ha in selection forests. In earlier studies (Pentek et al. 2008), in similar terrains, secondary forest road network optimization has been done for a various number of winch rope lengths: 30, 40, 50 and 60 meters. Due to terrain conditions, horizontal barriers, existing secondary forest road network and in order to provide work humanization, the winch rope length of 40 m was selected. The current secondary relative openness was 78.42%. By designing 23.03 km of new skid roads, an excellent secondary relative openness of 90% has been achieved with the traditional secondary openness of 113.47 m per ha. Comparing the results with previous studies (Pičman and Pentek 2003), taking into account the particularities of individual research areas, results in constantly developing methods for estimating the existing and optimizing future networks of secondary forest roads, thus achieving an excellent secondary relative openness with lower density of secondary forest roads. Planning a network of secondary forest roads is significantly determined by the existing secondary forest road infrastructure, which often directs further opening. The analysis of the current situation showed that the forest area is opened by the existing network of secondary roads two or even several times more than necessary, thus reducing the efficiency coefficient of the existing road network. This is the result of unsystematic forest opening in the past. It is therefore very important to use the modern methods of forest opening in the forest areas where the procedure of fine opening has been applied from the start, or where the secondary openness is very bad